SOK 24V Battery Bank only allowing 50% DOD

DM23322

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I'm a newbie here and new with solar design and setup. Any insight or guidance the group can provide is very much appreciated...

I installed a system in my RV and all has gone well except for an apparent battery limit to 50%-55% DOD, but I am still in the testing phase. My current setup: 8 SOK 12V LifePo4 batteries wired in series for 24V. I have a battery balancer for each pair of batteries. Using two Victron Multiplus 24V 3000 inverters and ten 215W panels on the roof of my RV. At this point, I have only tested the system without the panels as I am testing how long I can run the AC with a soft start installed from the battery bank without PV input.

I seem to have hit a major DOD discharge roadblock and I can't figure out why. I can only discharge to about 50%-55% before the voltage plummets and shuts the system down after about 9 to 10 hours of continuous run time. When the voltage plummets it still shows about 6 hours of run time left. My midpoint overs around 0.1-0.2 during normal discharge and then to 0.4 during the end of discharge. The load that I'm pulling while performing this test is about 42A and 1050W for the entire 9 to 10 hour period.

When I set up the system, I fully charged all the batteries and then let them settle for about 3 days so they were all at 13.5 before connecting them. At rest the midpoint is .0 or 0.1. The only time midpoint skyrockets is when recharging hits close to 98-100% then its very hard to control and will hit 3-6%.

What might be the cause of my limit to 50% DOD? Am I expecting too much? How should I proceed?

Thanks!

The most relevant Victron Settings are as follows:
Charger:
Charge current: 70A
Absorption voltage: 28.8
Float voltage: 27.20
Repeated absorption time: 3.00h
Absorption time: 3h

Inverter:
DC input low shutdown: 23.00V
DC input low restart: 24V
DC input low pre alarm 24V

Screenshot 2021-12-04 165012.png
AScreenshot_20211203-212021.jpg
 

FilterGuy

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My *Guess* is that you have 100AH SOK batteries. Is that correct?

With 8 of these set up for 24V, the total Ah would be 4 x 100 = 400Ah (Each series string of two only provides 100Ah and you have 4 parallel strings)

The load that I'm pulling while performing this test is about 42A and 1050W for the entire 9 to 10 hour period.
42A * 9.5H = 399Ah. That is exactly what I would expect with a 2S4P bank of 100Ah batteries.


It sounds to me like you have the Amp Hours set wrong in your SOC meter. Did you set it to 800Ah? (8 x 100)?

Edit: Added the following.
If the batteries are 100Ah, the SOC meter should be set to 400Ah
 

DM23322

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Thanks for the reply...

Yes, that is correct... I have 8 - SOK 100AH batteries in series. My battery capacity is set to 800AH in the Victron shunt settings.

So, if I understand correctly, I should set the Battery Capacity to 400? So, each series pair of a 100ah LifePo4 battery only provides 100ah? I didn't know that... Thanks for clearing that up!

BScreenshot_20211204-174225.jpg
 

FilterGuy

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Batteries in series.... the voltage adds and the amp-hours stay the same
Batteries in parallel... the voltage stays the same and the amp-hours add
 

rmaddy

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I have 8 - SOK 100AH batteries in series. My battery capacity is set to 800AH
Actually you have your batteries in 2S4P. That gives you 24V 400Ah, not 24V 800Ah. Update the Shunt battery settings to tell it your capacity is 400Ah.
 
Last edited:

DM23322

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Okay, that's what I just did. Here's a pic of my setup... I know, its a bit messy, but my its my first install.

Bank20211204_151412.jpg
 

DM23322

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Would there be any issues with me adding 2 more SOK 100AH LifePo4 batteries onto this string with the same wiring configuration?
 

rmaddy

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So you want 2S5P for 24V 500Ah? SOK doesn't show any max number of batteries in parallel but personally if I were to put 5 in parallel I would not use that wiring setup. I would add proper battery bus bars (separate from the main positive and negative bus bars that you already have) and wire the parallel setup that way. You will likely get better performance from the batteries than using the simple setup you have now.
 

740GLE

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No issues. You’d then only have 600ah if two more batteries are added, def help run time, but then do you have enough solar to charge the added capacity? You don’t want to struggle getting the batteries full.
 

DM23322

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So you want 2S5P for 24V 500Ah? SOK doesn't show any max number of batteries in parallel but personally if I were to put 5 in parallel I would not use that wiring setup. I would add proper battery bus bars (separate from the main positive and negative bus bars that you already have) and wire the parallel setup that way. You will likely get better performance from the batteries than using the simple setup you have now.
Yes. A total of 10 batteries rather than the 8 I have now. Regarding the bus bar suggestion... So, in the same wiring configuration but replacing the cables with bus bars?
 

rmaddy

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Yes. A total of 10 batteries rather than the 8 I have now. Regarding the bus bar suggestion... So, in the same wiring configuration but replacing the cables with bus bars?
Not quite. Please see:


and look at chapter 3. It shows various ways to connect batteries in parallel. I think with 5P you would want the battery bus bar solution. Right now you have the diagonal solution.
 

DM23322

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Not quite. Please see:


and look at chapter 3. It shows various ways to connect batteries in parallel. I think with 5P you would want the battery bus bar solution. Right now you have the diagonal solution.
Thanks. I'll take a look.
 

FilterGuy

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Would there be any issues with me adding 2 more SOK 100AH LifePo4 batteries onto this string with the same wiring configuration?
No there are no significant new issues.

However, the way you have these paralleled could be better.

Continuing with what you have would look like this:

1638744524357.png
Pulling the positive and negative off of the diagonal main + and Main Neg helps with balance but for every string after 2, the load/charge balance is a little worse.

1638745014286.png
 

FilterGuy

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BTW: the money page in the Wiring Unlimited e-book is this:

1638745131328.png
You are using 2 batteries in series for each of the single batteries in the diagrams above.


  • Your current system is 'Diagonally'. It works but is not great.
  • I am recommending 'Posts' with really short, high current bus bars.
  • 'Halfway' does not work with 5 strings
  • 'Busbars' is about the same as 'Diagonally' unless the busbars are *very* heavy duty.
 

DM23322

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No there are no significant new issues.

However, the way you have these paralleled could be better.

Continuing with what you have would look like this:

View attachment 74693
Pulling the positive and negative off of the diagonal main + and Main Neg helps with balance but for every string after 2, the load/charge balance is a little worse.

View attachment 74696
Very helpful. I really appreciate the guidance. This gives me something to consider for performance improvements. I may need to add telephone poles since I'm limited on wall space (just kidding of course)
 

DM23322

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BTW: the money page in the Wiring Unlimited e-book is this:

View attachment 74697
You are using 2 batteries in series for each of the single batteries in the diagrams above.


  • Your current system is 'Diagonally'. It works but is not great.
  • I am recommending 'Posts' with really short, high current bus bars.
  • 'Halfway' does not work with 5 strings
  • 'Busbars' is about the same as 'Diagonally' unless the busbars are *very* heavy duty.
Awesome. Thanks again!
 

Rocketman

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It is expensive but look at the Victron lynx distributor or power in. That it a very nice bus bar that can help you cleanup the wiring.
 
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