The rate at which a lead acid battery can accept charge falls as it is recharged. When fully discharged, it may accept currents of several multiples of its capacity. A typical vehicle starting battery, for example, may be able to accept several hundred amps. In many cases, these high currents can be damaging to the battery, so it is usual for the charging system to have some sort of current limit. Once the battery has recovered to around 70% to 80% of full charge, the current starts to tail off. So, although the battery may be restored to this point fairly quickly, to continue to full charge can take as long again. Boost charging raises the voltage, forcing more current into the battery and restoring it more quickly. However, if the boost voltage is sustained at a raised level after the battery is fully recharged, current will continue to flow causing electrolyte loss due to gassing. Some batteries are irreversibly damaged by continued overcharge. It is important, therefore, to reduce the voltage, once charged, to the float level. This is the voltage that the battery will sustain without suffering damage or significant gassing. A typical float voltage for a 12 volt battery is 13.6V, and around 14.4V for boost. More relevant in many cases, is the fact that a raised voltage at the charging source will offset voltage drop in the supply cables to the battery; this is often a more significant gain than the effect noted above.

Note that the appropriate boost and float voltages are dependent on temperature. See temperature compensation explanation for more details.