Originally Posted by Lightninrod
Not sure about your specific problem but I too worry a little about the constant drain on our AGM battery. Part of the time, I keep an AGM specific battery charger connected to the battery connections in the engine compartment. It's trickle charging right now.
The OP has a ZJ, where a good old fashioned non-AGM Die Hard is overkill...
I'd suggest springing for a cheap volt meter.
Before touching the starter, measure across the two posts.
Then, measure from the + post to the body right near the alternator. Any drop is good indication of connector crud.
Post both numbers.
Then, see if you can bribe someone to measure across the posts while starting to see how much you drop at the two posts. Reasonable indication of aging battery.
A bit trickier is to measure from the + post to the big terminal on the starter itself while starter is running. Some very small drop is normal, but is a good indication of poor connections.
Cleaning of connectors, including the ground to the body would be a starting point if not already done. Battery model and age also would help with advice.