How much damage did we do by having a battery reversed in our 48v bank?

So by flipping one of those 12V units, you essentially turned your 48V bank into a 24V bank. Since your inverter's LVD was likely set for somewhere around 42V to 44V, it saw that 24V and imediately nixed the connection. You actualy got incredibly lucky here. If the inverter had tried to 'recover' the bank, it would have been pushing current into that reversed cell in the wrong direction, which is a fast track to a thermal runaway or at least a very swollen, venting battery.

The main thing to worry about isn't actually the inverter, but rather the other parallel strings. If the strings were tied together before the LVD, the healthy 48V strings would have been trying to discharge into that 24V string the entire time. But if the inverter logs show zero current draw or charge and the site stayed up on grid power, it sounds like the whole DC bus stayed isolated. In that case, those batteries have just been sitting in a state of self-discharge for months. You definatly need to pull them all apart and test them individually. Don't just flip the battery and hit the 'on' switch. Check the resting voltage on every single block. If they've been sitting for months without a float charge, they're probaly sitting low—maybe 11.8V or 12.1V—and you've likely got some **sulfation** to deal with. AGM plates don't like sitting thirsty for that long. Before putting them back in service, I’d highly recommend charging each 12V battery individually with a dedicated charger to get them balanced. Then run your load test. If the reversed battery holds a load similar to the others, you're probably fine. But if it drops off a cliff immediately, the internal chemistry is likely shot. It’s a cheap insurance policy to just replace that one specific cell if the site is truly mission-critical.

That’s wild. I’m just starting to learn about DC systems and didn’t realize an inverter could be that smart. You mentioned sulfation—since they've been sitting so long, is that something a regular charger can fix? I've seen some units with a 'desulfation' or 'repair' mode, but I wasn't sure if that's actualy legit or just marketing fluff for AGMs.

Also, does the reversed battery take any permanent damage just from the 'pressure' of being in the circuit backwards even if the LVD kept the current from flowing? Or is the chemical damage only a thing once you actually try to charge it the wrong way?

Repair modes are mostly a gimmick for AGMs. You start hitting them with high-voltage pulses and you're more likely to dry out the glass mat or pop the pressure valves than actualy clean the plates. AGMs are 'starved' for a reason. As for that reversed block, the real killer is the potential for dendrites to form after sitting discharged that long. Even if it takes a charge now, it coud develop an internal short months later. If this site is remote and a pain to get to, just bin that one battery. It's cheaper than a second truck roll when the string fails again next winter.

If this is a remote site, I’d honestly just swap the whole string. Saving a few hundred bucks on one block isn't worth a second truck roll if the imbalance kills the rest of the units next month. Also, check your terminal hardware for any wierd oxidation. Even without a heavy load, those connections can get gross when left sitting in the damp. Prioritize the internal resistance readings during the load test; that’ll tell you way more about the plate health than a simple voltage check.