Chapter 4: Proper Charge Methods
To a large extent, the performance and longevity of rechargeable batteries depends on the quality of the chargers. Battery chargers are commonly given low priority, especially on consumer products. Choosing a quality charger makes sense. This is especially true when considering the high cost of battery replacements and the frustration that poorly performing batteries create. In most cases, the extra money invested is returned because the batteries last longer and perform more efficiently.
All About Chargers
Text Box: Battery chargers are commonly given low priority.There are two distinct varieties of chargers: the personal chargers and the industrial chargers. The personal charger is sold in attractive packaging and is offered with such products as mobile phones, laptops and video cameras. These chargers are economically priced and perform well when used for the application intended. The personal charger offers moderate charge times.
In comparison, the industrial charger is designed for employee use and accommodates fleet batteries. These chargers are built for repetitive use. Available for single or multi-bay configurations, the industrial chargers are offered from the original equipment manufacturer (OEM). In many instances, the chargers can also be obtained from third party manufacturers. While the OEM chargers meet basic requirements, third party manufacturers often include special features, such as negative pulse charging, discharge function for battery conditioning, and state-of-charge (SoC) and state-of-health (SoH) indications. Many third party manufacturers are prepared to build low quantities of custom chargers. Other benefits third party suppliers can offer include creative pricing and superior performance.
Not all third party charger manufacturers meet the quality standards that the industry demands, The buyer should be aware of possible quality and performance compromises when purchasing these chargers at discount prices. Some units may not be rugged enough to withstand repetitive use; others may develop maintenance problems such as burned or broken battery contacts.
Uncontrolled over-charge is another problem of some chargers, especially those used to charge nickel-based batteries. High temperature during charge and standby kills batteries. Over-charging occurs when the charger keeps the battery at a temperature that is warm to touch (body temperature) while in ready condition.
Text Box: High temperature during charge and standby kills batteries.Some temperature rise cannot be avoided when charging nickel-based batteries. A temperature peak is reached when the battery approaches full charge. The temperature must moderate when the ready light appears and the battery has switched to trickle charge. The battery should eventually cool to room temperature.
If the temperature does not drop and remains above room temperature, the charger is performing incorrectly. In such a case, the battery should be removed as soon as possible after the ready light appears. Any prolonged trickle charging will damage the battery. This caution applies especially to the NiMH because it cannot absorb overcharge well. In fact, a NiMH with high trickle charge could be cold to the touch and still be in a damaging overcharge condition. Such a battery would have a short service life.
A lithium-based battery should never get warm in a charger. If this happens, the battery is faulty or the charger is not functioning properly. Discontinue using this battery and/or charger.
It is best to store batteries on a shelf and apply a topping-charge before use rather than leaving the pack in the charger for days. Even at a seemingly correct trickle charge, nickel-based batteries produce a crystalline formation (also referred to as ‘memory’) when left in the charger. Because of relatively high self-discharge, a topping charge is needed before use. Most Li-ion chargers permit a battery to remain engaged without inflicting damage.
There are three types of chargers for nickel-based batteries. They are:
Slow Charger — Also known as ‘overnight charger’ or ‘normal charger’, the slow-charger applies a fixed charge rate of about 0.1C (one tenth of the rated capacity) for as long as the battery is connected. Typical charge time is 14 to 16 hours. In most cases, no full-charge detection occurs to switch the battery to a lower charge rate at the end of the charge cycle. The slow-charger is inexpensive and can be used for NiCd batteries only. With the need to service both NiCd and NiMH, these chargers are being replaced with more advanced units.
If the charge current is set correctly, a battery in a slow-charger remains lukewarm to the touch when fully charged. In this case, the battery does not need to be removed immediately when ready but should not stay in the charger for more than a day. The sooner the battery can be removed after being fully charged, the better it is.
A problem arises if a smaller battery (lower mAh) is charged with a charger designed to service larger packs. Although the charger will perform well in the initial charge phase, the battery starts to heat up past the 70 percent charge level. Because there is no provision to lower the charge current or to terminate the charge, heat-damaging over-charge will occur in the second phase of the charge cycle. If an alternative charger is not available, the user is advised to observe the temperature of the battery being charged and disconnect the battery when it is warm to the touch.
The opposite may also occur when a larger battery is charged on a charger designed for a smaller battery. In such a case, a full charge will never be reached. The battery remains cold during charge and will not perform as expected. A nickel-based battery that is continuously undercharged will eventually loose its ability to accept a full charge due to memory.
Quick Charger — The so-called quick-charger, or rapid charger, is one of the most popular. It is positioned between the slow-charger and the fast-charger, both in terms of charging time and price. Charging takes 3 to 6 hours and the charge rate is around 0.3C. Charge control is required to terminate the charge when the battery is ready. The well designed quick-charger provides better service to nickel-based batteries than the slow-charger. Batteries last longer if charged with higher currents, provided they remain cool and are not overcharged. The quick-chargers are made to accommodate either nickel-based or lithium-based batteries. These two chemistries can normally not be interchanged in the same charger.
