Checking SOC of AGM batts under load

[Bobodude]

Normally checking SOC on batteries requires the batteries to be in open circuit status for no less than 4 hrs to disapate plate charges. My installation does not lean favorably to that criteria due to the fact I have a Xantrex Sw 4048 inverter that doesn't take kindly to removing the 48v bank from the inverter as it will loose all of it's internal programming. So I wanted to exercise the batteries that have been in service about 9 months. I have had 4 or 5 incedents of the grid going down for a couple of hrs each during that time. I separated from the grid starting out at 50.0 volts full charge and in 12 hrs of discharge, I ended up at 48.8 volts. Now what is the remaining capacity of the bank? As close as I can determine, I have 80% remaining.

Now heres the rub, the batt voltage was recorded under about a 9 amp load. There is only one website on the net that I could find, the Rolls Royce site, that has a chart, Fig #2, that shows SOC under load values. But these are for FLA not AGM. I have 4 Deka 8g's in series for a 48v bank. The are rated at 210 ah.

What do you off griders do to measure the remaining capacity of your batteries that you obviously cannot take out of srvice to read the voltages? Comments anyone?

[Sunking]

Very simple SOC voltages are pretty much useless on a working system. Only way to determine the real SOC is with a hydrometer. You have AGM so you are SOL. You can try this chart on page 32 of 40 but it is still a WAG, however it is at least a AGM chart. You were about 50% DOD at 12.1 volts.

[Bobodude]

Very simple SOC voltages are pretty much useless on a working system. Only way to determine the real SOC is with a hydrometer. You have AGM so you are SOL. You can try this chart on page 32 of 40 but it is still a WAG, however it is at least a AGM chart. You were about 50% DOD at 12.1 volts.

Yes, I would agree, the only real solution would be to open circuit the battery bank before and after the discharge. But, my monitoring software shows that at the time of reconnect to the grid, through the point of full recharge, I show a total of 52 ah returned to the bank. With a 210 ah bank, it looks like about 26 % discharge. If we look at the rate of drop of about 0.24 volts (12.44 to 12.20 per battery) that also equates to a 25% discharge of the bank. I think if I had taken OCV readings, my estimate would bare out.

[inetdog]

Very simple SOC voltages are pretty much useless on a working system. Only way to determine the real SOC is with a hydrometer. You have AGM so you are SOL. You can try this chart on page 32 of 40 but it is still a WAG, however it is at least a AGM chart. You were about 50% DOD at 12.1 volts.

Lots of people have published tables or graphs of SOC versus voltage at various fractions of C charge or discharge. But those were measured with a particular type of battery and so may not apply well to you.

But if you are willing and able to take some time, you can produce some similar data sets specifically for your battery bank. Start by measuring the voltage after 4 or more hours resting, then draw a known load current or enable a known charge current for a minute or two (till the voltage stabilizes.) Write down the voltage.
Let the battery rest some more, but do not try to measure a new reference voltage for that SOC unless you want to wait another 4 hours.
Keep taking data points enough that you can use the shape of the published curves and your data points to construct your own graph and show it proudly to your friends.

Repeat every few months as the batteries age.

[Bobodude]

I appreciate the post Inetdog. When switching to off grid, the SW4048 picks up a critical subpanel in the house that has all of my refrigeration and selected lighting load on it. Getting a constant load is near impractical so I think I will check the voltage the old fashion way by separating the battery bank from the system. Then on the next test I will check them after discharge to be absolutely sure what I have. I hope I can fake out the inverter with the 54v feed from the Outback controller with sun on the panels.