Why Don't Parallel Battery Banks Balance?

Basspig · June 2022

I have three 16S LifePo4 battery banks with JK BMS's tied together in parallel.
They've been sharing the load and charging via solar for four days as of now, but bank 3 (at 65%) gets the least charge current during the day and sees the most load current at night. The other banks, with slightly higher voltage and higher states of charge, receive the majority of charge current and at night, a minority of the load. In other words, at night, bank 3 is the first to show negative current flow (discharge)
All three are connected in parallel with the same type and length of 2/0 AWG cables to the buss bar where the inverter takes its power.
I even tried charging bank 3 separately to 100%, but in a matter of hours it became the lowest of the three banks in terms of charge remaining.
I thought that parallel connected batteries were supposed to all equalize to the same voltage. What is going on here that I don't understand?

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BB. · June 2022

When charging (and disharging) at "high current", use a good quality DMM and measure the voltage at each battery termainal.

If you see a difference between the 3 battery terminal voltages, then there appears to be an issue with the wiring from the bus bar to the batteries... Bus bars do have voltage drop, but usually there is a lot more copper (lower resistance) in bus bars than the bar to battery wiring.

For charging... Connect the charging source (solar array, charge controller, etc.) "diagonally" to the bus bars. That will be a better "balanced" wiring scheme. Doing it this way, if there is bus bar voltage drop, it will be the same from Charging Source (and loads) to each battery.

If, for example, you have the charging source both connected to the "same ends" of the bus bars--Then you do have cumaltive bus resistance from Battery 1 to Battery 3.

http://www.smartgauge.co.uk/batt_con.html

With the bus bars and diagonal connections... You can put charging and loads on the "same diagonal" or the charger on on one "kitty corner" diagonal and the loads on the other diagonal.

-Bill

Basspig · June 2022

I've got equal lengths of 2/0 awg cable from each battery bank to oversized copper buss bars (10" wide, 1/2" thick, 1-1/4" wide) and the inverter load terminal is in the center of each buss bar, with the batteries connecting to points an inch apart on the buss, on either side of the load. The charging current and load currents are pretty low as I'm not using any large appliances when I'm observing this anomaly.

Is it possible that Bank 3 has the lowest resistance from battery to buss bar? Or is it the other way around?

This is how each battery bank is wired. Buss bars connecting each group of cells. Then at the bottom, the pos is connected to 2/0 cable and neg are connected to BMS and 2/0 cable and those cables go to the buss bars. I do have 200A breakers in each of the pos cable connections so there's a jump between the pos cable and the buss from each battery bank.

The wire at the far left comes from solar array. Then the next in sequence are: bank 1 batt, inverter, bank 2 batt, bank 3 batt, meter power. Hard to imagine that 1/2" of buss bar to bank 3 would have much of an effect.

BTW, I'm using method 3 from your article.

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littleharbor2 · June 2022

Where the 3 + or - connections are bolted to the buss bar, side by side it looks like the contact surface of the center lug is floating above the buss bar due to the left and right lugs being too wide to allow all 3 to lay flat. Is this the case?

mahendra · June 2022

Just a point to note .
Had the same issue with four overkill solar bms packs in parallel.
If the jk has a reset soc button try that then see what happens.

Basspig · June 2022

The connector that's on top (the second big lug from the left) is going to the inverter, not the batteries.

Basspig · June 2022

mahendra said:

Just a point to note .
Had the same issue with four overkill solar bms packs in parallel.
If the jk has a reset soc button try that then see what happens.

Trouble is, now that the system's installed, I can't access the BMS modules--they're in the back of the cabinet, due to cable length limitations.

Wheelman55 · June 2022

How about experimenting by moving the inverter cables over so that they are between batt 2 and 3. Then see how the battery banks charge.

Would the lugs lay flat get if you change the order to:
- batt 1
- array
- inverter
- batt 2
- meter
- batt 3

Dave Angelini · June 2022

Well if you can't access them you need to fix that!
I take it that the 3 BMS are not networked and that is also something you need to address. You have 3 open loop Batts that each need their own set-points, if I am reading this right. Closed loop is the way to fix this, if it can be done.

Basspig · June 2022

This inverter is a temporary unit until the 18kW unit arrives, so I didn't bother to file the edges of the terminal to make it sit flat against the bussbar. I plan to do that later, during the next shutdown.

Unfortunately, the design of the Treeline Systems battery cabinet makes maintenance impossible. If were to think this through again, I would have used some time of open metal shelving and mounted the batteries sideways with the terminals facing out toward me. Right now the only way to access the BMS units is to disconnect everything, move the 700lb cabinet away from the wall and gain access.

There's only a smartphone app which I monitor and control them with. There is CANBUS and GPS ports, but I don't know what their functions are.

mike95490 · June 2022

NO NO !
You cannot expect current to flow evenly when the lugs are not flat on the bus bar

mike95490 · June 2022

Basspig said:

This inverter is a temporary unit until the 18kW unit arrives, so I didn't bother to file the edges of the terminal to make it sit flat against the bussbar. I plan to do that later, during the next shutdown.

Modifying lugs "to fit" is the wrong way to solve this. you will still have unbalanced connections ( a smaller lug with less contact )
Larger buss bar is the proper way to go, and the lugs and bar should be factory tinned to reduce corrosion over the years

Basspig · June 2022

Yeah, I understand that's not a good way to connect. I just need to shave off 1mm on each side of lug to get it to fit between the other lugs.
What I did this morning was move the pos inverter cable to the opposite side of the bar (unused hole at right) and leave the neg on the left on its buss bar. That makes a more balanced current flow.

Last night, I had bank 3 down to 20%, bank 2 was 30% and bank 1 was 32% at 1am. Going down about 1% per 30 minutes. Figured that would be more than enough to last til morning sun, but wife woke me at 4am to tell me there was no power. Went down and checked and all three BMS report 0%! So the last 20% is not linear in terms of time. It's like 1% of run time left.

So I left the inverter off and switched to grid power. This morning with no load, I see all three batteries are at 11%, so it is probably that funky lug setup causing the imbalance.

BB. · June 2022

You can set a DMM for 2.0 or even 0.2 volts full scale and (for example) with high loads or charging current and measure the voltage drop for each section of electrical path such as bus bar to terminal on battery and see if there is a high drop somewhere.

Also if you have a current clamp meter, measure the current to each string and confirm proper current sharing or not.

Remember v=I*r. You can measure resistance by

R=V/I= measured voltage drop / measured current

Bill

Basspig · June 2022

That's a good idea. I think I need to treat this like I would and RF VHF circuit where even copper ground trace has impedance. I will measure voltage differences between buss bar and each connector terminal and then terminal to terminal and that will probably reveal some differences.

Basspig · June 2022

Today, I wanted to let the batteries recover. Mostly sunny today with some clouds. Three 14kWh banks. After a full day of charging with the inverter off, just power coming in from solar, we're almost halfway charged. There is still some differences in charge level between these parallel connected batteries. But as you can see, we stored about 20kWh of energy today. Obviously, I need a lot more than this.

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Basspig · June 2022

Today, I'm charging the batteries with the inverter off, no load.
The thing that doesn't make sense to me is why does the battery with 95% charge level take 25 amps from the batteries with 93% charge level in the other two banks? If it has a higher charge, it should be sending current to the lower charge batteries, not the other way around.

Basspig · June 2022

Today, on the second charge cycle, I'm seeing MUCH better balance between banks. Most of the day, all three were within 1-2% of each other. I'm starting to think this was a calibration issue and the BMS's took time to "learn" the system.

Why Don't Parallel Battery Banks Balance?

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