Fast Charger — The fast-charger offers several advantages over the other chargers; the obvious one is shorter charge times. Because of the larger power supply and the more expensive control circuits needed, the fast-charger costs more than slower chargers, but the investment is returned in providing good performing batteries that live longer.
The charge time is based on the charge rate, the battery’s SoC, its rating and the chemistry. At a 1C charge rate, an empty NiCd typically charges in a little more than an hour. When a battery is fully charged, some chargers switch to a topping charge mode governed by a timer that completes the charge cycle at a reduced charge current. Once fully charged, the charger switches to trickle charge. This maintenance charge compensates for the self-discharge of the battery.
Modern fast-chargers commonly accommodate both NiCd and NiMH batteries. Because of the fast-charger’s higher charge current and the need to monitor the battery during charge, it is important to charge only batteries specified by the manufacturer. Some battery manufacturers encode the batteries electrically to identify their chemistry and rating. The charger then sets the correct charge current and algorithm for the battery intended. Lead Acid and Li-ion chemistries are charged with different algorithms and are not compatible with the charge methods used for nickel-based batteries.
It is best to fast charge nickel-based batteries. A slow charge is known to build up a crystalline formation on nickel-based batteries, a phenomenon that lowers battery performance and shortens service life. The battery temperature during charge should be moderate and the temperature peak kept as short as possible.
It is not recommended to leave a nickel-based battery in the charger for more than a few days, even with a correctly set trickle charge current. If a battery must remain in a charger for operational readiness, an exercise cycle should be applied once every month.
A charger designed to service NiMH batteries can also accommodate NiCd’s, but not the other way around. A charger only made for the NiCd batteries could overcharge the NiMH battery.
While many charge methods exist for nickel-based batteries, chargers for lithium-based batteries are more defined in terms of charge method and charge time. This is, in part, due to the tight charge regime and voltage requirements demanded by these batteries. There is only one way to charge Li-ion/Polymer batteries and the so-called ‘miracle chargers’, which claim to restore and prolong battery life, do not exist for these chemistries. Neither does a super-fast charging solution apply.
The pulse charge method for Li-ion has no major advantages and the voltage peaks wreak havoc with the voltage limiting circuits. While charge times can be reduced, some manufacturers suggest that pulse charging may shorten the cycle life of Li-ion batteries.
Fast charge methods do not significantly decrease the charge time. A charge rate over 1C should be avoided because such high current can induce lithium plating. With most packs, a charge above 1C is not possible. The protection circuit limits the amount of current the battery can accept. The lithium-based battery has a slow metabolism and must take its time to absorb the energy.
Lead acid chargers serve industrial markets such as hospitals and health care units. Charge times are very long and cannot be shortened. Most lead acid chargers charge the battery in 14 hours. Because of its low energy density, this battery type is not used for small portable devices.
In the following sections various charging needs and charging methods are studied. The charging techniques of different chargers are examined to determine why some perform better than others. Since fast charging rather than slow charging is the norm today, we look at well-designed, closed loop systems, which communicate with the battery and terminate the fast charge when certain responses from the battery are received.
Charging the Nickel Cadmium Battery
Battery manufacturers recommend that new batteries be slow-charged for 24 hours before use. A slow charge helps to bring the cells within a battery pack to an equal charge level because each cell self-discharges to different capacity levels. During long storage, the electrolyte tends to gravitate to the bottom of the cell. The initial trickle charge helps redistribute the electrolyte to remedy dry spots on the separator that may have developed.
Some battery manufacturers do not fully form their batteries before shipment. These batteries reach their full potential only after the customer has primed them through several charge/discharge cycles, either with a battery analyzer or through normal use. In many cases, 50 to 100 discharge/charge cycles are needed to fully form a nickel-based battery. Quality cells, such as those made by Sanyo and Panasonic, are known to perform to full specification after as few as 5 to 7 discharge/charge cycles. Early readings may be inconsistent, but the capacity levels become very steady once fully primed. A slight capacity peak is observed between 100 and 300 cycles.
Most rechargeable cells are equipped with a safety vent to release excess pressure if incorrectly charged. The safety vent on a NiCd cell opens at 1034 to 1379 kPa (150 to 200 psi). In comparison, the pressure of a car tire is typically 240 kPa (35 psi). With a resealable vent, no damage occurs on venting but some electrolyte is lost and the seal may leak afterwards. When this happens, a white powder will accumulate over time at the vent opening.
Commercial fast-chargers are often not designed in the best interests of the battery.Commercial fast-chargers are often not designed in the best interests of the battery. This is especially true of NiCd chargers that measure the battery’s charge state solely through temperature sensing. Although simple and inexpensive in design, charge termination by temperature sensing is not accurate. The thermistors used commonly exhibit broad tolerances; their positioning with respect to the cells are not consistent. Ambient temperatures and exposure to the sun while charging also affect the accuracy of full-charge detection. To prevent the risk of premature cut-off and assure full charge under most conditions, charger manufacturers use 50°C (122°F) as the recommended temperature cut-off. Although a prolonged temperature above 45°C (113°F) is harmful to the battery, a brief temperature peak above that level is often unavoidable.
