If it is a 12v battery and it is big then it has to be a gell cell. You can use a car charger to charge these just trickle it at 1 or 2 amps. Some chargers also have a feature that will say PB or Lead Acid this is also a mode for chargine gell cells.

As for the LIFE pack. Just dont worry about the balance plug its just like charging lipos you dont have to use it. For the connector you need to invest in a squid, there are several ones out there common sense, racers edge just to name a couple.