This solution was to solve a triple problem: the rules require an on/off switch, secondly, how can one charge the battery in a sealed case without continually opening and closing the case. Lastly, this charge-switch needs to be impossible to plug in the wrong way round and impossible to short the battery.
As you can see from this diagram, the supply from the battery is broken.
By attaching the two red prongs we have an on/ off switch. It is impossible to attach the connecting plug to short the battery as this is too far away.
By attaching the black prong and the lower red prong to a charger, we can charge the battery. If this connection is plugged in the wrong way round it will not have a detrimental effect.