More advanced NiCd chargers sense the rate of temperature increase, defined as dT/dt, or the change in temperature over charge time, rather than responding to an absolute temperature (dT/dt is defined as delta Temperature / delta time). This type of charger is kinder to the batteries than a fixed temperature cut-off, but the cells still need to generate heat to trigger detection. To terminate the charge, a temperature increase of 1°C (1.8°F) per minute with an absolute temperature cut-off of 60°C (140°F) works well. Because of the relatively large mass of a cell and the sluggish propagation of heat, the delta temperature, as this method is called, will also enter a brief overcharge condition before the full-charge is detected. The dT/dt method only works with fast chargers.
Harmful overcharge occurs if a fully charged battery is repeatedly inserted for topping charge. Vehicular or base station chargers that require the removal of two-way radios with each use are especially hard on the batteries because each reconnection initiates a fast-charge cycle. This also applies to laptops that are momentarily disconnected and reconnected to perform a service. Likewise, a technician may briefly plug the laptop into the power source to check a repeater station or service other installations. Problems with laptop batteries have also been reported in car manufacturing plants where the workers move the laptops from car to car, checking their functions, while momentarily plugging into the external power source. Repetitive connection to power affects mostly ‘dumb’ nickel-based batteries. A ‘dumb’ battery contains no electronic circuitry to communicate with the charger. Li-ion chargers detect the SoC by voltage only and multiple reconnections will not confuse the charging regime.
More precise full charge detection of nickel-based batteries can be achieved with the use of a micro controller that monitors the battery voltage and terminates the charge when a certain voltage signature occurs. A drop in voltage signifies that the battery has reached full charge. This is known as Negative Delta V (NDV).
NDV is the recommended full-charge detection method for ‘open-lead’ NiCd chargers because it offers a quick response time. The NDV charge detection also works well with a partially or fully charged battery. If a fully charged battery is inserted, the terminal voltage raises quickly, then drops sharply, triggering the ready state. Such a charge lasts only a few minutes and the cells remain cool. NiCd chargers based on the NDV full charge detection typically respond to a voltage drop of 10 to 30mV per cell. Chargers that respond to a very small voltage decrease are preferred over those that require a larger drop.
To obtain a sufficient voltage drop, the charge rate must be 0.5C and higher. Lower than 0.5C charge rates produce a very shallow voltage decrease that is often difficult to measure, especially if the cells are slightly mismatched. In a battery pack that has mismatched cells, each cell reaches the full charge at a different time and the curve gets distorted. Failing to achieve a sufficient negative slope allows the fast-charge to continue, causing excessive heat buildup due to overcharge. Chargers using the NDV must include other charge-termination methods to provide safe charging under all conditions. Most chargers also observe the battery temperature.
The charge efficiency factor of a standard NiCd is better on fast charge than slow charge. At a 1C charge rate, the typical charge efficiency is 1.1 or 91 percent. On an overnight slow charge (0.1C), the efficiency drops to 1.4 or 71 percent.
At a rate of 1C, the charge time of a NiCd is slightly longer than 60 minutes (66 minutes at an assumed charge efficiency of 1.1). The charge time on a battery that is partially discharged or cannot hold full capacity due to memory or other degradation is shorter accordingly. At a 0.1C charge rate, the charge time of an empty NiCd is about 14 hours, which relates to the charge efficiency of 1.4.
During the first 70 percent of the charge cycle, the charge efficiency of a NiCd battery is close to 100 percent. Almost all of the energy is absorbed and the battery remains cool. Currents of several times the C-rating can be applied to a NiCd battery designed for fast charging without causing heat build-up. Ultra-fast chargers use this unique phenomenon and charge a battery to the 70 percent charge level within a few minutes. The charge continues at a lower rate until the battery is fully charged.
Once the 70 percent charge threshold is passed, the battery gradually loses ability to accept charge. The cells start to generate gases, the pressure rises and the temperature increases. The charge acceptance drops further as the battery reaches 80 and 90 percent SoC. Once full charge is reached, the battery goes into overcharge. In an attempt to gain a few extra capacity points, some chargers allow a measured amount of overcharge. Figure 4-1 illustrates the relationship of cell voltage, pressure and temperature while a NiCd is being charged.
Ultra-high capacity NiCd batteries tend to heat up more than the standard NiCd if charged at 1C and higher. This is partly due to the higher internal resistance of the ultra-high capacity battery. Optimum charge performance can be achieved by applying higher current at the initial charge stage, then tapering it to a lower rate as the charge acceptance decreases. This avoids excess temperature rise and yet assures fully charged batteries. [4.1]
Interspersing discharge pulses between charge pulses improves the charge acceptance of nickel-based batteries. Commonly referred to as ‘burp’ or ‘reverse load’ charge, this charge method promotes high surface area on the electrodes, resulting in enhanced performance and increased service life. Reverse load also improves fast charging because it helps to recombine the gases generated during charge. The result is a cooler and more effective charge than with conventional DC chargers.
