Well, I've officially joined the 'oops, I killed my batteries' club! Last July, I installed six East Penn 6V house batteries and set the Magnum remote to AGM type. Left my coach at a shop for 2.5 weeks and came back to completely dead batteries - we're talking 8V dead, folks. Nothing worked: steps, lights, even my fancy electric shades decided to take a permanent nap. Chassis batteries were still okay.
After plugging in at home, the Magnum charger went through its whole song and dance - bulk charging at 100A, then eventually settled into float at 13.2V. When I unplugged and fired up the generator briefly, voltage dropped from 13.0V to 12.7V.
So here's my million-dollar question: did my deep discharge adventure turn these batteries into expensive paperweights, or do these voltage readings suggest they might have survived my negligence? I'm cautiously optimistic but honestly have no idea what signs indicate whether AGM batteries bounced back from the dead or are just pretending to be okay!
Your East Penn 6V AGM batteries likely sustained some damage from the complete discharge to 8V, but your charging behavior suggests they may have recovered reasonably well. The key indicators are how they hold voltage under load and their actual capacity.
Your six East Penn 6V AGM house batteries have likely suffered some damage from being completely discharged to around 8V, but they may still be recoverable. The fact that they're accepting charge and holding voltage reasonably well (only dropping 0.1V overnight) is actually encouraging. AGM batteries can typically survive one or two deep discharge events, but repeated deep cycling will permanently reduce their capacity. The charging behavior you're seeing - progressing from bulk to float to absorb - suggests your Magnum charger is working correctly and the batteries are responding, though they may have lost some of their original capacity.
Deep discharge damage occurs when AGM batteries are drained below 50% of their capacity, and yours went much further - down to around 8V indicates they were nearly completely depleted. When AGM batteries sit at such low voltages for 2.5 weeks, the lead plates begin to sulfate heavily, which reduces capacity and can make them harder to charge. The repair shop should have disconnected your battery switch, as even minimal parasitic loads (like CO detectors, propane leak detectors, and inverter standby power) can drain batteries over time.
The good news is that your batteries are showing positive signs. The progression from bulk charging at 13.0V to float at 13.2V, then jumping to absorb at 14.2V when you reconnected, indicates the charger is reading the batteries' state correctly. AGM batteries typically need 14.1-14.4V for proper charging, so your Magnum's 14.2V absorb voltage is appropriate. The minimal voltage drop overnight (12.7V to 12.6V) suggests the batteries aren't heavily sulfated yet.
Contact a professional battery specialist or RV technician if individual batteries are reading below 6.0V after charging, if any battery fails the load test, or if the batteries continue to lose more than 0.2V per day when not in use. These symptoms indicate permanent sulfation damage that requires battery replacement. Also call a pro if your parasitic draw exceeds 0.5 amps, as this suggests an electrical fault that needs professional diagnosis.
Consider professional help if you're uncomfortable performing load testing, as improper testing can be dangerous with large battery banks. A battery specialist can also perform capacity testing to determine exactly how much capacity you've lost, helping you decide whether replacement is economically justified. Given that quality 6V AGM batteries like your East Penn units cost $200-300 each, professional testing might save you money by identifying which specific batteries need replacement rather than replacing the entire bank.
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