Chapter 10: Getting the Most from your Batteries
A common difficulty with portable equipment is the gradual decline in battery performance after the first year of service. Although fully charged, the battery eventually regresses to a point where the available energy is less than half of its original capacity, resulting in unexpected downtime.
Downtime almost always occurs at critical moments. This is especially true in the public safety sector where portable equipment runs as part of a fleet operation and the battery is charged in a pool setting, often with minimal care and attention. Under normal conditions, the battery will hold enough power to last the day. During heavy activities and longer than expected duties, a marginal battery cannot provide the extra power needed and the equipment fails.
Rechargeable batteries are known to cause more concern, grief and frustration than any other part of a portable device. Given its relatively short life span, the battery is the most expensive and least reliable component of a portable device.
In many ways, a rechargeable battery exhibits human-like characteristics: it needs good nutrition, it prefers moderate room temperature and, in the case of the nickel-based system, requires regular exercise to prevent the phenomenon called ‘memory’. Each battery seems to develop a unique personality of its own.
Memory: myth or fact?
The word ‘memory’ was originally derived from ‘cyclic memory’, meaning that a NiCd battery can remember how much discharge was required on previous discharges. Improvements in battery technology have virtually eliminated this phenomenon. Tests performed at a Black & Decker lab, for example, showed that the effects of cyclic memory on the modern NiCd were so small that they could only be detected with sensitive instruments. After the same battery was discharged for different lengths of time, the cyclic memory phenomenon could no longer be noticed.
The problem with the nickel-based battery is not the cyclic memory but the effects of crystalline formation. There are other factors involved that cause degeneration of a battery. For clarity and simplicity, we use the word ‘memory’ to address capacity loss on nickel-based batteries that are reversible.
The active cadmium material of a NiCd battery is present in finely divided crystals. In a good cell, these crystals remain small, obtaining maximum surface area. When the memory phenomenon occurs, the crystals grow and drastically reduce the surface area. The result is a voltage depression, which leads to a loss of capacity. In advanced stages, the sharp edges of the crystals may grow through the separator, causing high self-discharge or an electrical short.
Another form of memory that occurs on some NiCd cells is the formation of an inter-metallic compound of nickel and cadmium, which ties up some of the needed cadmium and creates extra resistance in the cell. Reconditioning by deep discharge helps to break up this compound and reverses the capacity loss.
The memory phenomenon can be explained in layman’s terms as expressed by Duracell: “The voltage drop occurs because only a portion of the active materials in the cells is discharged and recharged during shallow or partial discharging. The active materials that have not been cycled change in physical characteristics and increase in resistance. Subsequent full discharge/charge cycling will restore the active materials to their original state.”
When NiMH was first introduced there was much publicity about its memory-free status. Today, it is known that this chemistry also suffers from memory but to a lesser extent than the NiCd. The positive nickel plate, a metal that is shared by both chemistries, is responsible for the crystalline formation. [10.1]
In addition to the crystal-forming activity on the positive plate, the NiCd also develops crystals on the negative cadmium plate. Because both plates are affected by crystalline formation, the NiCd requires more frequent discharge cycles than the NiMH. This is a non-scientific explanation of why the NiCd is more prone to memory than the NiMH.
The stages of crystalline formation of a NiCd battery are illustrated in Figure 10-1. The enlargements show the negative cadmium plate in normal crystal structure of a new cell, crystalline formation after use (or abuse) and restoration.
Lithium and lead-based batteries are not affected by memory, but these chemistries have their own peculiarities. Current inhibiting pacifier layers affect both batteries — plate oxidation on the lithium and sulfation and corrosion on the lead acid systems. These degenerative effects are non-correctible on the lithium-based system and only partially reversible on the lead acid.
How to Restore and Prolong Nickel-based Batteries
The effects of crystalline formation are most pronounced if a nickel-based battery is left in the charger for days, or if repeatedly recharged without a periodic full discharge. Since most applications do not use up all energy before recharge, a periodic discharge to 1V/cell (known as exercise) is essential to prevent the buildup of crystalline formation on the cell plates. This maintenance is most critical for the NiCd battery.
All NiCd batteries in regular use and on standby mode (sitting in a charger for operational readiness) should be exercised once per month. Between these monthly exercise cycles, no further service is needed. The battery can be used with any desired user pattern without the concern of memory.
