ripple4 wrote:I'd like to learn more about this subject, and it may well be the case that the posts i see on this forum with the always be trickle/float charging comments may still be valid for normal temperatures. but there seems to be a warning against charging a lead acid battery that is very cold.
To the best of my knowledge, the warning against charging cold batteries is because a very discharged, very cold battery could have become frozen solid; and fast-charging it (especially trying to jump-start it) may cause it to explode.
I doubt that attempting to trickle-charge a frozen battery at 20 watts would be enough to cause an explosion (though a frozen battery would fail to charge). Regardless, if you know that the battery is not frozen, then I don't think you need to worry about damage from trickle-charging.
The act of trickle-charging a battery also creates some heat. So my method to heat the battery up has always been to hook-up a trickle charger on very cold nights. I use a BatteryMinder Jr. these days: while the battery is charging, it is warming-up; and once charged, when the charger is doing its "desulfation" thing, that should add a bit of heat also.
Insulating the battery would also minimize heat losses, so would be valuable with, or without a heat source.