Sears Craftsman chargers had several contacts too. They served a few purposes: over temperature detection, cell/voltage detection, charging. Knowing the exact model of battery may help. A voltmeter can help, but if you have a bad pack, tearing that apart is the easiest way. That's what I did with a Sears pack a few years ago.
If it's a lithium based battery it could be for cell balancing. If it's a NiCad or NiMH they could be related to temperature sensing and charge control.