Trickle charging, or float charging, means charging a battery at a similar rate as its self-discharging rate, 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. Care must be taken, however, that if a battery regulator is not employed, the charge rate is not greater than the level of self-discharge, or overcharging and possible damage or leakage may occur.
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A float charger differs from a trickle charger in having circuitry to prevent overcharging. It senses when the battery voltage is at the appropriate float level and temporarily ceases charging; it maintains the charge current at zero or a very minimal level until it senses that the battery output voltage has fallen, and then resumes charging. It is important to note that the appropriate float voltage varies significantly with the construction of the battery and the ambient temperature. With the appropriate voltage for the battery type and with proper temperature compensation, a float charger may be kept connected indefinitely without damaging the battery. With a 6-cell (nominal 12V) lead-acid battery the correct float voltage drops by about 0.15 V for a 5°C rise in ambient temperature. Not compensating for this will shorten battery life either by over- or under-charging.
A trickle charger, on the other hand, will charge no matter whether the battery is fully charged or not, so it needs to be connected and disconnected periodically. Trickle chargers will work to keep the battery charged, if used once a month or so for a day or two, but the float chargers can be left connected indefinitely without potential harm to the battery.
For example, a 24 volt battery pack, comprising twelve 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 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.
For a 12 V 60 Ah battery a C/300 rate would mean 60 A / 300 = 0,20 A = 200 mA
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.