one thing to remember about AGM battery is proper charging.
a regular battery charger won't charge them without a special process.
you ether attach a good battery to the AGM one with good jumper cables. then hook the charger to it or use a charger designed for them.
special AGM chargers have dropped in the last few years. getting the proper one is reccomended.

also if you have a low battery in your car the alternator might not charge it properly.