Guidelines For Discharging Batteries

Basics

The battery’s purpose is to store and release energy at the desired time and under controlled manner. The end-of-discharge voltage for lead acid is 1.75V/cell, for the nickel-based system is 1.00V/cell and for most Li-ion is 3.00V/cell. Roughly 95% of the energy is spent at this level and the voltage would drop quickly if the discharge continued. In order to protect the battery from over-discharging, most devices are preventing operation beyond the specified end-of-discharge voltage.

When you remove the load after discharge, the voltage of a healthy battery will gradually recover until it reaches its nominal voltage. The differences in the metal concentration of the electrodes enable the voltage potential when the battery is empty. The voltage of an aging battery with elevated self-discharge cannot be recovered due to the parasitic load.

The end-of-discharge voltage threshold should be set lower as the high load current lowers the battery voltage. The internal cell resistance, wiring, protection circuits and contacts add up to the overall internal resistance. Another thing to be lowered is the cut-off voltage when discharging at very cold temperatures as it compensates for the higher-than-normal internal resistance. On a high load, the lower end-of-discharge voltage compensates for the losses induced by the internal battery resistance.

Discharge/charge cycle is seen as delivering all stored energy, but in some cases it is not very accurate. Some manufacturers prefer rating the batteries at 80% depth of discharge (DoD), rather than a 100%. This means that only 80% of the available energy is being delivered and 20% remains in reserve. The less-than-full discharge increases service life and therefore, manufacturers argue that this is closer to a field representation as batteries are hardly fully discharged before recharging occurs. There are no standard definitions of what constitutes a discharge cycle as some smart batteries might require DoD of 70 % to define a discharge cycle while other batteries require less.

We at ReplaceUPSBattery.com think that the following tips and guidelines will be useful for you in attempting to discharge a battery.

Tips and Guidelines

• Cold temperature has a negative effect on battery performance while heat has a positive effect on it. However, heat affects battery life as it shortens it by a factor of two for every 10°C increase above 25–30°C (18°F above 77–86°F). Therefore, batteries have longer life under conditions of cold temperatures. Nonetheless, try to charge the battery at moderate temperatures to avoid problems and always check the manufacturer’s specifications and instructions. When charging at moderate temperatures is not possible, use heating blankets for rapid charging at cold temperatures.
• Avoid over-discharging as cell reversal can cause an electrical short.
• If repetitive deep discharge cycles cause stress, use a larger battery.
• A moderate DC discharge is better for the battery.
• When discharging at high frequency the battery exhibits capacitor-like characteristics, which allows higher peak currents compared to a DC load.
• Since lead acid is sluggish, it requires a few seconds of recovery between heavy load.

Bibliography:

Buchmann, I., 2013. Battery University - Basics About Discharging.