Trickle charging, or 
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.
 The difference between a float charger and a trickle charger is that the float has circuitry to prevent overcharging. It senses when the battery voltage is at the maximum level and temporarily shuts off the charge (floats voltage at zero or a very minimal charge until it senses that the battery output voltage has fallen, then resumes charging ). You can keep it connected indefinitely.
 
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. If left in place too long it'll eventually boil the electrolyte out of the cells or damage the plates. Trickle chargers will work to keep the battery charged if used once a month or so for a day or 2, but the float chargers can be left connected indefinitely without potential harm to the battery.
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