Much depends on the type of battery (- Lead-acid, gel, spiral gel, 'low maintenance lead-acid, etc). If it is an Optima battery they apparently require a different type of charger. I have an Optima 6V but no charger (yet) so can't provide much advice other than it is different technology used in these types of batteries so charging rate (current and time taken) is different to standard lead-acid. Optima info at
Optima Batteries Possibly the 20degree temperature is not very helpful to a battery when charging as the chemical reaction happening inside the cells at a very low temp might be altered by the cold and not happening 'normally'. Also it just might charge quicker if warmer (chemical reactions speed up with temp increases)so suggest taking it somewhere warmer, but others might have a better knowledge of this than I do! If it's lead acid don't forget to take the caps off to avoid a possible explosion.
A 50% discharged battery will possibly take 10 hours to fully recharge but this depends very much on the type of battery and also the type / capacity of the charger. Did you judge '50%' based on cranking speed as it got flatter?
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