Battery puzzle ...

[Slm14]

Hi everyone, new here and have been reading so much! What a great forum.
We have an off grid solar system comprising 24x270w panels, 8kw hybrid inverter and 16x12v 200ah batteries. Batteries are connected as 4 sets of 4 batteries in series to create 4x 48v 200ah, 800ah total. Average production is 6kw a day and average consumption is 3kw per day. The system has been running OK for 2 years but now we are showing very low voltage early in the morning before the sun comes up. The load is only about 200 or 300w per hour overnight. State of charge is never lower than 92% but voltage is low. This could have been gradually getting worse over time and we have only just noticed.

I have tested the batteries and 1 battery within each 48v bank were showing 10.5v and all the others were 12.8 to 13v, so 4 bad in total. The low ones were only one in each string. So I removed the bad and set up the system to have 3 sets of 4. Same problem overnight with low voltage so I tested again and there was again one low in each new set.. So I tested the first 4 low ones I checked and removed and they were all testing good again! I put them back in and the next night tested again. Once again, one low battery in each set but completely different batteries.

I have now cleaned all the connectors and tried again, same 4 showing low but I know if I move them around they will show normal voltage in different positions. It's driving me crazy. I only have a multi meter at present and we live off grid on a remote island in Fiji with no access to real shops or amazon! Has anyone got any suggestions before I replace the whole damn lot?

[Mike90250]

Batteries with voltage that recovers quickly, are either brand new, or sulfated.

Voltage testing can only accurately be done with the batteries idle for several hours after charging or discharging

If you get a hydrometer, you can accurately check the state of charge by sampling the electrolyte.

How many Absorption hours do the batteries get daily ?

Do you top off electrolyte with distilled water or tap water ?

> Average production is 6kw a day and average consumption is 3kw per day.
Whoa - that seems pretty unreasonable, that's worse than my NiFe bank. Are you having to add distilled water weekly ?

[Slm14]

Sorry i should have specified they gel acid batteries

[SunEagle]

More than likely some of those batteries have lost their life wired in a series / parallel configuration.

Also most gel batteries do not have a high cycle count so will die off after a couple of years especially if they are drained below 30% SOC.

If you really need a large Ah rated bank then going with low voltage (2, 4 or 6V) high Ah rated batteries wired all in series is a better bet.

[Mike90250]

Reading the latest - I think the batteries are toast. And unless you change something - add more solar, adjust the charger setpoints to what the battery mfg demands, the next set will last you about 2 years too.

I think you are at the danger point with the gel batteries, as they age, their efficiency drops and they consume more amps to recharge, causing heat, causing shorter life, causing more heat. eventually the vents pop open to relieve pressure and you have Hydrogen and Oxygen in your well vented battery box

New flooded lead acid batteries need about 20% more amps to recharge then was consumed. New AGM batteries only need 5% more than was consumed
No idea what gel battery ratio is, since gel is not really suitable for solar, it can't recharge fast enough to not get sulfated.
you state your set takes 100% more to recharge ( 3kwh used, 6kwh to recharge ) That's bad.


Suggestions.
Get AGM batteries, not gel.
Deep Cycle batteries, nothing with a CCA spec. Starting batteries can't take deep cycles
Confirm/Adjust the charger settings per the battery spec sheet,,