Charging with the reverse load method minimizes crystalline formation. The US Army Electronics Command in Fort Monmouth, NJ, USA, had done extensive research in this field and has published the results. (See Figure 10-1, Crystalline formation on NiCd cell). Research conducted in Germany has shown that the reverse load method adds 15 percent to the life of the NiCd battery.
After full charge, the NiCd battery is maintained with a trickle charge to compensate for the self-discharge. The trickle charge for a NiCd battery ranges between 0.05C and 0.1C. In an effort to reduce the memory phenomenon, there is a trend towards lower trickle charge currents.
Charging the Nickel-Metal Hydride Battery
Chargers for NiMH batteries are very similar to those of the NiCd system but the electronics is generally more complex. To begin with, the NiMH produces a very small voltage drop at full charge. This NDV is almost non-existent at charge rates below 0.5C and elevated temperatures. Aging and cell mismatch works further against the already minute voltage delta. The cell mismatch gets worse with age and increased cycle count, which makes the use of the NDV increasingly more difficult.
The NDV of a NiMH charger must respond to a voltage drop of 16mV or less. Increasing the sensitivity of the charger to respond to the small voltage drop often terminates the fast charge by error halfway through the charge cycle. Voltage fluctuations and noise induced by the battery and charger can fool the NDV detection circuit if set too precisely.
The popularity of the NiMH battery has introduced many innovative charging techniques. Most of today’s NiMH fast chargers use a combination of NDV, voltage plateau, rate-of-temperature-increase (dT/dt), temperature threshold and timeout timers. The charger utilizes whatever comes first to terminate the fast-charge.
NiMH batteries which use the NDV method or the thermal cut-off control tend to deliver higher capacities than those charged by less aggressive methods. The gain is approximately 6 percent on a good battery. This capacity increase is due to the brief overcharge to which the battery is exposed. The negative aspect is a shorter cycle life. Rather than expecting 350 to 400 service cycles, this pack may be exhausted with 300 cycles.
Similar to NiCd charge methods, most NiMH fast-chargers work on the rate-of-temperature-increase (dT/dt). A temperature raise of 1°C (1.8°F) per minute is commonly used to terminate the charge. The absolute temperature cut-off is 60°C (140°F). A topping charge of 0.1C is added for about 30 minutes to maximize the charge. The continuous trickle charge that follows keeps the battery in full charge state.
Applying an initial fast charge of 1C works well. Cooling periods of a few minutes are added when certain voltage peaks are reached. The charge then continues at a lower current. When reaching the next charge threshold, the current steps down further. This process is repeated until the battery is fully charged.
Known as ‘step-differential charge’, this charge method works well with NiMH and NiCd batteries. The charge current adjusts to the SoC, allowing high current at the beginning and more moderate current towards the end of charge. This avoids excessive temperature build-up towards the end of the charge cycle when the battery is less capable of accepting charge.
NiMH batteries should be rapid charged rather than slow charged. The amount of trickle charge applied to maintain full charge is especially critical. Because NiMH does not absorb overcharge well, the trickle charge must be set lower than that of the NiCd. The recommended trickle charge for the NiMH battery is a low 0.05C. This is why the original NiCd charger cannot be used to charge NiMH batteries. The lower trickle charge rate is acceptable for the NiCd.
It is difficult, if not impossible, to slow-charge a NiMH battery. At a C-rate of 0.1C and 0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full charge state accurately and the charger must depend on a timer. Harmful overcharge can occur if a partially or fully charged battery is charged on a charger with a fixed timer. The same occurs if the battery has lost charge acceptance due to age and can only hold 50 percent of charge. A fixed timer that delivers a 100 percent charge each time without regard to the battery condition would ultimately apply too much charge. Overcharge could occur even though the NiMH battery feels cool to the touch.
Some lower-priced chargers may not apply a fully saturated charge. On these economy chargers, the full-charge detection may occur immediately after a given voltage peak is reached or a temperature threshold is detected. These chargers are commonly promoted on the merit of short charge time and moderate price.
Figure 4-2 summarizes the characteristics of the slow charger, quick charger and fast charger. A higher charge current allows better full-charge detection.
Charging the Lead Acid Battery
The charge algorithm for lead acid batteries differs from nickel-based chemistry in that voltage limiting rather than current limiting is used. Charge time of a sealed lead acid (SLA) is 12 to 16 hours. With higher charge currents and multi-stage charge methods, charge time can be reduced to 10 hours or less. SLAs cannot be fully charged as quickly as nickel-based systems.
