I purchased a US3000C and hooked it up to my existing setup - Axpert MKS-4K + US2000 used without solar as a battery backup.
Everything seems to be working fine, I bumped my charging from the 20A set for the US2000 to 40A for US2000 + US3000C. I am now getting an ‘Abnormal’ status on one of the cells in the new battery.
Below is the output of the ‘soh’ command, should this be something to be concerned about or would it rectify itself over time? I would’ve thought the BMS would take care of balancing the internal cells if they are out of sync.
Battery Voltage SOHCount SOHStatus
0 3588 0 Normal
1 3589 0 Normal
2 3588 0 Normal
3 3339 1 Abnormal
4 3589 0 Normal
5 3595 0 Normal
6 3585 0 Normal
7 3592 0 Normal
8 3590 0 Normal
9 3593 0 Normal
10 3592 0 Normal
11 3586 0 Normal
12 3588 0 Normal
13 3590 0 Normal
14 3591 0 Normal
I have drained the battery to 70% 5 times and let it charge back up.
That one cell only ever goes up to 3.334V and the battery only reports 90% SOC.
I think I’m going to attempt to take it back for an exchange. I might be wrong, but IMO, a brand new battery shouldn’t have this issue.
My existing US2000 is happy with above settings with the DC charge rate set to 20A, this is the only setting I changed, so charge voltage should still be 53.2V, which is within spec for the battery.
I disconnected the US2000 and dropped charge rate to 20A. Attached a 3000w load to drop it to 60% then charge it back up. It gets to the point where it gets into this ‘Abnormal’ state on cell 3, and it stops charging.
My biggest concern is after dropping it to 60% 5 times, I still can’t get the whole pack to report above 92%.
I took the battery back, they’re keeping it there to test, but the technician said it sounds like a faulty cell, so they will probably replace the entire unit due to a manufacturing defect.
They’ll hook it up to their test system and check the stats for the battery and get back to me.
It is normal for a new battery to be a little out of balance. Irritating, but normal.
I know you sent the battery back, but herewith some advice anyway.
With such a battery it is important to lower your charge voltage to accommodate the high cell until the battery has had time to deal with the imbalance. In this case I would suggest lowering the charge voltage to 52.5V.
The trick to balancing the battery a little quicker is to charge at up to the point where the first cell exceeds 3.4V. Then slow the charge current way down, to as low as you can get it, and trickle it the rest of the way, so that the balancer has the maximum amount of time to do some balancing. When the highest cell hits 3.55V, discharge the battery again until all cells are below 3.4V, then repeat. This will have to be repeated a number of times, and the Axpert is not the best rig to do it with (with a Multiplus you can do things like running ESS and setting the grid setpoint to 60W, no loads on the output, so it charges really slowly).
Otherwise, just run the battery through several cycles and watch it daily.
Even for normal operation, I would not run at 53.2V. This battery wants 3.485V per cell, or thereabouts, that is where it is happiest. That’s around 52.4V. That’s what I’d go for.
Good to know, thanks @plonkster, with the heavy load shedding that is upon us, I don’t have the time to mess around with an unbalanced battery as I have critical loads that I need to keep up, so I will let the supplier sort that out for me.
The issue wasn’t a high cell, it was a low cell that wouldn’t go higher than 3.3V, other than that, all the other cells were balanced.
In the meantime, I have purchased an additional US3000C which is currently running fine on 53.2V and it’s BMS is reporting as ‘Normal’. I have dropped my charge voltage to 52.4V as you recommended and monitor the system in the coming days.
My understanding is that the problem isn’t the low cell. That cell is more than happy to still accept charge. The problem is the high cell, the cell that first blocks charge.
I think that is the reason why Izak said you want to accommodate the “high cell” and not charge the bank up too quickly, because then the high cell will just jump out every time and block charge for the low cell. At least, this is my understanding.
Titibit: To fix that issue on a DIY bank, as it does happen under frequent heavy loads or on a new bank, I connect a charger to that one cell for a few minutes at low SOC to boost it slightly. At high SOC it then tends to be a wee bit higher, the balancer sorts that over time.
Once the cells are settled over a period of time, the problem goes away UNLESS the cell is faulty.
LFP cells have a very flat charge curve. They get stuck at 3.3V for a really long time before they move up. Even a slightly low cell will appear to be grossly under voltage. Also, at 52.4V, the battery is basically 99% full, regardless of what it’s own estimate says. Simply running it at the lower voltage should not impact load shedding that much.