Batteries voltage test Ok, but problem under load

I’m seeing something weird I’ve never seen with batteries before. My batteries are fully charged and seem to be holding a full charge, actually a little over charge when all disconnected from each other and everything else. I’ve been noticing my inverter output shutting off and coming back on, no trouble codes and not all the time. I was in the power room today when it happened, the battery voltage drops suddenly from 54v down to 41v, I got low voltage alarms from the charge controllers and the inverter and the output shut down, then came back on over and over. The battery voltage keeps going from 56v down to 41v at the same time. If I switch my transfer switch to utility, the power is OK and the inverter shows 54-56v and seems normal, then if I wait a while and restart the inverter it seems to work OK for a while. The last day the online monitoring has shown overcharge and undercharge at the same time, but the power usage graph is flowing in a normal way up and down as things are being used, no spikes. The battery graph is showing 100% charge then a sudden discharge down to 10% charge then back to normal in a second or so. I would think it was the inverter, but the charge controllers are seeing the same thing. Any suggestions??

@the_uglydog it sounds a lot like a bad battery cable connection somewhere. It could be a terminal, a fuse or even a faulty crimp lug.
I was inspecting a system a few months ago that was randomly tripping off under load and it turned out to be a faulty battery cable lug. It had internal corrosion but you couldn’t see until I removed the heat-shrink. Are all your battery cables DC rated and sized correctly for the load?

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I actually found the problem, it was driving me crazy I checked everything, I thought… I’ve not seen this before. The batteries all tested OK, full voltage, held a charge just fine. I decided to do a load test on them, one of them had a dead short under load, but tests fine otherwise. I always thought if it had a short it would test low and not hold a charge. I switched it out with another used one and it works fine. Our Lithium batteries should be here in a couple of weeks so I’ll get rid of all the lead acid batteries.

Glad to hear you finally found the problem… Now that I think about it, I do recall something similar happening to me a long time ago. Sometimes lead-acid batteries may appear to be ok (based on voltage) but a load test is often the only way to tell if a battery is any good.

A battery load test is extremely important in order to run the batteries smoothly. When you planning to adopt solar energy for your home then you should plan the right solar system for home. Ideally, batteries should be tested in two different load conditions one under load and the other over the load.

Lead-acid batteries are a good option for small-medium scale storage applications.

Hi, My system is a bit large for the lead acid batteries. The best I’ve had only lasted 3 years, most 2. My new lithium batteries are in the container ready to leave so they should be here in PH in a couple of weeks. Those are 6, 200 ah 48 volt rack batteries, I should be able to run for 20 hrs with most things on and for 30 hrs if I am a bit conservative. I was lucky, the day after I ordered, the price went up by $300US/battery. I listened to you Svarky, thanks, instead of building my own I bought these premade with a 10 year warranty for only a little more than it would have cost to do it myself.