The difference between a float charger and a trickle 
charger is that the float has circuitry to prevent 
overcharging. It senses when the battery voltage is at the 
maximum level and temporarily shuts off the charge (floats 
voltage at zero or a very minimal charge until it senses 
that the battery output voltage has fallen, then resumes 
charging ). You can keep it connected indefinitely.
A trickle charger, on the other hand, will charge no matter 
whether the battery is fully charged or not. So it needs to 
be connected and disconnected periodically. If left in 
place too long it'll eventually boil the electrolyte out of 
the cells or damage the plates. Trickle chargers will work 
to keep the battery charged if used once a month or so for 
a day or 2, but the float chargers can be left connected 
indefinitely without potential harm to the battery.
For example, a 24 volt battery pack, comprising 12 2-volt 
flooded lead-acid cells, which has been deeply discharged, 
would normally be restored by a boost charge of 
approximately 2.4 volts per cell for a short time (perhaps 
around 72 hours). Once the collective cell voltage reaches 
a surface charge of 28.8 volts (2.4 volts x 12 cells), the 
charge rate would be switched to the sustained lower float-
charging rate of typically 2.23 volts.
Eventually, with the Boost charge removed, the surface 
charge will diminish slightly and the battery-bank voltage 
will stabilise at a preset float voltage, in the case of 
the example above to approximately 27 volts (2.23 volts x 
12).