The Heinzmann Power Pack looks like this:

The box has three main components - the battery cells, the battery meter, and the control board. The control board is described in more detail elsewhere. The basic electrical arrangement is this:

The cells have a positive and negative from them, along with two (usually) wires from a pair of thermistors which measure the temperature of the cells, and an additional wire from half-way along the pack. This “half voltage” is used by the controller.

The negative goes to the battery meter, which is of the amp shunt type - it counts exactly how many amps flow in or out of the cells, so it is very accurate. The negative then goes on to the controller, via a 30A fuse. The positive goes straight to the controller, with a thin wire running to the battery meter to power the meter’s electronics.

Three wires from the charging socket go to the controller - but the controller is not involved in the charging process. The charger itself uses the thermocouples to monitor the temperature of the cells.