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diy solar

Half of my battery bank is not discharging

JackArcher

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Joined
Aug 10, 2022
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13
Hello all,
I have four 12 v 200AH lifepo4 batteries, they are linked in series to make 24v and then in parallel. I have them connected to two MPP inverters in parallel and then to their loads in the breaker box.
My problem is that when I run the system off the batteries, it runs fine, but it drains the right two batteries fully to zero, and then doesn't use the left two batteries. I have connected the load cables at different terminals to try to find out what the problem is, but I can't figure it out. Why does the system not draw equally from all four batteries? And, if I did have a problem, why would it not pull from the top left and right...vs just the right side?
If anyone could help me configure the connection of these batteries in a way that would equally drain all four batteries to zero, that would be great.
Thanks in advance.

fyi, the cables are 1/0 and each cable is of equal length with its corresponding match on the other terminal. My power consumption draw is less than 500 watts.IMG_0295.jpgIMG_0296.jpgIMG_0297.jpgIMG_0297.jpg
 
Did they all start evenly charged? If not, maybe the one pair is at a lower SOC than the other.

The way you have it wired, the top pair should discharge quicker than the bottom. But, there should be no difference left to right.

I'd disconnect all the wiring, parallel them up and fully charge them to balance the batteries.
 
Do you have any way to get data from the batteries? What are the cell voltages when the right set drain? What do the batteries do when charging?
 
That doesn't make sense since you have to be using left and right to get you the 24 volts...
 
That doesn't make sense since you have to be using left and right to get you the 24 volts...
Yep! it does not make sense to me either. May be measurement error.
The bottom left battery is connected in series with the bottom right battery, so that 24V bottom bank.
The top left battery is connected in series with the to right attery, so that 24V top bank.
I also cannot see how you can drain the batteries down to zero unless the BMS are not working at all.
 
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Do you have any way to get data from the batteries? What are the cell voltages when the right set drain? What do the batteries do when charging?
so, update from test tonight...I ran the system for around 6 hours.
1st problem.
I was watching the inverter software on my laptop that shows the specs of the system while it is running. My load was under 500 watts the entire time, usually between 300 and 400. Those batteries are new, and are rated at 2.5 kWh a piece. So with four, I have around 10 kWh. Running my system on that load, should be able to sustain with all four batteries around 20 hours. However the monitoring software advised I had around 46% remaining charge after only 6 hours. I don't understand why the batteries are losing charge so quickly.
2nd problem. Again the two of the batteries discharged all the way to 1% in this test, while the other two remained at 100%. I am using a trickle charge battery charger to both charge the batteries to 100% and to check their state of charge after the tests.
The order of the batteries confounded me as well, with this wiring set up, the top right battery remained full at 100% and has the positive load cable attached to it. The bottom right battery was drained all the way. The top left battery was also drained all the way and had the negative load cable attached. The bottom left battery was left at 100%.

I have no idea what is going on. Anyone have any ideas?
In answer to your question 400bird, they aren't solar rack batteries..so they dont have monitoring screens or anything. How would I get info from the batteries like you're saying? Im wondering if I got a bad battery somehow?

The batteries do have a BMS, and when the two got to that low of a charge, the whole system shut off.
 
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That's the thing, its not an issue of two that are working, and two that are not...I have changed the load wire location before to different batteries and ended up with different batteries draining. Like the first test I mentioned, it was both the top right and the bottom right that drained to empty and the left two were 100%.
 
Don't move the load wire. Just remove the working set. Or better just remove the jumper between them. That load wire connection could well be the issue.

OK as an alternative the fully charged battery should be close to 27 volts while the low battery is less than 25. Start poking around until you find the 2v difference. Fix, repair, tighten, clean or adjust that point of 2v difference.
 
I'll be the one to ask the potentially dumb question here.

It sounds like you are using information from the inverter to decide the state of charge of the batteries. How does the inverter know the state of charge of each battery that is in series?

I don't see voltage sense wires going from each battery back to the inverter, so the inverter only knows the overall voltage of the two batteries together. Am I wrong?
 
That's the thing, its not an issue of two that are working, and two that are not...I have changed the load wire location before to different batteries and ended up with different batteries draining. Like the first test I mentioned, it was both the top right and the bottom right that drained to empty and the left two were 100%.

So the batteries are ok? I'd say be careful when hooking up drained with full batteries in parallel, as I understand assuming all the batteries are ok, the full ones will dump (high) current to the drained ones.

Remove the batteries that are working. Fix the issue with the remaining pair.
I call this divide and conquer.

This should be your mantra.
 
So the batteries are ok? I'd say be careful when hooking up drained with full batteries in parallel, as I understand assuming all the batteries are ok, the full ones will dump (high) current to the drained ones.



This should be your mantra.
The batteries seem to be ok, but I don't know how to tell if they are part of the problem. I fully charge each of them with a charger before I reconnect them in parallel and series.

And as I posted earlier, I am using the charger to see exactly what the state of charge for each individual battery is. I disconnect all the wires, connect it to the terminals, and take the reading. I can see what the altogether capacity of the batteries is through the inverter.

And as far as the voltage, again, these are 12v batteries not 24. I have them linked in series which is why I mentioned 24v earlier.

Lastly, I have run only two batteries instead of the four, and it seems to work just fine. The problem comes when I link all four up. That's what Im trying to figure out.
Does it matter which battery the load positive and negative cables are connected to? Are the cables somehow in the wrong place?
 
Your batteries don't have a screen, sounds like they don't have a BMS with Bluetooth, so you have no real idea on the SOC of each battery?

You are relying entirely on a trickle charger (and maybe the inverter) for SOC?
I think that's the issue. Neither of those components are going to give you a reliable SOC.
Most likely the SOC reading is set up for a lead acid battery and not the super flat voltage of lifepo4. Hence the weird (garbage) SOC numbers.

How about we switch to looking at voltage?
What is the voltage of each battery when "full"?
What is the voltage of each battery when one pair dies?
 
Your batteries don't have a screen, sounds like they don't have a BMS with Bluetooth, so you have no real idea on the SOC of each battery?

You are relying entirely on a trickle charger (and maybe the inverter) for SOC?
I think that's the issue. Neither of those components are going to give you a reliable SOC.
Most likely the SOC reading is set up for a lead acid battery and not the super flat voltage of lifepo4. Hence the weird (garbage) SOC numbers.

How about we switch to looking at voltage?
What is the voltage of each battery when "full"?
What is the voltage of each battery when one pair dies?
Correct, the batteries don't have a screen so the only way to find the SOC is through the charger which designates the percentage of charge it is. And yes I believe it is tracking the batteries like it would a lead acid not a lifepo4.

Ok, checking the voltage of the battery, would I just use a multimeter, and touch +prong to +terminal and -prong to -terminal of each battery, while having the settings on DC voltage?
I've never checked the voltage of a battery before.
 
I would highly recommend a Victron Smartshunt to track the battery capacity. It will keep track of all the amps going in or out of the battery. It is very accurate (once setup correctly).

If you don’t need the temperature sensor - it can track the midpoint of the batteries.
 
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