Basics about discharging batteries

This blog examines discharges under different C-rates and evaluates the depth to which a battery can safely be depleted. It also observes different discharge signatures and explores how certain patterns can affect battery life. But first, let’s look at charge and discharge rates, also known as C-rate.{nomultithumb}

Depth of Discharge
The end-of-discharge voltage for lead acid is 1.75 V/cell; nickel-based system is 1.00 V/cell; and most Li-ion is 3.00 V/cell. At this level, roughly 95% of the energy is spent and the voltage would drop rapidly if the discharge were to continue. To protect the battery from over-discharging, most devises prevent operation beyond the specified end-of-discharge voltage.

When removing the load after discharge, the voltage of a healthy battery gradually recovers and rises towards the nominal voltage. Differences in the metal concentration of the electrodes enable this voltage potential when the battery is empty. An aging battery with elevated self-discharge cannot recover the voltage because of the parasitic load.

A high load current lowers the battery voltage, and the end-of-discharge voltage threshold should be set lower accordingly. Internal cell resistance, wiring, protection circuits and contacts all add up to overall internal resistance. The cut-off voltage should also be lowered when discharging at very cold temperatures; this compensates for the higher-than-normal internal resistance. Table 1 shows typical end-of-discharge voltages of various battery chemistries.

Some battery analyzers apply a secondary discharge (recondition) that drains the battery voltage of a nickel-based battery to 0.5 V/cell and lower, a cut-off point that is below what manufacturers specify. These analyzers (Cadex) keep the discharge load low to stay within an allowable current while in sub-discharge range. A cell breakdown with a weak cell is possible and reconditioning would cause further deterioration in performance rather than making the battery better. This phenomenon can be compared to the experience of a patient to whom strenuous exercise is harmful.

What Constitutes a Discharge Cycle?
Most understand a discharge/charge cycle as delivering all stored energy, but this is not always the case. Rather than a 100% depth of discharge (DoD), manufacturers prefer rating the batteries at 80% DoD, meaning that only 80% of the available energy is being delivered and 20% remains in reserve. A less-than-full discharge increases service life, and manufacturers argue that this is closer to a field representation because batteries are seldom fully discharged before recharge.

There are no standard definitions of what constitutes a discharge cycle. A smart battery that keeps track of cycle count may require a depth of discharge of 70% to define a discharge cycle; anything less does not count as a cycle. There are many other applications that discharge the battery less. Starting a car, for example, discharges the battery by less than 5%, and the depth of discharge in satellites is 6 to 10% before the onboard batteries are being recharged during the satellite day. Furthermore, a hybrid car only uses a fraction of the capacity during acceleration before the battery is being recharged.

Pulse and High-current Discharge
A classic discharge is a battery that delivers a steady load at, say, 0.2C. A flashlight is such an example. Many applications demand momentary loads at double and triple the battery’s C-rating, and GSM (Global System for Mobile Communications) of a cellular phone is such an example (Figure 2).

GSM loads the battery with up to 2 A at a pulse rate of 577 microseconds. This is a large demand for a small 1000 mAh battery; however, with a high frequency the battery begins to behave like a capacitor and the characteristics change.

In terms of cycle life, a moderate current at a constant discharge is better than a pulsed or momentary high load. Figure 3 shows the decreasing capacity of a NiMH battery at different load conditions and includes a gentle 0.2C dc discharge, an analog discharge and a pulsed discharge. The cycle life of other battery chemistries is similar under such load conditions.

Figure 4 examines the number of full cycles a Li-ion battery with a cobalt cathode can endure when discharged at different C-rates. At a 2C discharge, the battery exhibits higher stress than at 1C, limiting the cycle count to about 450 before the capacity drops to half level.

For a long time, Li-ion had been considered fragile and unsuitable for high loads. This has changed, and today many lithium-based systems are more robust than the older nickel and lead chemistries. Manganese and phosphate-type Li-ion permit a continuous discharge of 30C. This means that a cell rated at 1500 mAh can provide a steady load of 45 A, and this is being achieved primarily by lowering the internal resistance through optimizing the surface area between the active cell materials. Low resistance keeps the temperature down, and running at the maximum permissible discharge current, the cells heat up to about 50ºC; the maximum temperature is limited to 60?C.

One of the unique qualities of Li-ion is the ability to deliver continuous high power. This is possible with an electrochemical recovery rate that is far superior to lead acid. The slow electrochemical reaction of lead acid can be compared to a drying felt pen than works for short marking but needs rest to replenish the ink.

Calculating Battery Runtime (Peukert Law)
If the battery were a perfect power source and behaved linearly, the discharge time could be calculated according to the in-and-out current. “What has been put in can be taken out in the same form over time” is the argument, and in our example a one-hour charge at 5 A should enable a one-hour discharge at 5 A, or a 5-hour discharge at 1 A. However, intrinsic losses impede the ideal working of a battery, and the relative discharge time becomes shorter when increasing the load. High discharge currents make the battery less efficient.

The efficiency factor of a discharging battery is expressed in the Peukert Law. W. Peukert, a German scientist (1897), was aware of this loss and devised a formula that expresses the loss at a given discharge rate in numbers. Because of sluggish behavior of lead acid, the Peukert numbers apply mostly to this battery chemistry and help in calculating the capacity when loaded at various discharge rates.

The Peukert Law takes into account the internal resistance and recovery rate of a battery. A value close to one (1) indicates a well-performing battery with good efficiency and minimal loss; a higher number reflects a less efficient battery. The Peukert Law of a battery is exponential and the readings for lead acid are between 1.3 and 1.4. Nickel-based batteries have low numbers and lithium-ion is even better. Table 5 illustrates the available capacity as a function of ampere drawn with different Peukert ratings.

The lead acid battery prefers intermittent loads to a continuous heavy d
ischarge. The rest periods allow the battery to recompose the chemical reaction and prevent exhaustion. This is why lead acid performs well in a starter application with brief 300A cranking loads and plenty of time to recharge in between. All batteries require recovery, and with nickel- and lithium-based system, the electrochemical reaction is much faster than with lead acid.

Simple Guidelines for Discharging Batteries
* The battery performance decreases with cold temperature and increases with heat.
* Heat increases battery performance but shortens life by a factor of two for every 10°C increase above 25–30°C.
* Although better performing when warm, batteries live longer when kept cool.
* Operating a battery at cold temperatures does not automatically permit charging under these conditions. Only charge at moderate temperatures.
* Some batteries accept charge below freezing but at a much-reduced charge current. Check the manufacturer’s specifications.
* Use heating blankets if batteries need rapid charging at cold temperatures.
* Prevent over-discharging. Cell reversal can cause an electrical short.
* Deploy a larger battery if repetitive deep discharge cycles cause stress.
* A moderate dc discharge is better for a battery than pulse and aggregated loads.
* A battery exhibits capacitor-like characteristics when discharging at high frequency. This allows higher peak currents than is possible with a dc load.
* Lead acid is sluggish and requires a few seconds of recovery between heavy loads.

Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For more information on batteries, visit www.batteryuniversity.com; product information is on www.cadex.com.