Float Battery Chargers in GRID/Power Plants

Bulk charging of batteries is done with current control. The charger will maintain a specified value of current and will adjust the voltage to achieve the desired charging current. This is a fast rate charge. It can be several amperes to tens of amperes or more depending on the capacity of the battery.

https://en.wikipedia.org/wiki/Battery_charger

A float charge maintains a specified voltage, e.g. 13.6 volts DC maybe for a six cell Pb-acid battery and adjusts the current up or down to maintain that 13.6 volts DC. For most batteries the float charge could be as little as a few milliamps or even less. It will depend on the size and the condition of the battery.

https://en.wikipedia.org/wiki/Trickle_charging

Many modern battery chargers have charging algorithms to transition from bulk charging to float or trickle charging.

(And then there is conditioning batteries, generally speaking of Pb-acid batteries where you charge to an even higher voltage, sometimes with pulses to eliminate sulfur from the plates.)

https://en.wikipedia.org/wiki/Battery_regenerator

A bit of history may help explain the term "floating". Back in the days before all this modern electronic stuff, the batteries that powered the POTS (Plain Old Telephone System" and the protective relaying and control systems were called station batteries. There were huge numbers of them connected in multiple series and parallel banks, all powered off of crude (by today's standards) DC battery chargers, but with no way to tell the SOC (State Of Charge) of any particular cell without going around and randomly sampling the SG (Specific Gravity) of each one.

Someone came up with the bright idea of placing a specially calibrated hydrometer that was small enough to fit into the top of a cell, so that a quick look would tell the technician if the cell was in need of charging. If enough of the hydrometers weren't seen, the technician would increase the charge voltage until the majority of the hydrometers were visible. This was known as a (drum-roll) floating charge, and was usually controlled by a timer to prevent overcharging of the bank. Any cell whose hydrometer wasn't floating was deemed to be defective and was replaced.