Trickle charging

Trickle charging, also called float charging, means charging a battery at a similar 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 pack, comprising 12 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 will 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 (ampere-hours), 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.

In low duty-cycle applications, where a relatively high current or power is required infrequently, charger costs can be minimized by applying trickle-charging principles. This can be an economy measure in cases where the charging method could be quite expensive if the full charging rate were employed, such as solar-cell installations. Full battery capacity can be achieved at a low charging current over a long period of time to provide a high-power load for a short period.