The NiMH battery is affected by memory also, but to a lesser degree. No scientific research is available that compares NiMH with NiCd in terms of memory degradation. Neither is information on hand that suggests the optimal amount of maintenance required to obtain maximum battery life. Applying a full discharge once every three months appears right. Because of the NiMH battery’s shorter cycle life, over-exercising is not recommended.
A hand towel must be cleaned periodically. However, if it were washed after each use, its fabric would wear out very quickly. In the same way, it is neither necessary nor advisable to discharge a rechargeable battery before each charge — excessive cycling puts extra strain on the battery.
Exercise and Recondition — Research has shown that if no exercise is applied to a NiCd for three months or more, the crystals ingrain themselves, making them more difficult to break up. In such a case, exercise is no longer effective in restoring a battery and reconditioning is required. Recondition is a slow, deep discharge that removes the remaining battery energy by draining the cells to a voltage threshold below 1V/cell. [10.2]
Tests performed by the US Army have shown that a NiCd cell needs to be discharged to at least 0.6V to effectively break up the more resistant crystalline formation. During recondition, the current must be kept low to prevent cell reversal. Figure 10-2 illustrates the battery voltage during normal discharge to 1V/cell followed by the secondary discharge (recondition).
Figure 10-3 illustrates the effects of exercise and recondition. Four batteries afflicted with various degrees of memory are serviced. The batteries are first fully charged, then discharged to 1V/cell. The resulting capacities are plotted on a scale of 0 to 120 percent in the first column. Additional discharge/charge cycles are applied and the battery capacities are plotted in the subsequent columns. The solid black line represents exercise, (discharge to 1V/cell) and the dotted line recondition (secondary discharge at reduced current to 0.4V/cell). On this test, the exercise and recondition cycles are applied manually at the discretion of the research technician. [10.3]
Battery A responded well to exercise alone and no recondition was required. This result is typical of a battery that has been in service for only a few months or has received periodic exercise cycles. Batteries B and C, on the other hand, required recondition (dotted line) to restore their performance. Without the recondition function, these two batteries would need to be replaced.
After service, the restored batteries were returned to full use. When examined after six months of field use, no noticeable degradation in the restored performance was visible. The regained capacity was permanent with no evidence of falling back to the previous state. Obviously, the batteries would need to be serviced on a regular basis to maintain the performance.
Applying the recondition cycle on a new battery (top line on chart) resulted in a slight capacity increase. This capacity gain is not fully understood, other than to assume that the battery improved by additional formatting. Another explanation is the presence of early memory. Since new batteries are stored with some charge, the self-discharge that occurs during storage contributes to a certain amount of crystalline formation. Exercising and reconditioning reverse this effect. This is why the manufacturers recommend storing rechargeable batteries at about 40 percent charge.
The importance of exercising and reconditioning NiCd batteries is emphasized further by a study carried out by GTE Government Systems in Virginia, USA, for the US Navy. To determine the percentage of batteries needing replacement within the first year of use, one group of batteries received charge only, another group was exercised and a third group received recondition. The batteries studied were used for two-way radios on the aircraft carriers USS Eisenhower with 1500 batteries and USS George Washington with 600 batteries, and the destroyer USS Ponce with 500 batteries.
With charge only (charge-and-use), the annual percentage of battery failure on the USS Eisenhower was 45 percent (see Figure 10-4). When applying exercise, the failure rate was reduced to 15 percent. By far the best results were achieved with recondition. The failure rate dropped to 5 percent. Identical results were attained from the USS George Washington and the USS Ponce. [10.4]
The GTE Government System report concluded that a battery analyzer featuring exercise and recondition functions costing $2,500US would pay for itself in less than one month on battery savings alone. The report did not address the benefits of increased system reliability, an issue that is of equal if not greater importance, especially when the safety of human lives is at stake.
Another study involving NiCd batteries for defense applications was performed by the Dutch Army. This involved battery packs that had been in service for 2 to 3 years during the Balkan War. The Dutch Army was aware that the batteries were used under the worst possible conditions. Rather than a good daily workout, the packs were used for patrol duties lasting 2 to 3 hours per day. The rest of the time the batteries remained in the chargers for operational readiness.