A multi-stage charger applies constant-current charge, topping charge and float charge (see Figure 4-3). During the constant current charge, the battery charges to 70 percent in about five hours; the remaining 30 percent is completed by the slow topping charge. The topping charge lasts another five hours and is essential for the well-being of the battery. This can be compared to a little rest after a good meal before resuming work. If the battery is not completely saturated, the SLA will eventually lose its ability to accept a full charge and the performance of the battery is reduced. The third stage is the float charge, which compensates for the self-discharge after the battery has been fully charged. [4.3]
Correctly setting the cell-voltage limit is critical. A typical voltage limit is from 2.30V to 2.45V. If a slow charge is acceptable, or the room temperature may exceed 30°C (86°F), the recommended voltage limit is 2.35V/cell. If a faster charge is required, and the room temperature will remain below 30°C, 2.40 to 2.45V/cell may be used. Figure 4-4 compares the advantages and disadvantages of the different voltage settings. [4.4]
The charge voltage limit indicated in Figure 4-4 is a momentary voltage peak and the battery cannot dwell on that level. This voltage crest is only used when applying a full charge cycle to a battery that has been discharged. Once fully charged and at operational readiness, a float charge is applied, which is held constant at a lower voltage level. The recommended float charge voltage of most low-pressure lead acid batteries is between 2.25 to 2.30V/cell. A good compromise is 2.27V.
The optimal float charge voltage shifts with temperature. A higher temperature demands slightly lower voltages and a lower temperature demands higher voltages. Chargers that are exposed to large temperature fluctuations are equipped with temperature sensors to optimize the float voltage.
Regardless of how well the float voltage may be compensated, there is always a compromise. The author of a paper in a battery seminar explained that charging a sealed lead acid battery using the traditional float charge techniques is like 'dancing on the head of a pin'. The battery wants to be fully charged to avoid sulfation on the negative plate, but does not want to be over-saturated which causes grid corrosion on the positive plate. In addition to grid corrosion, too high a float charge contributes to loss of electrolyte.
Differences in the aging of the cells create another challenge in finding the optimum float charge voltage. With the development of air pockets within the cells over time, some batteries exhibit hydrogen evolution from overcharging. Others undergo oxygen recombination in an almost starved state. Since the cells are connected in series, controlling the individual cell voltages during charge is virtually impossible. If the applied cell voltage is too high or too low for a given cell, the weaker cell deteriorates further and its condition becomes more pronounced with time. Companies have developed cell-balancing devices that correct some of these problems but these devices can only be applied if access to individual cells is possible.
A ripple voltage imposed on the charge voltage also causes problems for lead acid batteries, especially the larger VRLA. The peak of the ripple voltage constitutes an overcharge, causing hydrogen evolution; the valleys induce a brief discharge causing a starved state. Electrolyte depletion may be the result.
Much has been said about pulse charging lead acid batteries. Although there are obvious benefits of reduced cell corrosion, manufacturers and service technicians are not in agreement regarding the benefit of such a charge method. Some advantages are apparent if pulse charging is applied correctly, but the results are non-conclusive.
Whereas the voltage settings in Figure 4-4 apply to low-pressure lead acid batteries with a pressure relief valve setting of about 34 kPa (5 psi), the cylindrical SLA by Hawker requires higher voltage settings. These voltage limits should be set according to the manufacturer’s specifications. Failing to apply the recommended voltage threshold for these batteries causes a gradual decrease in capacity due to sulfation. Typically, the Hawker cell has a pressure relief setting of 345 kPa (50 psi). This allows some recombination of the gases during charge.
An SLA must be stored in a charged state. A topping charge should be applied every six months to avoid the voltage from dropping below 2.10V/cell. The topping charge requirements may differ with cell manufacturers. Always follow the time intervals recommended by the manufacturer.
By measuring the open cell voltage while in storage, an approximate charge-level indication can be obtained. A voltage of 2.11V, if measured at room temperature, reveals that the cell has a charge of 50 percent and higher. If the voltage is at or above this threshold, the battery is in good condition and only needs a full charge cycle prior to use. If the voltage drops below 2.10V, several discharge/charge cycles may be required to bring the battery to full performance. When measuring the terminal voltage of any cell, the storage temperature should be observed. A cool battery raises the voltage slightly and a warm one lowers it.
Plastic SLA batteries arriving from vendors with less than 2.10V per cell are rejected by some buyers who inspect the battery during quality control. Low voltage suggests that the battery may have a soft short, a defect that cannot be corrected with cycling. Although cycling may increase the capacity of these batteries, the extra cycles compromise the service life of the battery. Furthermore, the time and equipment required to make the battery fully functional adds to operational costs.
The Hawker cell can be stored at voltages as low as 1.81V. However, when reactivating the cells, a higher than normal charge voltage may be required to convert the large sulfite crystals back to good active material.
Caution: When charging a lead acid battery with over-voltage, current limiting must be applied once the battery starts to draw full current. Always set the current limit to the lowest practical setting and observe the battery voltage and temperature during the procedure. If the battery does not accept a normal charge after 24 hours under elevated voltage, a return to normal condition is unlikely.
The price of the Hawker cell is slightly higher than that of the plastic equivalent, but lower than the NiCd. Also known as the ‘Cyclone’, this cell is wound similar to a cylindrical NiCd. This construction improves the cell’s stability and provides higher discharge currents when compared to the flat plate SLA. Because of its relatively low self-discharge, Hawker cells are well suited for defibrillators that are used on standby mode.
