How to know which battery is bad?

[CostaRicaG]

Newer to Solar. My system is 48 volt. 8 batteries in 2 strings of 4x12. AGM 150AH.
I charge up to 100% and as soon as PV off, they drop within 30seconds to about 75% charged.
I separated the two strings, and same results, so assuming that 1of 4 on each string is bad.
Voltage and amps on each battery, all 8 test normally.
I have a load battery tester and hoped it would be very clear in finding the issue.
Under load test, 6 batteries do test at 11.0-11.2 on the tester screen above. The other 2, one from each string, do test a little bit lower at 10.8 on this screen. Not a big difference.
Would you consider this proof of the bad batteries?
They test so close. No hot terminals. Any other wisdom to share?

 

[sunshine_eggo]

First, all 12V should be individually charged to 14.1-14.4V and held there for four hours.

Second, after they are placed in series, you likely need to ignore the device reporting SoC. It is voltage based and is useless in an operating system.

The only way to accurately measure state of charge is with a battery monitor that counts current in and out of the battery and comparing it to the programmed total, 300Ah in your case.

The top three are viable:

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

 

[seneysolar]

You likely dont have a problem at all as sunshine_eggo mentions. Voltage on your batteries will drop when any load is placed on them. If you turn the inverter off, youll likely see it rise again. Resting voltage vs charging voltage vs load voltage are all going to be different.

Those old school automotive Load Testers aren't great for agm, as they aren't designed for cranking power, youll get marginal results.

X2 vote for a shunt based battery monitor

 

[CostaRicaG]

Thanks.
Update as I swapped out 2 of the batteries and put 4 in a string that all test "ok". But same results. I swapped out first one, then another.
My inverter shows batteries at 75%, turn on PV and within a minute are showing 100%. Turn off house load and staying at 100%. Turn off PV input and they drop within a minute to 75%.
The Inverter company Engineer advised it was battery failure on both strings
But with good batteries (I keep changing them up), results are the same. 100 down to 75% within a minute.

 

[CostaRicaG]

 

[CostaRicaG]

PV on...30 secs later when off

 

[sunshine_eggo]

Thanks.
Update as I swapped out 2 of the batteries and put 4 in a string that all test "ok". But same results. I swapped out first one, then another.
My inverter shows batteries at 75%, turn on PV and within a minute are showing 100%. Turn off house load and staying at 100%. Turn off PV input and they drop within a minute to 75%.
The Inverter company Engineer advised it was battery failure on both strings
But with good batteries (I keep changing them up), results are the same. 100 down to 75% within a minute.

I'm guessing you missed this:

Second, after they are placed in series, you likely need to ignore the device reporting SoC. It is voltage based and is useless in an operating system.

You are chasing a likely non-issue using a piece of information that is completely worthless.

 

[sunshine_eggo]

Try to think of this logically.

You KNOW you have batteries rated for 300Ah of capacity.

For you to lose 25% of your capacity in 60 seconds, that means you would need to discharge 25% * 300Ah = 75Ah in 60 seconds. 75Ah * 48V = 3600Wh

So, that's 3600Wh of energy consumed in 60 seconds.

60 seconds is 1/60th of an hour, thus that means you've sustained 3600Wh / (60 sec/3600 sec/hr) = 216,000W for 60 seconds.

Does it seem possible that you are discharging 216,000W?

Does it seem possible that you are charging with 216,000W when it goes back to 100% in 60 seconds?

That's what the gauge is telling you... you are discharging and charging at 216,000W.

Have I convinced you to NEVER use that gauge for anything at all?

 

[time2roll]

Thanks.
Update as I swapped out 2 of the batteries and put 4 in a string that all test "ok". But same results. I swapped out first one, then another.
My inverter shows batteries at 75%, turn on PV and within a minute are showing 100%. Turn off house load and staying at 100%. Turn off PV input and they drop within a minute to 75%.
The Inverter company Engineer advised it was battery failure on both strings
But with good batteries (I keep changing them up), results are the same. 100 down to 75% within a minute.

Inverter is a very poor method to measure capacity. Especially with a large load.

I agree with charge them all to 14.4 volts individually or in parallel and hold for several hours. Remove and separate the batteries and see if they hold 13+ volts standing disconnected 48 hours. If they all measure similar they are likely all good. Possibly all with reduced capacity.

To really know the state of charge a battery monitor with a shunt needs to be added.