There is little to no oxygen in the gas tank. If you tried to light a fire in a gas tank nothing would happen. The vapors burn but only with oxygen. You can actually weld on a gas tank that has gas in it, believe it or not. I'm not sure what effect gasoline has on batteries, though. If you have a whitness who can say they saw him put the batteries in the gas tank you can have a judge force him to pay for the removal of the batteries and the repair of any damage to your car. But, only if you have a whitness.