Lead acid batteries are preferred for UPS systems. During prolonged float charge, a periodic topping charge, also known as an ‘equalizing charge’, is recommended to fully charge the plates and prevent sulfation. An equalizing charge raises the battery voltage for several hours to a voltage level above that specified by the manufacturer. Loss of electrolyte through elevated temperature may occur if the equalizing charge is not administered correctly. Because no liquid can be added to the SLA and VRLA systems, a reduction of the electrolyte will cause irreversible damage. Manufacturers and service personnel are often divided on the benefit of the equalizing charge.
Some exercise, or brief periodic discharge, is believed to prolong battery life of lead acid systems. If applied once a month as part of an exercising program, the depth of discharge should only be about 10 percent of its total capacity. A full discharge as part of regular maintenance is not recommended because each deep discharge cycle robs service life from the battery.
More experiments are needed to verify the benefit of exercising lead acid batteries. Again, manufacturers and service technicians express different views on how preventive maintenance should be carried out. Some experts prefer a topping charge while others recommend scheduled discharges. No scientific data is available on the benefit of frequent shallow discharges as opposed to fewer deep discharges or discharge pulses.
Disconnecting the float charge while the VRLA is on standby is another method of prolonging battery life. From time-to-time, a topping charge is applied to replenish the energy lost through self-discharge. This is said to lower cell corrosion and prolong battery life. In essence, the battery is kept as if it was in storage. This only works for applications that do not draw a load current during standby. In many applications, the battery acts as an energy buffer and needs to be under continuous charge.
Important: In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.
Charging the Lithium Ion Battery
The Li-ion charger is a voltage-limiting device similar to the lead acid battery charger. The difference lies in a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge when full charge is reached.
While the lead acid battery offers some flexibility in terms of voltage cut-off, manufacturers of Li-ion cells are very strict on setting the correct voltage. When the Li- ion was first introduced, the graphite system demanded a charge voltage limit of 4.10V/cell. Although higher voltages deliver increased energy densities, cell oxidation severely limited the service life in the early graphite cells that were charged above the 4.10V/cell threshold. This effect has been solved with chemical additives. Most commercial Li-ion cells can now be charged to 4.20V. The tolerance on all Li-ion batteries is a tight +/-0.05V/cell.
Industrial and military Li-ion batteries designed for maximum cycle life use an end-of-charge voltage threshold of about 3.90V/cell. These batteries are rated lower on the watt-hour-per-kilogram scale, but longevity takes precedence over high energy density and small size.
The charge time of all Li-ion batteries, when charged at a 1C initial current, is about 3 hours. The battery remains cool during charge. Full charge is attained after the voltage has reached the upper voltage threshold and the current has dropped and leveled off at about 3 percent of the nominal charge current.
Increasing the charge current on a Li-ion charger does not shorten the charge time by much. Although the voltage peak is reached quicker with higher current, the topping charge will take longer. Figure 4-5 shows the voltage and current signature of a charger as the Li-ion cell passes through stage one and two.
Some chargers claim to fast-charge a Li-ion battery in one hour or less. Such a charger eliminates stage 2 and goes directly to ‘ready’ once the voltage threshold is reached at the end of stage 1. The charge level at this point is about 70 percent. The topping charge typically takes twice as long as the initial charge.
No trickle charge is applied because the Li-ion is unable to absorb overcharge. Trickle charge could cause plating of metallic lithium, a condition that renders the cell unstable. Instead, a brief topping charge is applied to compensate for the small amount of self-discharge the battery and its protective circuit consume.
Depending on the charger and the self-discharge of the battery, a topping charge may be implemented once every 500 hours or 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05V/cell and turns off when it reaches 4.20V/cell again. [4.5]
What if a battery is inadvertently overcharged? Li-ion batteries are designed to operate safely within their normal operating voltage but become increasingly unstable if charged to higher voltages. On a charge voltage above 4.30V, the cell causes lithium metal plating on the anode. In addition, the cathode material becomes an oxidizing agent, loses stability and releases oxygen. Overcharging causes the cell to heat up.
Much attention has been placed on the safety of the Li-ion battery. Commercial Li-ion battery packs contain a protection circuit that prevents the cell voltage from going too high while charging. The typical safety threshold is set to 4.30V/cell. In addition, temperature sensing disconnects the charge if the internal temperature approaches 90°C (194°F). Most cells feature a mechanical pressure switch that permanently interrupts the current path if a safe pressure threshold is exceeded. Internal voltage control circuits cut off the battery at low and high voltage points.
Exceptions are made on some spinel (manganese) packs containing one or two small cells. On overcharge, this chemistry produces minimal lithium plating on the anode because most metallic lithium has been removed from the cathode during normal charging. The cathode material remains stable and does not generate oxygen unless the cell gets extremely hot.
Important: In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.
