On Mon, 21 Jul 2003, Tim Demarest wrote:

> As long as the charging voltage is greater than the battery voltage (the
> battery is *always* putting out 12 volts), then some current will flow into
> the battery, and the battery will charge.

Actually, a *fully charged* "12 volt" battery will have a resting voltage
closer to 13 volts.

13.5 volts is a typical value for a "float charge".  This is used for
trickle-charging batteries that are left on charge all the time
("floated") for things like uninterruptable power supplies.  13.5 volts is
a compromise between getting a reasonable amount of capacity out of the
battery and not prematurely killing it by boiling off too much water.

For faster charging and higher capacity it's common to charge automotive
batteries at 14-15 volts, but this will boil off some water.  Too fast a
charging rate can also blow the vent plugs on sealed, gelled-electrolyte
type batteries.

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si