Chapter 5: Discharge Methods
The purpose of a battery is to store energy and release it at the appropriate time in a controlled manner. Being capable of storing a large amount of energy is one thing; the ability to satisfy the load demands is another. The third criterion is being able to deliver all available energy without leaving precious energy behind when the equipment cuts off.
In this chapter, we examine how different discharge methods can affect the deliverance of power. Further, we look at the load requirements of various portable devices and evaluate the performance of each battery chemistry in terms of discharge.
The charge and discharge current of a battery is measured in C-rate. Most portable batteries, with the exception of the lead acid, are rated at 1C. A discharge of 1C draws a current equal to the rated capacity. For example, a battery rated at 1000mAh provides 1000mA for one hour if discharged at 1C rate. The same battery discharged at 0.5C provides 500mA for two hours. At 2C, the same battery delivers 2000mA for 30 minutes. 1C is often referred to as a one-hour discharge; a 0.5C would be a two-hour, and a 0.1C a 10 hour discharge.
The capacity of a battery is commonly measured with a battery analyzer. If the analyzer’s capacity readout is displayed in percentage of the nominal rating, 100 percent is shown if 1000mA can be drawn for one hour from a battery that is rated at 1000mAh. If the battery only lasts for 30 minutes before cut-off, 50 percent is indicated. A new battery sometimes provides more than 100 percent capacity. In such a case, the battery is conservatively rated and can endure a longer discharge time than specified by the manufacturer.
When discharging a battery with a battery analyzer that allows setting different discharge C-rates, a higher capacity reading is observed if the battery is discharged at a lower C-rate and vice versa. By discharging the 1000mAh battery at 2C, or 2000mA, the analyzer is scaled to derive the full capacity in 30 minutes. Theoretically, the capacity reading should be the same as a slower discharge, since the identical amount of energy is dispensed, only over a shorter time. Due to energy loss that occurs inside the battery and a drop in voltage that causes the battery to reach the low-end voltage cut-off sooner, the capacity reading is lower and may be 97 percent. Discharging the same battery at 0.5C, or 500mA over two hours would increase the capacity reading to about 103 percent.
The discrepancy in capacity readings with different C-rates largely depends on the internal resistance of the battery. On a new battery with a good load current characteristic or low internal resistance, the difference in the readings is only a few percentage points. On a battery exhibiting high internal resistance, the difference in capacity readings could swing plus/minus 10 percent or more.
One battery that does not perform well at a 1C discharge rate is the SLA. To obtain a practical capacity reading, manufacturers commonly rate these batteries at 0.05C or 20 hour discharge. Even at this slow discharge rate, it is often difficult to attain 100 percent capacity. By discharging the SLA at a more practical 5h discharge (0.2C), the capacity readings are correspondingly lower. To compensate for the different readings at various discharge currents, manufacturers offer a capacity offset.
Applying the capacity offset does not improve battery performance; it merely adjusts the capacity calculation if discharged at a higher or lower C-rate than specified. The battery manufacturer determines the amount of capacity offset recommended for a given battery type.
Li-ion/polymer batteries are electronically protected against high discharge currents. Depending on battery type, the discharge current is limited somewhere between 1C and 2C. This protection makes the Li-ion unsuitable for biomedical equipment, power tools and high-wattage transceivers. These applications are commonly reserved for the NiCd battery.
Depth of Discharge
The typical end-of-discharge voltage for nickel-based batteries is 1V/cell. At that voltage level, about 99 percent of the energy is spent and the voltage starts to drop rapidly if the discharge continues. Discharging beyond the cut-off voltage must be avoided, especially under heavy load.
Since the cells in a battery pack cannot be perfectly matched, a negative voltage potential (cell reversal) across a weaker cell occurs if the discharge is allowed to continue beyond the cut-off point. The larger the number of cells connected in series, the greater the likelihood of this occurring.
A NiCd battery can tolerate a limited amount of cell reversal, which is typically about 0.2V. During that time, the polarity of the positive electrode is reversed. Such a condition can only be sustained for a brief moment because hydrogen evolution occurs on the positive electrode. This leads to pressure build-up and cell venting.
If the cell is pushed further into voltage reversal, the polarity of both electrodes is being reversed, resulting in an electrical short. Such a fault cannot be corrected and the pack will need to be replaced.
