Mystery 10 amp battery draw has me stumped

[OffGridCA]

Hi all,

I've lived off grid for about 5 1/2 years and our lead acid battery bank seems to be failing. We have a 24 volt system consisting of 12 2-volt LA batteries in a single string installed by a local solar company. It's our first experience with off grid solar electric systems and of course the learning curve has been and continues to be steep.

Here's the mystery I'd love to get some some feedback on: while troubleshooting our system it became clear that there is a consistent 10 amp draw on the battery bank even when the main panel is shut off. You would think when you "throw the mains" the amps will be 0 (at night) or more (during the day) as the batteries receive input from sun. But even shutting down the main AC panel, the PV input to the charge controllers, and the inverters (at the mate), there is STILL this mystery draw. Of course when these things are on there is more than 10 amps drawing down as the batteries meet the demands of our house load.

In addition to the steps above we also experimented with turning off the inverter circuit breakers, the inverters themselves, and the charge controllers. Even with all these turned off the trimetric shows the 10 amp draw. Originally our solar tech thought the trimetric was not calibrated correctly, but we've replaced that and it still shows the 10 amp draw.

The solar professional who installed the system is coming back to investigate further, but I also would like to hear feedback from this community since it seems to be such an unusual problem.

Many thanks for any ideas you might have, and apologies if I've left out key info.

[ButchDeal]

try shutting off the DC disconnect which should totally isolate the batteries from everything.
Possibly one of the cells is starting to go after 5.5 years.

[Mike90250]

I agree, 5.5 years on your learning set of batteries is pretty good. But the 10A would trouble me too.
When powering down everything isn't an option (do all the pieces of gear have non-volatile memory for their set-points and programming) having a clip on DC ampmeter is handy. Sears has a good one for about $70 or so, be sure you get the DC clip on version, Many AC clip on meters exist, DC are more rare. Very handy for off-grid systems to troubleshoot array issues, battery issues, and of course, stray losses.

[Sunking]

Pretty easy to find with a clamp-on amp meter. However I think you might be chasing a ghost. You said your batteries are 5 years old which means they are due for replacement, At that age I would be willing to bet at least 1 battery of your 12 has a shorted plate. As batteries age the wear from charging and discharging causes corrosion, part of the charging process. This over time will cause the plates to shed and flake off debris. If the battery jar is clear you can see the debris collecting in the bottom of the jar. As the debris sheds off the plates and falls, some may and will over time get wedged between the plates and/or grids causing a short circuit.

Disconnect the load cable from the batteries. If you still see the mystery current, it is your batteries. If the current goes away, trace it down with a meter. Current cannot flow in an open circuit.

Now what I am impressed with is your system is a very rare breed. Whoever installed it knew not to parallel batteries and knew what they were doing . Must be a very large system to use 2 volt batteries. So when you get new batteries, accept nothing else. If those are say 2000 AH make sure you go back in with 2000 AH batteries. Do not let anyone convince you to use say 500 AH batteries with 4 parallel strings to make 2000 AH.

Bottom line here is your batteries are in need of replacement. My guess is that will fix your problem. If there is a phantom load, your installer will find it before he allows new batteries to be put in service if brought to his attention. .

[OffGridCA]

Thank you all so much for your input! Our solar tech is coming out next week and I will share your responses. He will especially appreciate the props for his system.

I will look into getting that clamp-on meter. There is one particularly weak battery and he might disconnect it from the system and reprogram it all to be 22 volts. that might see us through the summer since it might take a while for the replacement batteries to arrive.

I'll also try shutting off the DC disconnect.

Again, can't thank you enough.