Hello Paul Davies, and SRS,
It is best to use a charger designed to charge batteries at
their correct charging rate.
There are gel cell battery chargers used by ride-on toys
which are made for either 6 volt or 12 volt batteries. These
chargers output a Direct Current (DC) at a voltage
compatible with the size battery being charged.
It is important not to overcharge batteries, so it is good
if a charger can shut off when the battery is fully charged.
Toys-r-us , and Radio Shack carry gel cell battery chargers.
Radio Shack sells a paperback book about batteries and their
proper charging and discharging.
Bryan E. Daniel