After the war, the batteries were sent to the Dutch Military Headquarters and were tested with Cadex 7000 Series battery analyzers. The test technician found that the capacity of some packs had dropped to as low as 30 percent. With the recondition function, 90 percent of the batteries restored themselves to full field use. The Dutch Army set the target capacity threshold for field acceptability to 80 percent. This setting is the pass/fail acceptance level for their batteries.
Based on the successful reconditioning results, the Dutch Army now assigns the battery maintenance duty to individual battalions. The program calls for a service once every two months. Under this regime, the Army reports reduced battery failure and prolonged service life. The performance of each battery is known at any time and any under-performing battery is removed before it causes a problem.
NiCd batteries remain the preferred chemistry for mobile communications, both in civil and defense applications. The main reason for its continued use is dependable and enduring service under difficult conditions. Other chemistries have been tested and found problematic in long-term use.
During the later part of the 1990s, the US Army switched from mainly non-rechargeable to the NiMH battery. The choice of chemistry was based on the benefit of higher energy densities as compared to NiCd. The army soon discovered that the NiMH did not live up to the expected cycle life. Their reasoning, however, is that the 100 cycles attained from a NiMH pack is still more economical than using a non-rechargeable equivalent. The army’s focus is now on the Li-ion Polymer, a system that is more predictable than NiMH and requires little or no maintenance. The aging issue will likely cause some logistic concerns, especially if long-term storage is required.
Do not leave a nickel-based battery in a charger for more than a day after full charge is reached.
- Apply a monthly full discharge cycle. Running the battery down in the equipment may do this also.
- Do not discharge the battery before each recharge. This would put undue stress on the battery.
- Avoid elevated temperature. A charger should only raise the battery temperature for a short time at full charge, and then the battery should cool off.
- Use quality chargers to charge batteries.
The Effect of Zapping
To maximize battery performance, remote control (RC) racing enthusiasts have experimented with all imaginable methods available. One technique that seems to work is zapping the cells with a very high pulse current. Zapping is said to increase the cell voltage slightly, generating more power.
Typically, the racecar motor draws 30A, delivered by a 7.2V battery. This calculates to over 200W of power. The battery must endure a race lasting about four minutes.
According to experts, zapping works best with NiCd cells. NiMH cells have been tried but they have shown inconsistent results.
Companies specializing in zapping NiCd for RC racing use a very high quality Japanese NiCd cell. The cells are normally sub-C in size and are handpicked at the factory for the application. Specially labeled, the cells are delivered in a discharged state. When measuring the cell in empty state-of-charge (SoC), the voltage typically reads between 1.11 to 1.12V. If the voltage drops lower than 1.06V, the cell is considered suspect and zapping does not seem to enhance the performance as well as on the others.
The zapping is done with a 47,000mF capacitor that is charged to 90V. Best results are achieved if the battery is cycled twice after treatment, then is zapped again. After the battery has been in service for a while, zapping no longer seems to improve the cell’s performance. Neither does zapping regenerate a cell that has become weak.
The voltage increase on a properly zapped battery is between 20 and 40mV. This improvement is measured under a load of 30A. According to experts, the voltage gain is permanent but there is a small drop with usage and age.
There are no apparent side effects in zapping, however, the battery manufacturers remain silent about this treatment. No scientific explanations are available why the method of zapping improves battery performance. There is little information available regarding the longevity of the cells after they have been zapped.
How to Restore and Prolong Sealed Lead Acid Batteries
The sealed version of the lead acid battery is designed with a low over-voltage potential to prevent water depletion. Consequently, the SLA and VRLA systems never get fully charged and some sulfation will develop over time.
Finding the ideal charge voltage limit for the sealed lead acid system is critical. Any voltage level is a compromise. A high voltage limit produces good battery performance, but shortens the service life due to grid corrosion on the positive plate. The corrosion is permanent and cannot be reversed. A low voltage preserves the electrolyte and allows charging under a wide temperature range, but is subject to sulfation on the negative plate. (In keeping with portability, this book focuses on portable SLA batteries. Due to similarities between the SLA and VRLA systems, references to the VRLA are made where applicable).
Once the SLA battery has lost capacity due to sulfation, regaining its performance is often difficult and time consuming. The metabolism of the SLA battery is slow and cannot be hurried.
A subtle indication on whether an SLA battery can be recovered is reflected in the behavior of its discharge voltage. A fully charged SLA battery that starts its discharge with a high voltage and tapers off gradually can be reactivated more successfully than one on which the voltage drops rapidly when the load is applied.
