I think I have a battery issue. I have 2 x 250ah 12v AGM batteries in parallel ( LFP12250 ) and 480w array. Yes I know that I am ‘over batteried’ or ‘under arrayed’ but my theory was this. I actually use very little power under normal circumstances, 8 x 6w LED lights, 4W water pump, LED TV using 46W, and a few phone chargers etc. Maybe I’d draw 16A with everything going at once which would happen rarely and even if they were it wouldn’t be for long. On a big night perhaps I’d draw out 50ah from the battery or 10% according to the Trimetric. Usually that’s replaced by the solar even on an average day. So they don’t get a hard life.
These AGM batteries have a Bulk Charge recommend by the manufacturer of 14.8v, a float of 13.6 and when are 100% full sit at 12.8 to 12.9 at rest. Even by the next morning they are still at or around 12.6v to 12.7v. Yes I know they are not ‘Solar Batteries’ as such and are generally used for UPS but I get them cheap 2nd hand and are usually about 2-3 years old when I get them.
I’ve had them installed now for almost 4 years and have worked flawlessly and to be honest haven’t been watching them too closely lately, however this week when I came up on the Friday night (I’m usually up there 2 to 3 nights every week) I happened to switch the Trimetric over to Volts (From % , it was showing 100%) and noticed it was showing 12.6v, this was after about half an hour of being there and had a few lights on and TV on. I didn’t think too much of it but watched it over the next few hours and noted it dropped to 12.5, then, 12.4 and finally down to 12.3 by the end of the night. Under load 4a it would drop to 12.2v. Switching it back to % on the Trimetric show around 92%. I had never seen the voltage drop this low before. The only constant load is a 4w fan in the composting toilet and I’m fanatical about turning everything else off before i leave.
The next day was reasonably sunny and I put about 90ah back into the battery according to the CC. Now i could have sworn it was flashing as Charged Status on the CC and was sitting on 13.2v. The Trimetric was showing 103% charged, but now I’m not sure it was in Float or charged. I use very little power during the day, so the majority went into the battery. Anyway I watched the voltage again and noticed it dropped almost immediately to 12.8v under load, around 6a and half an hour later 12.7v. I watched it like the night before and it acted almost in the exact same manner, dropping to 12.2v again by the end of the night. Trimetric was showing 92% again.
At this staged i was beginning to think my batteries were stuffed, but I thought I’d give it a blast with the generator the next day. By the time I had to leave, I had pumped about 130ah into the battery via gen and solar at about 20a on the gen and 10 to 15a on the solar, so up to about 35a going in at any one time. This time i watched the CC and noticed it always stayed in Bulk and the battery only ever got to about 13.6v even with 35a going in. Bulk is set via CC at 14.8v and Float 13.6v. The % on the Trimetric was showing 105% I think. I reset it back to 100. It now has 3 to 4 days to recharge via the solar before i get back to check. Even on a bad to average day it puts back 15 to 30ah and the fan uses maybe 6ah.
Anyone want to hazard a guess as to what’s happening here? Have I somehow discharged my batteries badly and the Trimetric hasn’t picked it up? Even though my solar array is about half the desired or recommended size for my battery, i use only about 10% of its capacity a night/day and it usually is fully charged every day. Occasionally after a series of really cloudy days its only ever been down to 79% according to the Trimetric and sitting at 12.5v. I’ve never been concerned about it and by the time i get back after 3-4 days its behaving as normal.
Is it possible that it has developed a surface charge that is fooling the CC into thinking it’s charged because of insufficient charging current over time? We’re just into winter over here now and there hasn’t been much sun. If its had a low but constant charging current of only 4 or 5a per hour for the last few weeks with only the occasional sunny day could this happen?
Secondly are my batteries junk or will a few days of heavy gen charging cure things eventually?
Obviously I’ll see how things are next time I’m up there and report back. In the meantime any obvious ideas?
Off Grid shack - Victoria Australia. 480W array, 500Ah AGM at 12V. 30A PWM Manison CC. Trimetric 2030. 300W Pure Sine Inverter. 120lt Dometic Gas Fridge. Composting Toilet. 5000lt water tank with 12v 35psi pump. Bosch Hydropower 16 for nice hot water. 4kw Fuji Micro Generator (dead after 7 years) 5kva Subaru Generator.