Trickle charging

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Trickle charging, also called float charging, means charging a battery at the same rate as it is self-discharging, thus maintaining a full capacity battery. Most rechargeable batteries, particularly nickel-cadmium batteries or nickel metal hydride batteries, have a moderate rate of self-discharge, meaning they gradually lose their charge even if they are not used in a device. One must be careful, however, that if a battery regulator is not employed, the charge rate isn't greater than the level of self-discharge, or overcharging and possible damage or leakage may occur.

For example, a 24 volt battery bank, comprised of 12 x 2 volt flooded lead-acid cells which has been deeply discharged, would normally be restored by a boost charge of approximately 2.4 volts per cell for a short time (perhaps around 72 hours). Once the collective cell voltage reaches a surface charge of 28.8 volts (2.4 volts x 12 cells), the charge rate would be switched to the sustained lower float charging rate of typically 2.23 volts.

Eventually, with the Boost charge removed, the surface charge will diminish slightly and the battery bank voltage stabilise at a preset float voltage, in the case of the example above to approximately 27 volts (2.23 volts x 12).

Charging rates for a trickle charge are very low. For example, if the normal capacity of a battery is C (amperehours), the battery may be designed to be discharged at a rate of C/8 or an 8-hour rate. The recharge rate may be at the C/8 rate or as fast as C/2 for some types of battery. A float or trickle charge might be as low as C/300 ( a 300-hour discharge rate) to overcome the self-discharge. Allowable trickle charging rates must conform to the battery manufacturer's recommendations.