Usually you use a 5-7 amp sealed lead acid battery, also called a gel cell, that fits inside the starter box. It's rectangular, like a small version of a car battery. In fact you wil get the best deal at an automotive battery store.
On the outside of the starter box you wil find a red and a black post. You unscrew the plastic caps and to these posts you attach the positive and negative clamps of what's called a float charger. These can be found at the same battery store you get the SLA, recreational vehicle shops, and of course, your local hobby shop. The advantage with a float charger is you can leave it attached whenever you're not using the starter box and it will always have a strong charge. The voltage varies with the state of charge and there's no danger of overcharging the battery.
It can be charged by other methods, but be very cautious if you try to use a full-size car battery charger. Most of those charge at 2 amps on "slow charge." An SLA should be charged at no more than 1700 mah (1.7 amps.)
Some find it easier to use two 6-cell stick packs in series, and you can use the same methods to charge them, but these generally don't provide the torque you will get with a 7 amp SLA.
The dual-motor and single-motor debate rages on, but in my experience the dualies work out better. The large single motors can reach a higher RPM, but it's not RPM you need, it's torque. The two motors combined provide more than enough torque to start anything up to a .28 engine.
