Simply Question

I have a 12v 7.2ah battery hooked up to a old 16mm projector.
This gives me the power I need but soon flatens the battery.
If I used a 12v car battery would I get a longer run time, since they have a higher a/h.

It is measured in AH and the bigger the battery the more ampere hours there are in it. So a big car or truck battery may have 60 to 90 AH a small alarm panel battery only has 7 to 17 AH So parallel a bunch of them or get a big Gel Cell like a Optima Yellow Top or Blue Top batteries and it can be charged a number of times. Automotive batteries can be charged maybe a dozen times and it is ruined. If you use two batteries in Parallel AH add if they are in series they remain the same but voltage will add up. So a 12 volt 20 AH battery pack using two batteries is either 12 volt 40 AH or 24 volt 20 AH.

Thanks to Peukert effect, doubling the AH capacity of the battery will much more than double the power you can get out of it. The crucial difference is

1. AH capacity
2. Battery design; make sure you get a "Deep cycle" battery for best life.

Mark

of course, much longer, as a car battery has >50amp hours