Parallel Battery Setup Not Discharging Evenly – Need Advice

[Glen Bush]

Hello everyone,

I’m experiencing some issues with my parallel battery setup and could use some input. I have two LiTime 12V 100Ah LiFePO4 batteries connected in parallel with 4 AWG wires, spaced 18 inches apart. They’ve been in use for over two weeks now, and I’ve occasionally plugged my charger into shore power to bring them both up to a full charge.

Here’s what I’ve observed: once they reach full charge, one battery will enter "StandbyFull" mode, as shown in the LiTime app, and it won’t exit this state until I manually disconnect the other battery. This morning, for example, one battery was reading 46Ah while the other was still in "StandbyFull." It seems that they aren’t discharging evenly, which defeats the purpose of having them in parallel.

Could this issue be due to cabling resistance, or is there something specific to LiFePO4 battery chemistry in parallel setups that I need to account for? Any insights or recommendations would be greatly appreciated.

Thanks in advance for your help!

 

[sunshine_eggo]

Line #2 in my signature.

1. The 4awg wires are the same length?
2. Is 4awg wire adequate for the application?
3. You have verified definitively that both batteries can individually attain peak charge voltage (14.4V-ish) without engaging BMS protection?

 

[42OhmsPA]

How are they wired in parallel and to the inverter? Post a diagram or photo.

Welcome to the forum.

 

[wpns]

Pictures are really helpful, we might see something that not obvious to you.

 

[Glen Bush]

Yes, both 4 AWG wires are the same length, measuring about 18 inches between the batteries. I wanted to ensure they would share the load as evenly as possible, so I kept the cable lengths identical. Then I connected the negative to the negative terminal on one battery while the positive is connected to the other battery also with 4 AWG wires.

Based on my setup and the expected current draw, 4 AWG should be adequate, with a general amp rating around 85-95 amps for the short 18-inch runs in my system. This should minimize resistance and voltage drop. However, if anyone has insights on higher gauge requirements for similar setups, I’d appreciate the advice.

I have my Victron BSC IP22 12/30 charger absorption voltage set to 14.4; but it seems toward the end of the absorption the voltage bumps to 14.7 and trips both the BMSs.

it seems that neither battery does get to 14.4 without tripping bms.

I currently have no inverter installed, i am using only 12 volt at this time.

 

[GXMnow]

it seems that neither battery does get to 14.4 without tripping bms.

This seems to be a common issue. The cells in the batteries are not wll balanced so one of the cells is hitting the over voltage protection. Does the app show the cell voltages?

14.4 volts is very optimistic. That would be getting all 4 cells to 3.6 volts. That is well into the top knee, so even a small imbalance will cause a big problem. Se the absorb voltage lower, try just 13.8 volts and see if the batteries can get that high without the BMS tripping. With decent sun, the Victron charge controller should hold it at a steady 13.8 volts for a while and give the balancer some time to pull the cells closer. If it still trips at 13.8 volts, you may also need to set the charge current much lower to give the low balance current more time to do it's job.

As the cells top balance better, you can creep up the absorb voltage a bit at a time. But I doubt you will get them to 14.4 without a cell over voltage trip. The internal balancer is likely only around 100 milliamps. While you are charging, it only has that tiny balance current to keep the high cell from going too high and tripping the fault. You should be able to see this if the app can show you cell voltages.

Once you get both batteries to all balance p, they should share the current a little better. But it is not too big of a deal if they are off up to 20% or so. In a perfect world, the current drawn from the parallel batteries will end up tracking close to the battery capacity. My newer battery bank is consistently running about 10% more current than my older bank. The cells are all rated at the same capacity, but it is obvious the new set has more capacity and this shows as it always holds that higher current in both the charge and discharge direction. It just takes a bit more current to pull the higher capacity cells to the same voltage change.

 

[Glen Bush]

I’ve reset both BMS units and adjusted the AC and solar chargers to 13.8 volts to see if that helps trigger the cell balancing. I haven’t been drawing much 12-volt power yet, as I just put this system together and want to get everything working reliably before investing further. I’m thinking of this as a trial run to prove it’s worth the investment.

Currently, I can unplug the AC charger for about two days and rely on the single 250-watt panel. With minimal power usage, I’m seeing a loss of only 2 to 5 Ah daily, so it’s working well in that regard. However, this balancing issue has made me question whether the setup will meet my long-term goals.

My plan is to gradually increase the absorption voltage above 13.8 over the next few days to see if that stabilizes the balance. Unfortunately, the app doesn’t provide individual cell voltage readings, which is a drawback. All I get during balancing is a vague message: "Your battery is doing a balancing act." I wouldn’t recommend the app for anyone who needs detailed monitoring.

 

[Koyaanisqatsi]

My plan is to gradually increase the absorption voltage above 13.8 over the next few days to see if that stabilizes the balance. Unfortunately, the app doesn’t provide individual cell voltage readings, which is a drawback. All I get during balancing is a vague message: "Your battery is doing a balancing act." I wouldn’t recommend the app for anyone who needs detailed monitoring.

Pretty much what I'm doing on my new Epoch 460Ah, and for the same reason. But the 150mA balancing circuit is just ridiculous. After 5 hours of floating the voltage in the cell balancing range, it shaved off about 10mV of imbalance. Stupid. The balancing circuit needs to run at like 2 or 3 amps at least, especially on such a large battery. Right now, it's still over 100mV between the lowest and highest cells. When it was new, they were more than 150mV out.

I _think_ you're on the right track. But I haven't seen much success myself yet. Keeping fingers crossed that this is the right solution.