NB: The Battery system is being updated (late 2020).

Previously the vehicle was powered by a 36 Volt battery pack consisting of 6 small 12 Volt batteries connected in series-parallel.
The pack (pictured) is mounted on the floor between the driver's legs.

The only instrumentation used is an ammeter, although a cheap multimeter is often attached to monitor the battery voltage under load.

Charging is effected off-vehicle using a 12 Volt automotive charger to charge one battery at a time.
The charger has been modified to cut out at 14.4 Volts so as not to cook the batteries.

Charging of batteries is well documented by experts, here's a brief overview:
Charging 6 in parallel is possible but will result in uneven charge as the 'best' battery will take all the charge.
Charging the 36 Volt pack with a 36 Volt charger is possible but one needs to build said charger.

Some history:
Sydney Electrathon rules originally stipulated a 30 Kg lead-acid battery pack.
We used 3 flooded-cell automotive batteries weighing 10 Kg each.
A range of around 90 minutes at a speed of around 40 km/h was achievable.
When I acquired the vehicle its 30 Kg pack was mounted well forward of the front axle.
This led to severe understeer and a tendency to bend the delicate stubaxle carriers.
(of course, I have been known to lift the inside front wheel in cornering occasionally).
The battery pack was soon moved to a more central location.

Later rules prescribed a 6 pack of 7.2 AmpHour 12 Volt batteries weighing 2.5 Kg each.
(these sealed lead-acid batteries are commonly used in burglar alarm systems).
These gave a much smaller range, typically 40 km at 40 km/h on a velodrome circuit.
5 years later I'm still running this pack but they are beginning to age.