Help on problem with my 48v solar battery bank

[Destroymix]

So I think I have a bad battery....

I have a 1000 watts on the roof with a 40a charge controller and 4 12v AGM batteries (they are solar UPS) linked together in series for 48v. When I fire up my inverter, everything is good, until i have to start something over 500 watts....The inverter kicks off with a low voltage code.

This is my second inverter (both wagan 3000W), same issue. I've changed cables and fuses, no effect. my charge controller confirms what my DVM says of 52-54volts input to the inverter and my charge controller reads the batteries as full and is in float mode.

When i tested the individual batteries while connected in series, i found that one battery, which I'll refer to as battery #2 was only reading 8v! Note, this was while connected in series. I took all my fuses out and disconnected all the batteries. Now #2 is reading 14v open circuit, just like all the others. I have a small 200w inverter and hooked that up to a known good battery, #3. it ran a 150w fan just fine. battery #2 when hooked up to the inverter came down to almost 6 volts when turned on and turned my fan about 1 inch

It sounds like I have a bad battery, HOWEVER, they are all under 1 year old, and have always been connected in the 48v series with a charge controller. all connecting cables are the same quality, length, and size.

My question is there any other way i can test this battery to confirm if its bad. Note: I have had it on a 12v trickle charge for approx. 2 days, but I have not retested yet

Any help or suggestions would be greatly appreciated. Thanks!

[family wagon]

With the batteries wired in series you could have equalization problems, ie the bank as a whole appears fully charged but some cells are under-charged while others are over-charging. This could be a case of that, and if so, then breaking it down as much as possible and charging each element individually is the thing to do. Sounds like you've already done this by separating the four batteries and charging them, or at least the suspect one, with that trickle charger. Do check on the charge specs from the battery manufacturer and verify that the trickle or whatever charger brings the voltage high enough, for long enough, to assure full charge. Some trickle chargers are junk.. Then load test.

The O'Reilly Auto stores around me have a battery tester, but it's a conductance type and probably won't be reliable for your batteries. The AutoZones here, and Harbor Freight, have a carbon pile load that simply draws a heavy current. That's the kind of test you'd want to do. It's similar to your 150W inverter but probably even higher current. While the tester draws power, and also after disconnecting the tester, check the battery voltage at the posts. If it drops too low then definitely at least one cell in the battery has failed. I'm not sure what number to call out as a threshold for "too low" however... Below 9.0, maybe? Perhaps it should stay even higher.

It sounds like you've done all you can to build a good bank. Manufacturing defects and "infant mortality" (early demise of a unit) can happen, and it sounds like you've got one.

[Destroymix]

Thanks for the advice! You're dead on, I do have a bad battery. It won't take a charge, when I set my charger on AGM the voltage just shoots up to 16volts and says charging complete, like all that's there is simply a surface charge.

I measured that battery while reconnected in series, it goes open on hard loads, it's definitely bad...sounds like I need 4 new batteries, ugh! Any advice and/leads, lol?!

[family wagon]

Sorry, I don't have any advice or leads for new batteries. I'm thinking real hard about building a bank from LiFePo prismatic cells, perhaps, but in all honesty it's probably a year away. If your other 3 batteries seem to be in good shape you might consider buying just one new AGM to go with them. Being a series string I think it'd get along alright, better than a parallel set of old and new would. IMHO the charge equalization things the LiFePo electric vehicle people (and others) do would also help extend the life of other battery types too, such as your AGM. Depending how comfortable you are seeing their idea, adapting it, and modifying some electronics to suit, that might be a good idea to help avoid over-discharging or over-charging your setup.

[dannewtobusland]

Heres the link to the separate thread! Think its in the right spot now?

http://www.skoolie.net/forums/f28/ca...tor-11525.html