Hummer battery quick-charger/compressor question
 

I have a quick-charger/air compressor from a late nineties or so Hummer. It needs to be charged and I do not know the polarity of the unit. I have a universal charger to connect to it but want to ensure correct polarity before I attempt to use it. I need to know if the charging pin in the Hummer unit is positive on the outside and negative on the inside of the pin which is what the universal charger will accept. I don't want to fry this thing trying to re-charge it. Any advise at all on re-charging these units. I do not have the original charger for it. No model number on this unit that I can see, either. Thanks for your replies.