How to Parallel Balancing. (YEP 99% of us is doing it wrong)(PART#1)

[Fred]

I decided to bottom balance since my operating range at the top end is limited by my midnite classics. I can set eq voltage, absorb voltage and float voltage. I set them at 56.8 (EQ is also disabled), 56.8, and 40. When the solar charging pushes the bank voltage to 56.8 I am done but my SOC (based on a coulomb counter indicates about 80%. I will be discharging down to 10% or so. Yesterday I let my bank discharge while monitoring all cells. I stopped the discharge when the lowest cell was at 2.7v (these are new cells and I had not balanced them yet). The difference between the highest and lowest cell was .25 volts when at the low SOC. (At 50% SOC it was only .01). I took the whole bank apart and connected all 64 100 ah cells in parallel for 18 hours. After that I checked all the cell voltages and they all read exactly 3.010. I reinstalled them and monitored all cell voltages as I reconnected the load. They were within .01v under load and as charging with the genset was started. Maybe not perfect but plenty good enough as far as I can tell.
I will say that being able to log individual cell voltages during discharge and charge and see them on a graph is really helpful in understanding cell charge curves and the degree of cell balance you have. The breaks in the graph happened when the load changed (Heat gun switched off, on) or monitoring was paused for a bit.


discharge curve, before balancing :


A short discharge curve was made from cell data collected right after the pack was reinstalled in my system, before charging, under load:

 

[MisterSandals]

being able to log individual cell voltages during discharge and charge and see them on a graph

Very cool. How are these being logged?

 

[Fred]

I have an Orion Jr 2 BMS. I can connect a laptop and monitor/log cell parameters and program BMS settings such as LVD, HVD, high current disconnect, voltage level to begin balance, temperature shut off, state of charge drift points, etc. It has pros and cons but the data logging is the reason I chose it over the Chargery BMS.

 

[nebster]

I decided to bottom balance since my operating range at the top end is limited by my midnite classics. I can set eq voltage, absorb voltage and float voltage. I set them at 56.8 (EQ is also disabled), 56.8, and 40. When the solar charging pushes the bank voltage to 56.8 I am done but my SOC (based on a coulomb counter indicates about 80%.

That doesn't sound quite right. Your counter should be at 100% when your bank voltage is 56.8. (What was it at when you started, and what was the voltage then?) Have you configured it to sync to 100% at a specific threshold voltage and current?

Most BMSes are going to want to balance at the top of the charge curve. Your weakest cell is going to be roughly 0.25V higher than the strongest cell when you reach the absorb voltage (just like it was 0.25V lower at the bottom, before).

Are you going to disable the balancing feature on your BMS somehow?

If no, it is going to "undo" your bottom balance over time. You will end up with a top-balanced pack. (Which would probably work out better given that high stopping voltage.)

If yes, you are going to be stressing that weak cell at the top of the charge.

You have good datalogging capability and are clearly being diligent about setting up your pack. If you can, try setting your absorb threshold lower as a starting point, use your counter to measure the total energy put in, and use your cell-level voltage graphs to verify that you see the opposite behavior now with the spread forming at the top.

 

[Fred]

Good questions. I too expected to be at a higher SOC when the voltage reached the absorb voltage setting. Once charging stopped, the measured voltage dropped to around 54 something. I need to do a close monitoring during charge now to see better what is happening on the high end.
I would probably disable the balancing feature but need a bit more testing.
Once I log data for a solar charge up to 56.8 volts. I may disconnect the bank and let the voltage stabilize to get a better idea of SOC. I think the shape of the curve will help determine that too.
I will post my data logs once I do that. It may be that my SOC is higher than previously thought when solar charging hits the absorb setpoint.

 

[nebster]

Good questions. I too expected to be at a higher SOC when the voltage reached the absorb voltage setting. Once charging stopped, the measured voltage dropped to around 54 something.

Yeah, that sounds like basically 100%. You need to calibrate your counter so that it resets itself to 100% at that point (or do it manually, at least, for now).

I would probably disable the balancing feature but need a bit more testing.

If you've no choice but to charge that high, I think you will get better longevity with top balancing. (I personally bottom balance. But then I charge to a much lower voltage.)

Once I log data for a solar charge up to 56.8 volts. I may disconnect the bank and let the voltage stabilize to get a better idea of SOC. I think the shape of the curve will help determine that too.

The best way is to put a small, known load (0.05C is good) on the pack to force the voltage down to discharge voltage. Then you can use any reliable discharge curve as a lookup into SOC, because all healthy cells have roughly the same one.

 

[orangeminnie]

Personally, I won't charge below 2 degrees celsius. If it's that cold, how much sunshine is actually available? I'd rather stay on the safe side. But yeah, it sure is interesting to imagine pushing the limits of this chemistry. So cool.

Will: There can be a lot of sun at locations well below freezing. Many of them are in your state of residence.