Reasonably good results in regaining lost capacity are achieved by applying a charge on top of a charge. This is done by fully charging an SLA battery, then removing it for a 24 to 48 hour rest period and applying a charge again. This is repeated several times, then the capacity of the battery is checked with a full discharge. The SLA is able to accept some overcharge, however, too long an overcharge could harm the battery due to corrosion and loss of electrolyte.
The effect of sulfation of the plastic SLA can be reversed by applying an over-voltage charge of up to 2.50V/cell for one to two hours. During that time, the battery must be kept cool and careful observation is necessary. Extreme caution is required not to raise the cell pressure to venting point. Most plastic SLA batteries vent at 34 kPa (5 psi). Cell venting causes the membrane on some SLA to rupture permanently. Not only do the escaping gases deplete the electrolyte, they are also highly flammable!
The VRLA uses a cell self-regulating venting system that opens and closes the cells based on cell pressure. Changes in atmospheric pressure contribute to cell venting. Proper ventilation of the battery room is essential to prevent the accumulation of hydrogen gas.
Cylindrical SLA — The cylindrical SLA (made by Hawker) resembles an enlarged D sized cell. After long storage, the Hawker cell can be reactivated relatively easily. If affected by sulfation, the cell voltage under charge may initially raise up to 5V, absorbing only a small amount of current. Within about two hours, the small charging current converts the large sulfate crystals back into active material. The internal cell resistance decreases and the charge voltage eventually returns to normal. At a voltage between 2.10V and 2.40V, the cell is able to accept a normal charge. To prevent damage, caution must be exercised to limit the charge current.
The Hawker cells are known to regain full performance with the described voltage method, leaving few adverse effects. This, however, does not give credence to store this cell at a very low voltage. It is always best to follow the manufacturer’s recommended specifications.
Improving the capacity of an older SLA by cycling is mostly unsuccessful. Such a battery may simply be worn out. Cycling would just wear down the battery further. Unlike nickel-based batteries, the lead acid battery is not affected by memory.
SLA batteries are commonly rated at a 20-hour discharge. Even at such a slow rate, a capacity of 100 percent is difficult to obtain. For practical reasons, most battery analyzers use a 5-hour discharge when servicing SLA batteries. This typically produces 80 to 90 percent of the rated capacity. SLA batteries are normally overrated and manufacturers are aware of this.
Caution: When charging an SLA with over-voltage, current limiting must be applied to protect the battery. Always set the current limit to the lowest practical setting and observe the battery voltage and temperature during charge. Prevent cell venting.
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.
- Always keep the SLA charged. Never store below 2.10V/cell.
- Avoid repeated deep discharges. Charge more often.
- If repeated deep discharges cannot be avoided, use a larger battery to ease the strain
- Prevent sulfation and grid corrosion by choosing the correct charge and float voltages.
How to Prolong Lithium-based Batteries
Today’s battery research is heavily focused on lithium chemistries, so much so that one could assume that all future batteries will be lithium systems. Lithium-based batteries offer many advantages over nickel and lead-based systems. Although maintenance free, no external service is known that can restore the battery’s performance once degraded.
In many respects, Li-ion provides a superior service to other chemistries, but its performance is limited to a defined lifespan. The Li-ion battery has a time clock that starts ticking as soon as the battery leaves the factory. The electrolyte slowly ‘eats up’ the positive plate and the electrolyte decays. This chemical change causes the internal resistance to increase. In time, the cell resistance raises to a point where the battery can no longer deliver the energy, although it may still be retained in the battery. Equipment requiring high current bursts is affected most by the increase of internal resistance.
Battery wear-down on lithium-based batteries is caused by two activities: actual usage or cycling, and aging. The wear-down effects by usage and aging apply to all batteries but this is more pronounced on lithium-based systems.
The Li-ion batteries prefer a shallow discharge. Partial discharges produce less wear than a full discharge and the capacity loss per cycle is reduced. A periodic full discharge is not required because the lithium-based battery has no memory. A full cycle constitutes a discharge to 3V/cell. When specifying the number of cycles a lithium-based battery can endure, manufacturers commonly use an 80 percent depth of discharge. This method resembles a reasonably accurate field simulation. It also achieves a higher cycle count than doing full discharges.
