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If you mean like AA or AAA, nothing. Absolutely nothing happens. If they both have an electrical current running, I don't know, it might be dangerous.
When you take the battery out nothing really happens. I tried it myself and it did nothing.
If the new battery is 12 volt and fits into the battery tray and you connect the cables correctly, nothing will happen. If the new battery doesn't have the required CCA needed for the vehicle, it may crank over the engine slower especially in cold weather.
Defective, corroded, or loose battery cable.
Nothing good.
Nothing. The battery is sealed an no harm will be done.
Possibly the ignition switch is defective.
I would start by checking the battery and fuses.
get a new car
100 % think it is Nothing
Sometimes nothing and sometimes you will just blow the main fuse. Sometimes you can do serious electrical damage to other components. Depends on the vehicle.
nothing really, unless the batteries had battery acid all over them, other than that nothing