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What is "float-charge"?
A typical UPS battery is almost always operating in float-charge mode. In this mode
the overall battery voltage is regulated to a fixed voltage that equals the optimum
cell voltage multiplied by the number of cells in a battery string, for example; The optimum cell voltage is chosen very precisely so that the cell is kept fully charged without overcharging. If the voltage is too high, the cell may start leaking gas from the venting valve, eventually leading to dry-out. A voltage too low will cause sulfation and permanent loss of capacity. (click/unclick) Each cell voltage will "float" around a voltage that equals the total string voltage divided by the number of cells in the string. The meaning of "float" is that the cell is not forced to this voltage, instead it is fed a small current and left to find its own optimal voltage that will keep it in equilibrium, which may vary ±50mV from average. 
Floating in a state of equilibrium.
By definition, float-charge implies that the same current flows through all the cells. The float-charge current in a healthy battery can be very low, typically 0.1 to 1 mA per 1Ah capacity (C10). However, this varies heavily with voltage, temperature, age and cell type. In a 100 Ah battery string a normal float-charge current can be 40mA. Any current that is bypassing a cell will cause an imbalance between the cells and thereby disturb the self-regulating voltage distribution. It is therefore essential to keep the cells dry and clean from any conducting contamination around the terminals. Consequently, for the float-charge mode to work, with the same current through all cells, no external connections should be made to any terminal within a string. Any battery current, whether charging or discharging must be connected at the string level only.
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