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Basics About Discharging
The purpose of a battery is to store and release energy at the desired time and in a controlled manner. This section examines discharges under different C-rates and evaluates the depth to which a battery can safely be depleted. Chapter 5 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.Depth of Discharge
The end-of-discharge voltage for lead acid is 1.75V/cell; nickel-based system is 1.00V/cell; and most Li-ion is 3.00V/cell. At this level, roughly 95 percent 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 devices 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.
Table 1: Recommended end-of-discharge voltage under normal and heavy load
The lower end-of-discharge voltage on a high load compensates for the losses induced by the internal battery resistance.
Some battery analyzers apply a secondary discharge (recondition) that drains the battery voltage of a nickel-based battery to 0.5V/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.
(extract from www.batteryuniversity.com)
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