On battery analyzers that apply a secondary discharge (recondition), the current is controlled to assure that the maximum allowable current, while in sub-discharge range, does not exceed a safe limit. Should a cell reversal develop, the current would be low enough as not to cause damage. A cell breakdown through recondition is possible on a weak or aged pack.
If the battery is discharged at a rate higher than 1C, the more common end-of-discharge point of a nickel-based battery is 0.9V/cell. This is done to compensate for the voltage drop induced by the internal resistance of the cell, the wiring, protection devices and contacts of the pack. A lower cut-off point also delivers better battery performance at cold temperatures.
The recommended end-of-discharge voltage for the SLA is 1.75V/cell. Unlike the preferred flat discharge curve of the NiCd, the SLA has a gradual voltage drop with a rapid drop towards the end of discharge (see Figure 5-1). Although this steady decrease in voltage is a disadvantage, it has a benefit because the voltage level can be utilized to display the state-of-charge (SoC) of a battery. However, the voltage readings fluctuate with load and the SoC readings are inaccurate. [5.1]
°C (77°F) with respect to the depth of discharge is:
- 150 – 200 cycles with 100 percent depth of discharge (full discharge)
- 400 – 500 cycles with 50 percent depth of discharge (partial discharge)
- 1000 and more cycles with 30 percent depth of discharge (shallow discharge)
The SLA should not be discharged beyond 1.75V per cell, nor can it be stored in a discharged state. The cells of a discharged SLA sulfate, a condition that renders the battery useless if left in that state for a few days.
The Li-ion typically discharges to 3.0V/cell. The spinel and coke versions can be discharged to 2.5V/cell. The lower end-of-discharge voltage gains a few extra percentage points. Since the equipment manufacturers cannot specify which battery type may be used, most equipment is designed for a three-volt cut-off.
Caution should be exercised not to discharge a lithium-based battery too low. Discharging a lithium-based battery below 2.5V may cut off the battery’s protection circuit. Not all chargers accommodate a recharge on batteries that have gone to sleep because of low voltage.
Some Li-ion batteries feature an ultra-low voltage cut-off that permanently disconnects the pack if a cell dips below 1.5V. This precaution prohibits recharge if a battery has dwelled in an illegal voltage state. A very deep discharge may cause the formation of copper shunt, which can lead to a partial or total electrical short. The same occurs if the cell is driven into negative polarity and is kept in that state for a while. A fully discharged battery should be charged at 0.1C. Charging a battery with a copper shunt at the 1C rate would cause excessive heat. Such a battery should be removed from service.
Discharging a battery too deeply is one problem; equipment that cuts off before the energy is consumed is another. Some portable devices are not properly tuned to harvest the optimal energy stored in a battery. Valuable energy may be left behind if the voltage cut-off-point is set too high.
Digital devices are especially demanding on a battery. Momentary pulsed loads cause a brief voltage drop, which may push the voltage into the cut-off region. Batteries with high internal resistance are particularly vulnerable to premature cut-off. If such a battery is removed from the equipment and discharged to the appropriate cut-off point with a battery analyzer on DC load, a high level of residual capacity can still be obtained.
Most rechargeable batteries prefer a partial rather than a full discharge. Repeated full discharge robs the battery of its capacity. The battery chemistry which is most affected by repeat deep discharge is lead acid. Additives to the deep-cycle version of the lead acid battery compensate for some of the cycling strain.
Similar to the lead acid battery, the Li-ion battery prefers shallow over repetitive deep discharge cycles. Up to 1000 cycles can be achieved if the battery is only partially discharged. Besides cycling, the performance of the Li-ion is also affected by aging. Capacity loss through aging is independent of use. However, in daily use, there is a combination of both.
The NiCd battery is least affected by repeated full discharge cycles. Several thousand charge/discharge cycles can be obtained with this battery system. This is the reason why the NiCd performs well on power tools and two-way radios that are in constant use. The NiMH is more delicate with respect to repeated deep cycling.
Battery chemistries react differently to specific loading requirements. Discharge loads range from a low and steady current used in a flashlight, to intermittent high current bursts in a power tool, to sharp current pulses required for digital communications equipment, to a prolonged high current load for an electric vehicle traveling at highway speed. Because batteries are chemical devices that must convert higher-level active materials into an alternate state during discharge, the speed of such transaction determines the load characteristics of a battery. Also referred to as concentration polarization, the nickel and lithium-based batteries are superior to lead-based batteries in reaction speed. This reflects in good load characteristics.
