One cell always higher

larrylwill

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Aug 22, 2021
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I put together 4 lipo4 280a cells for my motor home. The cell voltage was about 3.2v +/- 0.5v per cell when I got them.
I wired them in series and charged them with a 10 amp charger. All cells were 3.3v +/- 0.1v. Next I charged them with my Motor home generator (extra Alternator) which was putting out about about 60 amps, there was also a AGM 100A battery connected in parallel. After about 5 minuets 3 cells were about 3.3v and one was 4.1v.
I removed the BMS and AGM battery and connected them in parallel and they settled back to 3.3v all cells +/- 0.1v. I felt the wire getting hot on the 4.1 cell for about 5 minuets until the voltage dropped. I then connected them back in series and put on a lipo cell charger/balancer at 1.5 amp. In the balance mode. After an hour there back to reading the same as before. 3.3v 4.1v 3.3v 3.3v. How can I balance them so they stay the same? I could wire them back in series and hook up a 3.6v supply like in wills video but I would have thought the Lipo charger/balancer would have balanced them. What do you suggest
Link below.
tinyurl.com/ysw9ayc7

 

larrylwill

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In the post I said I removed the BMS and agm battery. So yes. With the BMS and wired in series and using my motor home generator one cell was 4.1v BMS connected.
 

Just John

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In the post I said I removed the BMS and agm battery. So yes. With the BMS and wired in series and using my motor home generator one cell was 4.1v BMS connected.
Then you have your BMS set wrong for high cell voltage disconnect. What type of BMS do you have?
 

Just John

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You should read this and follow the directions. The resource section here is very useful.

 
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larrylwill

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Aug 22, 2021
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Yes I read it and watched Wills balancing method also. I do not have a bench supply although I do have a $500,000 Ic tester that draws 14 amps idling. I guess Im going to have to buy one just to balance these cells. I might be able to find a 5v power supply I can change to 3.65v though. I have to look. Tomorrow Im going to try charging 2 cells in parallel using my lipo charger. The setting is 3.7v . The batteries are about 3.35v with 2 in parallel.

As for the BMS It is a smart one maybe I can reset the upper V limit. I have to look. Its a Dykbcells or Xiaoxiang or other names 150a. Im using the CarplugloungeBMS app. Ill check tomorrow.

Strange the cells were at 3.35 and 3.36 until I started charging with the alternator then that 1 went to 4.21v.

 

Short_Shot

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Due to the nature of lithium cells connecting them in parallel will not provide sufficient voltage top balancing. The current that flows between the cells at your 3.3xx volts is almost nothing.

Even if there is a very large difference in SoC between them, you will still read only mv difference and experience negligible current.

I would not recommend using a 3.7v charger. Going much beyond 3.65 risks damaging the cells.

I'm guessing you have some experience with this stuff but not a lot with lifepo4 specifically?
 

larrylwill

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Yes you are right. I haven't done much with Lithium cells except use them and replace a few Lion cells in laptops or drill battery's. These lipo4 and BMS is totally out of my experience. I did notice the first time I hooked them in parallel the wire from the 4.1 cell got hot as the cell came down. I had the lipo charger on for a while and it was reading 3.55v I suppose it will go up as the cells charge. Since that one cell read 4.1 it may already be damaged.
 

Short_Shot

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Could be, but you might be lucky enough to get good life from it.

Lifepo4 isn't really any more complex, if anything it's safer, a bit less density, and that's about it. You just need to understand the voltage difference and you seem to have that handled.

Depending where you are located you might be able to find a generous forum member to assist with the power supply issue via borrowing one out.
 

Just John

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Yes you are right. I haven't done much with Lithium cells except use them and replace a few Lion cells in laptops or drill battery's. These lipo4 and BMS is totally out of my experience. I did notice the first time I hooked them in parallel the wire from the 4.1 cell got hot as the cell came down. I had the lipo charger on for a while and it was reading 3.55v I suppose it will go up as the cells charge. Since that one cell read 4.1 it may already be damaged.
It's likely that cell is fine, but I wouldn't tempt fate by repeating that. Your BMS should be set to stop charging when a cell hits 3.65v, if it doesn't, something is wrong. You need either an active balancer or a cheap power supply to top balance cells.


 

time2roll

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Even if an active balancer is used, the BMS needs to be connected to stop the charging when any cell reaches 3.65v.
Do you know what happened that allowed the BMS to ignore the overvolt in the first trial?
 

larrylwill

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I now have the cells in parallel. Im charging them to 3.6v at 10A they have been on for about 3 hours now and are reading 3.36. 3.37, 3.36, 3.36
How long do you suppose it will take to charge to 3.65v?
 

noenegdod

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Expect several hours at a minimum. Could very easily be a day or more.

Im assuming you got a power supply? Please make sure when you set the voltage you did so with the power supply not connected to the cells (open circuit) and not with the leads connected to the cells.
 

Just John

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I now have the cells in parallel. Im charging them to 3.6v at 10A they have been on for about 3 hours now and are reading 3.36. 3.37, 3.36, 3.36
How long do you suppose it will take to charge to 3.65v?
Assuming one is full, and the other 3 are at 50%. So, 140 x 3 = 420 and you can add 10 amp hours via your supply, so 42 hours maybe. Just depends.
 

ericgrand

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That means your power supply is not properly set...Amp should decrease significantly when cells reach 3.6X.
Also, with cells in parallel, reading such a diffecence between cells is not consistent. Stop your charger immediately and check wiring and charger voltage: 3.65 without any load connected!
 

larrylwill

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Aug 22, 2021
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Sorry 3.36 and 3.37, between the garage and house I switched the numbers. I also check the PS unloaded each time I check. I think when I hooked the PS up they read 3.33 and 3,34, really slow going, its been 8 hours so far.
 

noenegdod

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Sorry 3.36 and 3.37, between the garage and house I switched the numbers. I also check the PS unloaded each time I check. I think when I hooked the PS up they read 3.33 and 3,34, really slow going, its been 8 hours so far.
Ya, Its painfully slow with the small power sources that most of us have. What ever you do, dont deviate. Just check them 2 or 3 times each day and all of a sudden they are done.
 
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