In addition to cycling, the battery ages even if not used. The amount of permanent capacity loss the battery suffers during storage is governed by the SoC and temperature. For best results, keep the battery cool. In addition, store the battery at a 40 percent charge level. Never fully charge or discharge the battery before storage. The 40 percent charge assures a stable condition even if self-discharge robs some of the battery’s energy. Most battery manufacturers store Li-ion batteries at 15°C (59°F) and at 40 percent charge.
- Charge the Li-ion often, except before a long storage. Avoid repeated deep discharges.
- Keep the Li-ion battery cool. Prevent storage in a hot car. Never freeze a battery.
- If your laptop is capable of running without a battery and fixed power is used most of the time, remove the battery and store it in a cool place.
- Avoid purchasing spare Li-ion batteries for later use. Observe manufacturing date when purchasing. Do not buy old stock, even if sold at clearance prices.
Battery Recovery Rate
The battery recovery rate by applying controlled discharge/charge cycles varies with chemistry type, cycle count, maintenance practices and age of the battery. The best results are achieved with NiCd. Typically 50 to 70 percent of discarded NiCd batteries can be restored when using the exercise and recondition methods of a Cadex battery analyzer or equivalent device.
Not all batteries respond equally well to exercise and recondition services. An older battery may show low and inconsistent capacity readings with each cycle. Another will get worse when additional cycles are applied. An analogy can be made to a very old man for whom exercise is harmful. Such conditions indicate instabilities caused by aging, suggesting that this pack should be replaced. In fact, some users of the Cadex analyzers use the recondition cycle as the acid test. If the battery gets worse, there is strong evidence that this battery would not perform well in the field. Applying the acid test exposes the weak packs, which can no longer hide behind their stronger peers.
Some older NiCd batteries recover to near original capacity when serviced. Caution should be applied when ‘rehiring’ these old-timers because they may exhibit high self-discharge. If in doubt, a self-discharge test should be carried out.
The recovery rate of the NiMH is about 40 percent. This lower yield is, in part, due to the NiMH’s reduced cycle count as compared to the NiCd. Some batteries may be afflicted by heat damage that occurs during incorrect charging. This deficiency cannot be corrected. Permanent loss of battery capacity is also caused by prolonged storage at elevated temperatures.
The recovery rate for lead acid batteries is a low 15 percent. Unlike nickel-based batteries, the restoration of the SLA is not based on reversing crystalline formation, but rather by reactivating the chemical process. The reasons for low capacity readings are prolonged storage at low terminal voltage, and poor charging methods. The battery also fails due to age and high cycle count.
Lithium-based batteries have a defined age limit. Once the anticipated cycles have been delivered, no method exists to improve the battery. The main reason for failure is high internal resistance caused by oxidation. Operating the battery at elevated temperatures will momentarily reduce this condition. When the temperature normalizes, the condition of high internal resistance returns.
The speed of oxidation depends on the storage temperature and the battery’s charge state. Keeping the battery in a cool place can prolong its life. The Li-ion battery should be stored at 40 percent rather than full-charge state.
An increasing number of modern batteries fall prey to the cut-off problem induced by a deep discharge. This is especially evident on Li-ion batteries for mobile phones. If discharged below 2.5V/cell, the internal protection circuit often opens. Many chargers cannot apply a recharge and the battery appears to be dead.
Some battery analyzers feature a boost, or wake-up function, to activate the protection circuit and enable a recharge if discharged too low. If the cell voltage has fallen too low (1.5V/cell and lower) and has remained in that state for a few days, a recharge should not be attempted because of safety concerns on the cell(s).
It is often asked whether a restored battery will work as good as a new one. The breakdown of the crystalline formation can be considered a full restoration. However, the crystalline formation will re-occur with time if the battery is denied the required maintenance.
When the defective component of a machine is replaced, only the replaced part is new; the rest of the machine remains in the same condition. If the separator of a nickel-based battery is damaged by excess heat or is marred by uncontrolled crystalline formation, that part of the battery will not improve.
Other methods, which claim to restore and prolong rechargeable batteries, have produced disappointing results. One method is attaching a strong magnet on the side of the battery; another is exposing the battery to ultrasound vibrations. No scientific evidence exists that such methods will improve battery performance, or restore an ailing battery.