As well as the different Charging techniques, there are also a number of different types of discharging methods which should be considered in order to prolong battery life.

The capacity of a battery can be measured with a battery analyser. If the analyser's capacity readout is displayed in percentage of the nominal rating, 100% is shown if a 1000mAh battery can provide this current for one hour. If the battery only lasts for 30 minutes before cut-off, 50% is indicated. A new battery sometimes provides more than 100% capacity.

When discharging a battery with a battery analyser that allows the setting of 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 analyser is scaled to derive the full capacity in 30 minutes. Theoretically, the capacity reading should be the same as with a slower discharge, since the identical amount of energy is dispensed, only over a shorter time. Due to internal energy losses and a voltage drop that causes the battery to reach the low-end voltage cut-off sooner, the capacity reading may be lowered to 95%. Discharging the same battery at 0.5C, or 500mA over two hours may increase the capacity reading to about 105%. The discrepancy in capacity readings with different C-rates is related to the internal resistance of the battery.

Depth of discharge

The typical end-of-discharge voltage for nickel-based batteries is 1V/cell. At that voltage level, roughly 99% of the energy is spent and the voltage starts to drop rapidly if the discharge continued. 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, also known as cell reversal, will occur across a weaker cell if the discharge is allowed to continue uncontrolled. The more cells that are connected in series, the greater the likelihood of cell reversal occurring.

Nickel-cadmium can tolerate some 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 on the positive electrode leads to pressure build-up and possible cell venting. If the cell is pushed further into voltage reversal, the polarity of both electrodes is being reversed and the cell produces an electrical short. Such a fault cannot be corrected.

Nickel-cadmium is least affected by repeated full discharge cycles.
Lithium-ion typically discharges to 3.0V/cell, since the equipment manufacturers do not specify the battery type; most equipment is designed for a 3-volt cut-off. Some lithium-ion batteries feature an ultra-low voltage cut-off that permanently disconnects the pack if a cell dips below 1.5V. 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. Manufacturers rate the lithium-ion battery at an 80% depth of discharge. Repeated full (100%) discharges would lower the specified cycle count. It is therefore recommended to charge lithium-ion more often rather than letting it discharge down too low. Periodic full discharges are not needed because lithium-ion is not affected by memory.

The recommended end-of-discharge voltage for lead-acid is 1.75V/cell. The discharge does not follow the preferred flat curve of nickel and lithium-based chemistries. Instead, Lead-acid has a gradual voltage drop with a rapid drop towards the end of discharge.

The cycle life of sealed lead-acid is directly related to the depth of discharge. The typical number of discharge/charge cycles at 25°C (77°F) with respect to the depth of discharge is:

• 150 - 200 cycles with 100% depth of discharge (full discharge)
• 400 - 500 cycles with 50% depth of discharge (partial discharge)
• 1000 and more cycles with 30% depth of discharge (shallow discharge)

The lead-acid battery should not be discharged beyond 1.75V per cell, nor should it be stored in a discharged state. The cells of a discharged lead-acid battery, sulfate; a condition that renders the battery useless if left in that state for a few days. Always keep the open terminal voltage at 2.10V and higher.

What is a discharge cycle?

There are no standard definitions that constitute a discharge cycle. Generally, anything less then a 70% depth-of-discharge is not regarded as a discharge cycle.

Batteries often receive short discharge/recharge cycles, this has the most damaging effect on nickel-based batteries, and they require periodic deep discharge to prolong battery performance.

Lithium and Lead-based batteries do not require full discharge, and is generally better not to discharge them too deeply and it is recommended to charge them more often.

Deep Discharge Rechargeable batteries have an ‘end-of-discharge’ voltage; this voltage level varies for each battery type. At this voltage level, roughly 99% of the energy of the battery is spent and the voltage starts to drop rapidly if the discharge continued. Deep discharging is where the discharge cycle continues past this cut-off voltage, this can damage the battery, possibly irreversibly if discharging continues for too long.