The lead acid battery performs best at a slow 20-hour discharge. A pulse discharge also works well because the rest periods between the pulses help to disperse the depleted acid concentrations back into the electrode plate. In terms of capacity, these two discharge methods provide the highest efficiency for this battery chemistry.
A discharge at the rated capacity of 1C yields the poorest efficiency for the lead acid battery. The lower level of conversion, or increased polarization, manifests itself in a momentary higher internal resistance due to the depletion of active material in the reaction.
Different discharge methods, notably pulse discharging, also affect the longevity of some battery chemistries. While NiCd and Li-ion are robust and show minimal deterioration when pulse discharged, the NiMH exhibits a reduced cycle life when powering a digital load.
In a recent study, the longevity of NiMH was observed by discharging these batteries with analog and digital loads. In both tests, the battery discharged to 1.04V/cell. The analog discharge current was 500mA; the digital mode simulated the load requirements of the Global System for Mobile Communications (GSM) protocol and applied 1.65-ampere peak current for 12 ms every 100 ms. The current in between the peaks was 270mA. (Note that the GSM pulse for voice is about 550 ms every 4.5 ms).
With the analog discharge, the NiMH wore out gradually, providing an above average service life. At 700 cycles, the battery still provided 80 percent capacity. By contrast, the cells faded more rapidly with a digital discharge. The 80 percent capacity threshold was reached after only 300 cycles. This phenomenon indicates that the kinetic characteristics for the NiMH deteriorate more rapidly with a digital rather than an analog load.
Discharging at High and Low Temperature
Batteries function best at room temperature. Operating batteries at an elevated temperature dramatically shortens their life. Although a lead acid battery may deliver the highest capacity at temperatures above 30°C (86°F), prolonged use under such conditions decreases the life of the battery.
Similarly, a Li-ion performs better at high temperatures. Elevated temperatures temporarily counteracts the battery’s internal resistance, which is a result of aging. The energy gain is short-lived because elevated temperature promotes aging by further increasing the internal resistance.
There is one exception to running a battery at high temperature — it is the lithium polymer with dry solid polymer electrolyte, the true ‘plastic battery’. While the commercial Li-ion polymer uses some moist electrolyte to enhance conductivity, the dry solid polymer version depends on heat to enable ion flow. This requires that the battery core be kept at an operation temperature of 60°C to 100°C.
The dry solid polymer battery has found a niche market as backup power in warm climates. The battery is kept at the operating temperature with built-in heating elements. During normal operation, the core is kept warm with power derived from the utility grid. Only on a power outage would the battery need to provide power to maintain its own heat. To minimize heat loss, the battery is insulated.
The Li-ion polymer as standby battery is said to outperform VRLA batteries in terms of size and longevity, especially in shelters in which the temperature cannot be controlled. The high price of the Li-ion polymer battery remains an obstacle.
The NiMH chemistry degrades rapidly if cycled at higher ambient temperatures. Optimum battery life and cycle count are achieved at 20°C (68°F). Repeated charging and discharging at higher temperatures will cause irreversible capacity loss. For example, if operated at 30°C (86°F), the cycle life is reduced by 20 percent. At 40°C (104°F), the loss jumps to a whopping 40 percent. If charged and discharged at 45°C (113°F), the cycle life is only half of what can be expected if used at moderate room temperature. The NiCd is also affected by high temperature operation, but to a lesser degree.
At low temperatures, the performance of all battery chemistries drops drastically. While -20°C (-4°F) is threshold at which the NiMH, SLA and Li-ion battery stop functioning, the NiCd can go down to -40°C (-40°F). At that frigid temperature, the NiCd is limited to a discharge rate of 0.2C (5 hour rate). There are new types of Li-ion batteries that are said to operate down to -40°C.
It is important to remember that although a battery may be capable of operating at cold temperatures, this does not automatically mean it can also be charged under those conditions. The charge acceptance for most batteries at very low temperatures is extremely confined. Most batteries need to be brought up to temperatures above the freezing point for charging. The NiCd can be recharged at below freezing provided the charge rate is reduced to 0.1C.