The green one on the right is their original NiMh which was discontinued long ago. The left hand one with blue wrapped rectangular cells is their current lithium polymer one. I doubt the NiMh one will be much use at its age, having short range if it is usable at all.
They use different chargers and one will not do both. The correct NiMh charger has the third connection used for charge monitoring by temperature via an integral battery sensor which matches the charger control circuit. It also has to use a higher charge voltage which would damage the lithium unit. The charger for the lithium battery just delivers the current, the battery's integral circuitry controls the charge.
So it looks like you need to buy their lithium charger, assuming that battery is ok, they usually only last about two years.
.
