Troubleshooting Charging/Capacity - Overkill BMS + Growatt AIO

[gregcron]

A few questions regarding my setup, which includes 2*280ah 24v batteries in parallel with Overkill BMS and a Growatt AIO charger/inverter.

1. When I store the van in my garage, usually I keep the Growatt plugged into the house to keep things topped up. I have Utility as first charge & load priority. However, the batteries still seem to drain while it's plugged in. When I unplug/replug, it seems to restart the charging process. Is this normal? I expected it to just keep my batteries at 100%.

2. In preparation for this weekend's trip, I unplugged/replugged the Growatt to house power to top off the batteries. After quite a while, the batteries were only reading ~52ah and 56ah remaining from the Overkill BMS. At one point the charge light on the Growatt was solid green, indicating batteries full. When I plugged back in today, the charge light is blinking, but is only sending 1-4 amps to the batteries, while the BMS still read ~55ah remaining.

3. In case it just needed to be synced, I did 'reset capacity' on both the BMS's. For whatever reason, this only brought the remaining ah up to 162ah and 165ah on the BMS display, while the Growatt continues to only charge at 1-4amps from Utility input.

I can confirm I've seen in the past both the Growatt push a reasonable charge rate, and have also seen the BMS's display 280ah remaining as expected.

Related screenshots:

https://imgur.com/a/Ns2VNi2

 

[DThames]

The BMS can't determine what is in the battery, regarding amp hours. There are a lot of settings on that BMS that might give some confusing information about what is left in the battery. I think you only do the reset capacity when it is fully charged or fully discharged. I am not 100%. Also, the BMS has some voltage level = to X% values so you can tweak the % curve. I would trust the amp hours left more than the % values, as they are voltage dependent.

If the display on the Growatt is on, it is using some power. When I am not using mine, I turn the inverter off, turn the AC feed off, and turn the solar input off. The battery will settle at about 3.4v per cell. LiFePo4 don't self discharge much at all so turning it all off would maybe be the best. If you want to charge it right before a trip, that would make sense. You shouldn't see a lot of charge needed.

I forget what Growatt program setting codes are for Absorption and Float. But if the charger is in float mode and the battery is largely charged (regardless of what the BMS says) there will be almost no charging current.

Hope that helps a little.

 

[gregcron]

Thanks for the reply. I'm guessing from the lack of cycling in the past month or two, the BMS ah counter is out of sync. It looks like the ah remaining after the 'reset capacity' is likely related to the %/voltage rules you mentioned - as the BMS settings are currently set at 3.3V = 60% capacity (which would bring the capacity reset to around 165ah which it's displaying).
I'm wondering if my cells just settled at full charge at 3.3v.

The cell data sheet doesn't seem to show any rules regarding voltage curve: https://www.alibaba.com/product-det...a2756.order-detail-ta-bn-b.0.0.4f792fc2H6rvdU

I had Growatt's charge/float voltage on default settings, which was 28.2v bulk charging (CV) voltage, and 27v Float voltage.
Given the data sheet shows 3.65v charge voltage, I increased CV on the Growatt to 29.2 (3.65*8).
I'm not sure what the correct Float charge voltage should be in this case, but I think you're correct that since the battery is >27v, the Growatt is in float mode and barely charging it.

I wonder if once I'm on the road and use capacity and then recharge, if everything will get back in sync. I'm thinking it may just be that since they've been hovering mostly full with minor drain (fridge) which should be charging from Utility, if it just threw things out of sync.

I'm not sure if only other option is to get on the road and see what they sync. Would just feel better if I could confirm with certainty I'm not embarking with 60% capacity.

 

[DThames]

I can't say (at work right now) but I think mine normally setting just under or right at 3.4v. Less than 3.35v, I think is concern of state of charge, but don't take that to the bank. I have a battery with new cells and an older battery in parallel, so baby mine a bit. I have absorb at 27.2v and float about 26.8v (as I recall) and that is high enough for what I normally do. But if it goes to float not fully charged, it will indeed park right there and not fully charge until it goes back to absorb.

 

[MisterSandals]

I'm wondering if my cells just settled at full charge at 3.3v

Mostly an observation: 3.30V is about 75% charge difference from 3.39V. So saying 3.3V is fairly meaningless.

And fwiw, I consider my settle from full charge voltage to be my 100% charge voltage (3.35V).
That has been my story for almost 2 years. I have over 40 cells of different sizes and makes that somehow all settle quickly to 3.35V.
I just received 19 280Ah cells and their settle voltage is about 3.52V. My world has been turned on it’s side!

So I would base YOUR 100% charge on where YOUR CELLS settle.

 

[beckkl]

Mostly an observation: 3.30V is about 75% charge difference from 3.39V. So saying 3.3V is fairly meaningless.

And fwiw, I consider my settle from full charge voltage to be my 100% charge voltage (3.35V).
That has been my story for almost 2 years. I have over 40 cells of different sizes and makes that somehow all settle quickly to 3.35V.
I just received 19 280Ah cells and their settle voltage is about 3.52V. My world has been turned on it’s side!

So I would base YOUR 100% charge on where YOUR CELLS settle.

When you say settle, do you mean the resting voltage some time after a charge?

 

[MisterSandals]

When you say settle, do you mean the resting voltage some time after a charge?

Yes, i'd say within an hour without any load. Most of my 206Ah cells would snap to 3.35V alarmingly fast but stay there for days. Its a number i kept seeing repeatedly and tried 3 different DMMs because it was so reliable in a strange way.

I am still learning how my 280Ah cells behave. They settle to 3.50-ish and after a couple hours they were all 3.48V - 3.52V from what my notes say. They stayed here overnight. I applied a 40W load (i had a grow light handy, would have liked about 100W for a little more stress) for a day and took notes. They came down to the 3.3x's within a few hours and that was where the bulk of the charge is so it didn't move much for the rest of the 24h test. Most of the cells finished around 3.32V and 3.33V (and this was for 4 different 4S packs, 16 cells - a good sign in terms of cells being well matched and healthy).
280Ah x 12.8V = 3584Wh
40W x 24h = 960Wh

Sorry, mostly rehashing my notes and seeing if i learned anything. The 280's look pretty normal to me.