So, my brother has a '98 ZJ, 5.2L. If he leaves it sitting turned off for more than a few minutes, the battery gets drained. We can jump it and get it started, but it won't stay running. The battery has been replaced by two different Optimas with the problem still happening, so we know that's not it. He disconnected all his aftermarket stuff, so they wouldn't pull from the battery. Sometimes, it'll get drained while he's driving, so I know the alternator needs replaced. What could be draining the battery when it's turned off, though? Any ideas would be appreciated. He can't hardly drive it because of this.

 

For the vehicle to be draining a fully charged battery in a matter of minutes, it must be draining an extremely large amount of current (during this time). In theory, a 70Ah battery could withstand a continuous drain of 70A for 1 hour, to fully drain the battery - or 140A for ½ an hour, and so on. The are no electrical loads within the vehicle which should be draining the battery so fast.

However you have pointed to a possible fault current route, and that is the alternator. Should a diode within the alternator fail, this can result in a low impedance path that could be causing your problem.
Try disconnecting the alternator, then connect a charged battery and see how you get on.

If this is not the fault current route, then I would suggest you purchase a DC current clamp (available from ebay sellers) and use it to trace the battery drain current path. Cheaper current clamps are available, but they do not measure DC current via the clamp.

http://cgi.ebay.co.uk/UNI-T-UT203-AC...item3a612efd12

 

also if the alternator is not charging the battery won't have an hour of reserve when you park or as you stated run the car fix the alternator first

 

Easy and Cheap!! Dissconect the battery. When you reconnect, you should only have a small spark as the various components memorys pick up current. If you have a fat spark, disconnect the alternator (ALL leads) and check again. By process of elimination, you should be able to find out where you juice is going. Good luck!

 

To test for electrical drain... You need a cheap test light... Disconect the negitive cable... hook one end of the test light to the battery.. the other to the neg cable... the test light makes the circut... if any draw it will light up... take in mind that clocks and radios draw power for memory.. but its pretty simple.. the brighter it lights up.. the bigger the draw... In your case.. disconect alt first... if the light goes out.. thats your draw... if not... start unpluging fuses... and what ever you can find till light goes out...
there are other ways but this is a simple easy way to track down shorts of any kind...but sometimes you get lucky... dude brought a old car to shop said its poping radio and interior lights or something every now and then... replace fuse it was good hit bump and poof... blow fuse... took to two shops then brought to us... I jumped in... sat in car.. was old neat looking car... clasic... cant remember what.. but anyway.. I reached in back of the center console and opened the ashtray.. Was a penny in the cig lighter... :} sometimes you just get lucky... LOL

 

I do not know electrical work at all. Lol. But, Hommer, yours seems like the least challenging, so I'll probably try that first. Lol.