The high resistance on the seatbelt tensioners has come up before. The yellow plugs underneath the seats can develop bad connections so this will turn on the airbag light. Make sure the ignition is off. Keys out .
Look under the drivers seat and there is a yellow multiplug facing you, you have to move a bar at the side of the plug to disconnect the plug. Spray the connections with solvent contact cleaner ( not WD40 ! ) leave for 5 mins until it has dried, refit the plug back together. Repeat on the passenger seat.
Reset the airbag light.
It's a common problem on all cars with Airbags.
As for the battery terminal if it had been activated I would have thought it would be open circuit, so the car would not have any power to it, I believe you have to disconnect the negative lead first, so not to trigger the charge inside it, try checking the resistance with a meter to see if its faulty.
Maybe its breaking down under load. I'd be wary of messing with it as I believe it's got an explosive charge in it to disconnect the power in an accident. ( X5 Sport will know if that's the case, as he' the electrical Guru on here

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