[acropolite (Phil)]

It's fine to join them together as banks of 12V in fact it's common practice. Give them a good charge, 14 volts or even a little higher is acceptable, normal charge voltage for lead acid is around 2.35 V per cell which equates to a little over 14 volts on a 12 Volt bank. Boosting to a slightly higher voltage at the end of charge is beneficial as it will bubble the electrolyte slightly and circulate it, preventing stratification of the electrolite, which is a major cause of premature failure especially at higher temperatures. The normal car charger will be OK but don't expect the rating to be anywhere near accurate as battery charger manufacturers have a habit of grossly exaggerating charge outputs. The charge rate can be fairly modest, for example I charge a similar sized (600+ AH) lead acid bank with solar panels with output of 8 amps, the solar regulator terminates charge at 14.2V floats at 13.8V and occasionally (once a month) boosts to around 15V for a simgle cycle.

Once the batteries have reached full charge then maintain them at 13.8 to 14 Volts and keep an eye on electrolyte levels. When you do start to use them, their service life will be maximised if you limit discharge to around 30% of capacity.