A guy who owns a neighboring car in the communal parking garage I park in disconnects the positive side of the battery when leaving his car for a few days at a time.
Is this dangerous? If so, under what circumstances could this cause an accident?
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If he has a classic car with a positive ground (as opposed to almost all if not all modern cars that have a negative ground) then there is no danger and that is the proper terminal to disconnect.
On the other hand, if his car has a negative ground this can be dangerous. The danger is present while disconnecting and reconnecting the battery. If while loosening/tightening the terminal the wrench contacts the body of the car or another grounded metal component, a completed circuit is created. This will just about instantly heat up the wrench and terminal and other contacted metal (like maybe a ring he is wearing). I have seen battery terminals melt in seconds when someone wasn't paying attention. Should the short circuit not be immediately terminated, this poses a serious safety risk to him as he is doing it.