AGM Battery Testing

First test is to disconnect it for at least 3 hours. Look up the specs for your battery to find out the resting voltage. For most lead acid batteries it is right around 12.7 but those higher end batts sometimes are different. See how close your battery is to the spec. It should be very close. If not it may be shorted out internally, or sulphated badly so it won't accept a charge.

Next is a load test. Hook up the battery with the pod unplugged from AC power. If you still have incandescent bulbs in your fixtures turn on 8-9 bulbs (not fixtures - many of them have 2 bulbs). That is about a 10-11 amp load. If you have LED lights an alternative is to turn the fridge on DC mode, if the fridge is warm inside and the pod's interior is warm (not the middle of winter), and leave the door open. That is a 10-11 amp load. Leave the lights or fridge on for one hour, then turn it all off and disconnect the battery. Measure the voltage. Let it rest again for at least 3 hours. Measure the voltage again. Go to the link below and scroll 2/3 of the way down to the colored chart of voltages. Your first measurement (right after disconnecting the battery) should be somewhere in the green. The 2nd one should be around the 80-90% marks. If either/both are well below then it is probably sulphated. It may show a good voltage in the first test (the very first test - resting voltage) but there is little lead plate left to accept a charge so the voltage will crash quickly with the 2nd and 3rd measurements in this test.

http://www.solar-electric.com/deep-cycle-battery-faq.html