one of my friends' car needed a jump earlier, I got there just after another friend did so they didnt need my car anymore. But, my question is about how they jumped the car... they connected both terminals on both batteries which I know you have to ground the car being jumped (I believe). They decided not to listen and it worked. My question is, what happens when you do that? does it damage the battery?

 

Not to my knowledge..............I believe it really is to lessen the chances of explosion................which can happen. I've also seen where battery terminals can melt into the casing, but I'm not sure it was caused by battery to battery connection.

 

when you jump a battery alot of times it vents hydrogen gas, and keeping connections separated lessens the chance of a spark occurring near the gas venting from the battery.