Charging the Lithium Polymer Battery
The charge process of a Li-Polymer is similar to that of the Li-ion. Li-Polymer uses dry electrolyte and takes 3 to 5 hours to charge. Li-ion polymer with gelled electrolyte, on the other hand, is almost identical to that of Li-ion. In fact, the same charge algorithm can be applied. With most chargers, the user does not need to know whether the battery being charged is Li-ion or Li-ion polymer.
Almost all commercial batteries sold under the so-called ‘Polymer’ category are a variety of the Li-ion polymer using some sort of gelled electrolyte. A low-cost dry polymer battery operating at ambient temperatures is still some years away.
Charging at High and Low Temperatures
Rechargeable batteries can be used under a reasonably wide temperature range. This, however, does not automatically mean that the batteries can also be charged at these temperature conditions. While the use of batteries under hot or cold conditions cannot always be avoided, recharging time is controlled by the user. Efforts should be made to charge the batteries only at room temperatures.
In general, older battery technologies such as the NiCd are more tolerant to charging at low and high temperatures than the more advanced systems. Figure 4-6 indicates the permissible slow and fast charge temperatures of the NiCd, NiMH, SLA and Li-ion. [4.6]
NiCd batteries can be fast-charged in an hour or so, however, such a fast charge can only be applied within temperatures of 5°C and 45°C (41°F and 113°F). More moderate temperatures of 10°C to 30°C (50°F to 86°F) produce better results. When charging a NiCd below 5°C (41°F), the ability to recombine oxygen and hydrogen is greatly reduced and pressure build up occurs as a result. In some cases, the cells vent, releasing oxygen and hydrogen. Not only do the escaping gases deplete the electrolyte, hydrogen is highly flammable!
Chargers featuring NDV to terminate full-charge provide some level of protection when fast-charging at low temperatures. Because of the battery’s poor charge acceptance at low temperatures, the charge energy is turned into oxygen and to a lesser amount hydrogen. This reaction causes cell voltage drop, terminating the charge through NDV detection. When this occurs, the battery may not be fully charged, but venting is avoided or minimized.
To compensate for the slower reaction at temperatures below 5°C, a low charge rate of 0.1C must be applied. Special charge methods are available for charging at cold temperatures. Industrial batteries that need to be fast-charged at low temperatures include a thermal blanket that heats the battery to an acceptable temperature. Among commercial batteries, the NiCd is the only battery that can accept charge at extremely low temperatures.
Charging at high temperatures reduces the oxygen generation. This reduces the NDV effect and accurate full-charge detection using this method becomes difficult. To avoid overcharge, charge termination by temperature measurement becomes more practical.
The charge acceptance of a NiCd at higher temperatures is drastically reduced. A battery that provides a capacity of 100 percent if charged at moderate room temperature can only accept 70 percent if charged at 45°C (113°F), and 45 percent if charged at 60°C (140°F) (see Figure 4-7). Similar conditions apply to the NiMH battery. This demonstrates the typically poor summer performance of vehicular mounted chargers using nickel-based batteries.
Another reason for poor battery performance, especially if charged at high ambient temperatures, is premature charge cutoff. This is common with chargers that use absolute temperature to terminate the fast charge. These chargers read the SoC on battery temperature alone and are fooled when the room temperature is high. The battery may not be fully charged, but a timely charge cut-off protects the battery from damage due to excess heat.
The NiMH is less forgiving than the NiCd if charged under high and low temperatures. The NiMH cannot be fast charged below 10°C (45°F), neither can it be slow charged below 0°C (32°F). Some industrial chargers adjust the charge rate to prevailing temperatures. Price sensitivity on consumer chargers does not permit elaborate temperature control features. [4.7]
The lead acid battery is reasonably forgiving when it comes to temperature extremes, as in the case of car batteries. Part of this tolerance is credited to the sluggishness of the lead acid battery. A full charge under ten hours is difficult, if not impossible. The recommended charge rate at low temperature is 0.3C.
Figure 4-8 indicates the optimal peak voltage at various temperatures when recharging and float charging an SLA battery. Implementing temperature compensation on the charger to adjust to temperature extremes prolongs the battery life by up to 15 percent. This is especially true when operating at higher temperatures.
An SLA battery should never be allowed to freeze. If this were to occur, the battery would be permanently damaged and would only provide a few cycles when it returned to normal temperature. [4.8]
To improve charge acceptance of SLA batteries in colder temperatures, and avoid thermal runaway in warmer temperatures, the voltage limit of a charger should be compensated by approximately 3mV per cell per degree Celsius. The voltage adjustment has a negative coefficient, meaning that the voltage threshold drops as the temperature increases. For example, if the voltage limit is set to 2.40V/cell at 20°C, the setting should be lowered to 2.37V/cell at 30°C and raised to 2.43V/cell at 10°C. This represents a 30mV correction per cell per 10 degrees Celsius.
