Float charging of a battery involves charging the battery at a reduced voltage. This reduced voltage reduces the possibility of overcharging.The Float charger ensures that the battery is always in the charged condition and is therefore considered "floating". The Float charger starts by applying a charging volltage to the battery. As the battery gets charged, its charging current reduces gradually. The float charger senses the reduction in charging current and reduces the charging voltage.
If the battery gets drained, the float charger will again increase the charging voltage and process continues. Float chargers can be connected indefinitely to the batteries.
Boost charging involves a high current for short period of time to charge the battery. It is generally if the battery has been discharged heavily. Boost charge enables the quick charging of depleted batteries.
For instance, a two volt lead acid battery which has been discharged will initially be boost charged with a charging voltage of around 2.35-2.4 volts. However, as the battery voltage rises, the charger will switch over to the float charge mode with a float voltage of 2.25 volts.
Most battery chargers come equipped with provisions for both boost and float charging.