diy solar

diy solar

Bad cell?

the_uglydog

New Member
Joined
Sep 11, 2020
Messages
123
Location
Luzon, Philippines + Orlando FL, USA
I have a bank of 6 200ah Lifepo4 batteries. In 2 of them I notice that they shut off prematurely, one at 52.5v the other at 50.9v. The rest of the batteries go down to my 47v switchback to utility with no problems and all cells closely matching their voltages. I see that one cell in each of the 2 that shut off seems to match the voltage of the others when it gets to 54v charging but when discharging the voltage drops more than the other cells. Example all the cells are 3.33v that one is 2.6v. I've tried charging that cell up to match the others but after the battery charges up to 54v it does the same thing. I'm assuming the cell is defective, right? Also one of the batteries is the opposite, the cells match at the low point before the switchback to utility but when reaching about 54v one cell seems to be charged higher. I see that the balancer switches on and off balancing the cells but it's always the same cells that go out of balance. Thanks for the suggestions, the dog
 
I have a bank of 6 200ah Lifepo4 batteries. In 2 of them I notice that they shut off prematurely, one at 52.5v the other at 50.9v. The rest of the batteries go down to my 47v switchback to utility with no problems and all cells closely matching their voltages. I see that one cell in each of the 2 that shut off seems to match the voltage of the others when it gets to 54v charging but when discharging the voltage drops more than the other cells. Example all the cells are 3.33v that one is 2.6v. I've tried charging that cell up to match the others but after the battery charges up to 54v it does the same thing. I'm assuming the cell is defective, right? Also one of the batteries is the opposite, the cells match at the low point before the switchback to utility but when reaching about 54v one cell seems to be charged higher. I see that the balancer switches on and off balancing the cells but it's always the same cells that go out of balance. Thanks for the suggestions, the dog

If you're saying the same cells are hitting peak voltage first AND dropping out first, those cells are definitely the weakest and may be bad.

Cells that hit the peak or the bottom at the same time as the others, but fail on the other end of the curve are the weakest and may be bad.

The question now becomes... are you getting 200Ah from each battery? If yes, no cells are bad.

Lastly, if these are 16S batteries, charging to 54V is not sufficient to fully charge these batteries, and you are likely not charging high enough to enable balancing. 54V is a float voltage. Charge to at least 3.45V/cell to ensure the BMSs are allowed to balance. The issues you describe may simply be due to significant imbalance developed over time from charging to insufficient voltage. If these are 15S batteries, 54V is a peak charge voltage.
 
Last edited:
Have you checked for loose connections and corrosion on that cell?

I had an issue of a single cell outage being off in one of my two batteries and found a cable had come loose from the terminal, and ended up taking the cable off cleaning, putting it back on with oxguard paste to protect from corrosion and then torqued down. I left balancing on the BMS for several days not running any loads, and the cell is within specs now.
 
Actually I guess I wasn't clear about it. 2 batteries have cells that are lower when discharged, they also shut off at about 54v when charging, then cycle on and off balancing, I assume. The lower cells gradually come up to the others, but drop down again the next time the battery discharges. There are 2 others that the cells match Ok when discharged but when they reach about 54v one cell in each of those goes up higher than the others and the BMS shuts it off till the system voltage drops back to about 53.5v. I've checked all the connections. Everything is only about 9 months old and resides in an airconditioned room. I've had it set to charge higher, 58v before, but it ended up that everything shut down with it showing overvoltage. That's why I'm thinking the cells are bad, or maybe weak.
 
Actually I guess I wasn't clear about it. 2 batteries have cells that are lower when discharged, they also shut off at about 54v when charging, then cycle on and off balancing, I assume. The lower cells gradually come up to the others, but drop down again the next time the battery discharges. There are 2 others that the cells match Ok when discharged but when they reach about 54v one cell in each of those goes up higher than the others and the BMS shuts it off till the system voltage drops back to about 53.5v. I've checked all the connections. Everything is only about 9 months old and resides in an airconditioned room. I've had it set to charge higher, 58v before, but it ended up that everything shut down with it showing overvoltage. That's why I'm thinking the cells are bad, or maybe weak.

It's still not entirely clear. Can you get specific with cell numbers?

If you're running a system where you are routinely running the batteries down to near 0% and only charging to 54V, you're likely in a losing battle. The only thing I can say with certainty is that your cells appear to be substantially out of balance. It's impossible to say if anything is actually bad without more information.

Are these DIY batteries? If so, were they ever top balanced?
Are you getting rated capacity when discharging from 54 to 47V?
 
Hi, Sorry. The cells have been balanced to a full charge at 58v until the BMS shut off all except one one, the one with no low or high cells. It caused problems for the one remaining battery, the BMS burned out and the main board on the inverter burned out when the load was slowly added back onto the system only reaching about 3kw. The load was added slowly but since only one battery was working I think it overloaded it. They still acted the same afterward. The only problem was, I did that while all the load was removed so I could get as much charge as possible into them (during our rainy season). The last one to shut off did so at 56v, other than that one the others shut off at about 54v when all of the cells seem to be the same. I've even opened the batteries as the manufacturer told me (before this last covid, now I can't reach them) and charged up the individual cells. it seems to not make a difference, they still act the same.
 
Charging to 58 volts until the BMSs trip is not a top balance, that is simply charging a battery. The BMS trips because a cell is out of tolerance.

If you can provide the data about the in dividing cell voltage that will help.

A top balance is done charging each cell to 3.6 or 3.65 volts.
 
Back
Top