The short answer is ZERO. Well, not exactly. Float charging is to
replace the charge lost by internal discharge, which is on the order of
2-3% per month.

The voltage mentioned by others is valid only at 25 C. This goes up
significantly in cold weather and drops in hot. This means your battery
is under charged when it is cold and you are over charging it when hot
- not particularly good for battery life. Panasonic specifies a voltage
of 14.1 V @ 0 C, 13.7 V @ 25 C and 13.4 V @ 40 C. A temperature
compensated battery charger should be used if you are charging at
extreme temperatures.

Tom