The Li-ion batteries offer good cold and hot temperature charging performance. Some cells allow charging at 1C from 0°C to 45°C (32°F to 113°F). Most Li-ion cells prefer a lower charge current when the temperature gets down to 5°C (41°F) or colder. Charging below freezing must be avoided because plating of lithium metal could occur.
Some charger manufacturers claim amazingly short charge times of 30 minutes or less. With well-balanced cells and operating at moderate room temperatures, NiCd batteries designed for fast charging can indeed be charged in a very short time. This is done by simply dumping in a high charge current during the first 70 percent of the charge cycle. Some NiCd batteries can take as much a 10C, or ten times the rated current. Precise SoC detection and temperature monitoring are essential.
The high charge current must be reduced to lower levels in the second phase of the charge cycle because the efficiency to absorb charge is progressively reduced as the battery moves to a higher SoC. If the charge current remains too high in the later part of the charge cycle, the excess energy turns into heat and pressure. Eventually venting occurs, releasing hydrogen gas. Not only do the escaping gases deplete the electrolyte, they are also highly flammable!
Several manufacturers offer chargers that claim to fully charge NiCd batteries in half the time of conventional chargers. Based on pulse charge technology, these chargers intersperse one or several brief discharge pulses between each charge pulse. This promotes the recombination of oxygen and hydrogen gases, resulting in reduced pressure buildup and a lower cell temperature. Ultra-fast-chargers based on this principle can charge a nickel-based battery in a shorter time than regular chargers, but only to about a 90 percent SoC. A trickle charge is needed to top the charge to 100 percent.
Pulse chargers are known to reduce the crystalline formation (memory) of nickel-based batteries. By using these chargers, some improvement in battery performance can be realized, especially if the battery is affected by memory. The pulse charge method does not replace a periodic full discharge. For more severe crystalline formation on nickel-based batteries, a full discharge or recondition cycle is recommended to restore the battery.
Ultra-fast charging can only be applied to healthy batteries and those designed for fast charging. Some cells are simply not built to carry high current and the conductive path heats up. The battery contacts also take a beating if the current handling of the spring-loaded plunger contacts is underrated. Pressing against a flat metal surface, these contacts may work well at first, and then wear out prematurely. Often, a fine and almost invisible crater appears on the tip of the contact, which causes a high resistive path or forms an isolator. The heat generated by a bad contact can melt the plastic.
Another problem with ultra-fast charging is servicing aged batteries that commonly have high internal resistance. Poor conductivity turns into heat, which further deteriorates the cells. Battery packs with mismatched cells pose another challenge. The weak cells holding less capacity are charged before those with higher capacity and start to heat up. This process makes them vulnerable to further damage.
Many of today’s fast chargers are designed for the ideal battery. Charging less than perfect specimens can create such a heat buildup that the plastic housing starts to distort. Provisions must be made to accept special needs batteries, albeit at lower charging speeds. Temperature sensing is a prerequisite.
The ideal ultra-fast charger first checks the battery type, measures its SoH and then applies a tolerable charge current. Ultra-high capacity batteries and those that have aged are identified, and the charge time is prolonged because of higher internal resistance. Such a charger would provide due respect to those batteries that still perform satisfactorily but are no longer ‘spring chickens’.
The charger must prevent excessive temperature build-up. Sluggish heat detection, especially when charging takes place at a very rapid pace, makes it easy to overcharge a battery before the charge is terminated. This is especially true for chargers that control fast charge using temperature sensing alone. If the temperature rise is measured right on the skin of the cell, reasonably accurate SoC detection is possible. If done on the outside surface of the battery pack, further delays occur. Any prolonged exposure to a temperature of 45°C (113°F) harms the battery.
New charger concepts are being studied which regulate the charge current according to the battery's charge acceptance. On the initial charge of an empty battery when the charge acceptance is high and little gas is generated, a very high charge current can be applied. Towards the end of a charge, the current is tapered down.
Charge IC Chips
Newer battery systems demand more complex chargers than batteries with older chemistries. With today’s charge IC chips, designing a charger has been simplified. These chips apply proven charge algorithms and are capable of servicing all major battery chemistries. As the price of these chips decreases, design engineers make more use of this product. With the charge IC chip, an engineer can focus entirely on the portable equipment rather than devoting time to developing a charging circuit.
The charge IC chips have some limitations, however. The charge algorithm is fixed and does not allow fine-tuning. If a trickle charge is needed to raise a Li-ion that has dropped below 2.5V/cell to its normal operating voltage, the charge IC may not be able to perform this function. Similarly, if an ultra-fast charge is needed for nickel-based batteries, the charge IC applies a fixed charge current and does not take into account the SoH of the battery. Furthermore, a temperature compensated charge would be difficult to administer if the IC chips do not provide this feature.
Using a small micro controller is an alternative to selecting an off-the-shelf charge IC. The hardware cost is about the same. When opting for the micro controller, custom firmware will be needed. Some extra features can be added with little extra cost. They are fast charging based on the SoH of the battery. Ambient temperatures can also be taken into account. Whether an IC chip or micro controller is used, peripheral components are required consisting of solid-state switches and a power supply.