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Parralel bank with uneven SOC.

littlepea

New Member
Joined
Jan 6, 2025
Messages
3
Location
NJ
Hey people,

First time poster. I have lurked a while and watched Will's videos on YT which brought me here to try and diagnose an issue with my campers battery bank.

I recently (3 months ago) purchased 3 x LiTime 100ah BT lithium. I have wired them all in parallel with exact equal lengths of 4AWG and a 100a ANL fuse between the battery positive posts and a large 600a bus bar.

The positive bus bar also shares connection with my 3000w inverter (never uses high current draw, max application will be 1400w for a hot water heater however this is not currently running), main load connecting wire to my 12v fuse panel and Renogy 50a DCDC MPPT controller.
All negative terminals are wired similarly with exact same length 4AWG to a negative bus bar which shares the negative for the Renogy unit, the inverter and fuse panel.

For the first several weeks things were running fine. A few weeks ago however I noticed fairly different SOC's as the batteries were used (5-10% between batteries).
This last week or two it has gotten significantly worse. I currently have two batteries at 100% SOC and one battery at 41% SOC.

I have emailed LiTime customer support and they seem to think everything is normal as the very limited bluetooth app says balancing, cells and BMS are all operating normal.

Can anyone advise as to what to look at from here?

Have a wired this setup incorrectly? Is this a manufacturing/warranty issue? How may I test my bank to troubleshoot this further?

I thought I had wired this correctly and in the best possible way for equalisation of the bank and decided to use bus bars instead of simply going terminal to terminal from opposite directions.

If it matters, I have limited load on the batteries since installing, a 12v Engel fridge which only runs sporadically at this current time, around 5a of load going to heating pads to keep my batteries warm when temps get low, a diesel heater which draws 0.5-4a while running depending on heat setting and 4 LED 12v puck lights.

Any help or advice would be greatly appreciated!

I also hope I have posted this in the right area, apologies if I haven't!
 
I have a similar set up. The cells in each battery will have different resistance to charge and discharge inside the battery but the BMS will take care of this. The individual batteries will do the same but do not have a balancer to keep them in check. This means the batteries will not stay synced over time. This is where top balancing comes into play. Many articles on here telling you how to do this but basically you want to charge all of them up to 100% from time to time to keep them close to the same SOC. They will always wander off if you don't however. Nature of the beast and totally normal.
 
I have a similar set up. The cells in each battery will have different resistance to charge and discharge inside the battery but the BMS will take care of this. The individual batteries will do the same but do not have a balancer to keep them in check. This means the batteries will not stay synced over time. This is where top balancing comes into play. Many articles on here telling you how to do this but basically you want to charge all of them up to 100% from time to time to keep them close to the same SOC. They will always wander off if you don't however. Nature of the beast and totally normal.
Thank you for the response. I topped up all batteries to 100% SOC via DCDC and then shut off all load to allow the batteries to equalise without load applied I would say 4, maybe 5, nights ago. I should have mentioned this in my OP, sorry.

Would this change your assessment on what is happening? I can certainly try charge all your 100% SOC again and see how I go.

I had topped them up as I was concerned with the 5-10% difference happening, however it seems to have only gotten worse.
 
My camper batteries do the same thing and they were both built from the same batch of cells with the same make & models of BMS and all the wiring to within a couple mm length. Never figured out why they drain and fill at such different rates, but they do catch up with each other when the sun comes out fairly quickly.
 
I had two DIY LiFePO4 batteries in parallel, using equal length cables. They stayed within a couple SOC % of each other for four years.

I think you need to observe the problem battery through the app as it is being charged. Something is not working properly. A few percentage points of difference in SOC is normal. But not a 60% difference.

Check the voltage at the battery's terminals with a multimeter. Check the amps going through that battery's cables. Compare those to the other battery.

It could something as simple as you fat fingered the charge button in the app.
 
I had two DIY LiFePO4 batteries in parallel, using equal length cables. They stayed within a couple SOC % of each other for four years.

I think you need to observe the problem battery through the app as it is being charged. Something is not working properly. A few percentage points of difference in SOC is normal. But not a 60% difference.

Check the voltage at the battery's terminals with a multimeter. Check the amps going through that battery's cables. Compare those to the other battery.

It could something as simple as you fat fingered the charge button in the app.

Thanks, I will try and give them a test today.
The response I got from Litime was 'they are OK, run 50a through them once fully charged'.
I didn't have any 50a devices handy but did have a 1200w heat gun so I did that and each battery was drawing around 35-7a while it was on which seemed fine, now though I have my diesel heater running and two batteries have current drawing from them and one doesn't 🤦‍♂️
Maybe this is the price I pay for going cheap lithium!
 
What are your charge parameters? Charge to atleast 54.5v, and hold it for a few hours. That will give cells time to balance.
 
I have a similar set up. The cells in each battery will have different resistance to charge and discharge inside the battery but the BMS will take care of this. The individual batteries will do the same but do not have a balancer to keep them in check. This means the batteries will not stay synced over time. This is where top balancing comes into play. Many articles on here telling you how to do this but basically you want to charge all of them up to 100% from time to time to keep them close to the same SOC. They will always wander off if you don't however. Nature of the beast and totally normal.
if the batteries are in parallel ( if i read the OP's first post correct) , there shouldnt be a need for a balancer, but charging the batteries to full on a regular basis is....
at a higher voltage/SOC they should balance themselves out if they are in parallel
 
The battery SOC is a calculation done by the BMS, using data from imperfect sensors and integration over time.
Expect it (and ones of multiple batteries in parallel) to drift away from reality until a reset is done.

Usually a reset is done by the BMS from a full charge as detected by voltage. Some BMS' require some amount of time at full voltage (ie. a float period).
 
The built in bms's in my chins batteries are on crack. None of them ever are remotely close. I just ignore them. The cells are even between each other but all of the batteries read differently soc. I have had some of the bms's show 10% when all of the batteries are fully charged including the one showing 10%. Once the voltage hits that magic spot that it resets itself it goes to 100%.
 

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