12V 200Ah wired 4s3p for 48V 600Ah
Cycling 25% DoD would be 150 Ah, 7.5 kWh
The AGM brand I use is supposed to give a bit over 2000 cycles to 25% DoD. Some brands might be half that, some FLA much longer life.
You said the loss of capacity was observed a few months after lightning strike. Previously, had you observed the battery still had full capacity by occasionally cycling it deeply? Or did you measure rest voltage after having drawn 7.5 kWh overnight, and observed that it still looked like 75% SoC? Because AGM, you couldn't measure SG like you would for FLA.
If you weren't aware while it was happening, could be capacity was continually degrading. Initially 600 Ah so your 150 Ah draw was just 25%, but later 50% and 80%, eventually drained at 4:00 AM.
Once capacity dropped to the point where cycling was deep, they would degrade more rapidly. Once a cell was imbalanced, it would remain undercharged and degrade faster.
Some AGM vendors do recommend equalization when capacity loss is observed. I have some automotive AGM which had low resting voltage and this process brought it up a bit. Not sure if they actually equalized, or if the batteries just weren't being properly charged by alternator or battery charger.
Alternatively, could lightning have affected charge controller? Have you confirmed that charge voltages are correct? If it stopped charging to the correct absorption voltage, then battery wouldn't be maintained as well.
4x TS MPPT 60
If any under-charged the battery, others should still deliver full charge. If any overcharged the battery it would be overcharged.
TriStar MPPT, Morningstar Corporation, Solar, Charge Controller, Maximum Power Point Tracking
www.morningstarcorp.com
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Some charge controllers are more sophisticated than others. Some work by voltage alone, while others track amp-hours as well. Certainly the settings (volts, absorption time) need to be correct for the batteries. Default VRLA settings of my inverter/charger were close, but I adjusted them slightly for what battery vendor recommends.
Batteries were $1200 (each?, 200 Ah at 12V)
What brand/model? Link to data sheet?
My SunXtender are 400 Ah at 6V, so same capacity. L16 size, 120 lbs each. They cost $600 each. I have 8 of them, 8s for 48v.
When you switch to lithium, you'll no longer have the problem that less than full charge causes degradation. So long as you set max/min charge correctly, SoC won't harm the battery.
What temperature will the batteries live at? If LiFePO4 is kept at moderate temperatures, it can accept full charge current.
How big a back are you putting in? Likely only half as big as the AGM or less, since they can be cycled more deeply.
Maybe 16, 208 Ah cells for 13,400 Wh?
You've got 40 x 200 = 8000W PV panels.
That was about 0.25C for your AGM. Hopefully within recommended charge rate, which does vary by brand/type.
That would be around 0.5C of 13,400 Wh LiFePO4 bank, which should be OK at room temperature.
If battery gets close to zero degrees C but not quite to low-temperature disconnect, acceptable charge current is probably around 0.1C, 1340W, 28 A. If you exceed that while LiFePO4 is very cold, would shorten life.
Probably easiest way to avoid that is keep battery in a warm location, and set low-temperature (charging) disconnect at a temperature where 0.5C is acceptable.