Bottom balance a way-undercharged LiFePO4 cell

[jholovacs]

Ordered a 200AH LiFePO4 12v system from China... but due to the insanity in the world, one of the cells arrived 45 days after the other 3. Seller assures me they are all of the same manufacturer and chemistry (absolutely no markings on the battery whatsoever), but failed to disclose they were used and way out of balance with each other.

Voltages are currently:
C1: 3.284v
C2: 3.336v
C3: 3.339v
C4: 2.640v

Needless to say, not especially thrilled with this, outside of the dispute window and definitely won't be buying from him again and will get a rather unhappy review, but... I have these cells now.

C4 voltage is way under the other 3 cells. How can I balance these to at least get some use out of this battery?

 

[BiduleOhm]

Why bottom balance?

What I'd do is recharge C4 and C1 to 3.3-3.4 V, then connect the 4 cells in parallel and top balance them to 3.65 V.

But before that I'd do a capacity test of each cell because you can't be sure they are the same as the seller wasn't honest.

 

[jholovacs]

I don't really have specialized equipment, like a charger that can charge to 3.65v, and I haven't bought an inverter for a good capacity test

 

[Vitosik]

Witch sells is better? Lower or higher resistance?
Thank you

 

[BiduleOhm]

Lower is better.

But to be a proper comparative test they need to be at the same SoC.

 

[BiduleOhm]

I don't really have specialized equipment, like a charger that can charge to 3.65v, and I haven't bought an inverter for a good capacity test

So then I'd connect C1, C2 and C3 in parallel (which should bring C1 closer to C2 and C3) with bus bars, and connect them to C4 with long (a few meters) relatively thin wire (14 or 16 AWG) to limit the current until C4 is close enough (let's say no more than 0.2 V difference) to connect it directly with busbars to the other cells. Then you just let the 4 cells in parallel like that for at least a few days, more if you can.

It's certaintly not perfect but it doesn't require any special equipment and it's far better than nothing.

The BMS you plan to use has balancing capability?

 

[jholovacs]

yeah, recommended by Will, straight out of the decontamination chamber in China lol...

I'll try your suggestion, thx

 

[BiduleOhm]

Ok, so then you should be fine. It may take days or even weeks for the BMS to balance the cells to the best it can but you'll be ok.

You'll also see if a cell is weaker than the others as it'll discharge and charge faster, hopefully they are more or less matched.

 

[ghostwriter66]

So then I'd connect C1, C2 and C3 in parallel (which should bring C1 closer to C2 and C3) with bus bars, and connect them to C4 with long (a few meters) relatively thin wire (14 or 16 AWG) to limit the current until C4 is close enough (let's say no more than 0.2 V difference) to connect it directly with busbars to the other cells. Then you just let the 4 cells in parallel like that for at least a few days, more if you can.

It's certainty not perfect but it doesn't require any special equipment and it's far better than nothing.

The BMS you plan to use has balancing capability?

SO not trying to bust you out but that way won't work at all --- It takes current to balance -- if you hook all the cells up in parallel YES for a few minutes you will see some current going from one battery to another but soon - VERY SOON -- that all stops ...

The OP does not have any equipment to do it right so honestly he is out of luck at this stage BUT UNLESS the 4th battery is damaged or something then just through normal use they should balance out close enough to be good ...

BUT to do this right --he needs to hook all 4 batteries in parallel and then apply constant current across all four simultaneously in order to balance them all correctly and get them all synchronized...

 

[BiduleOhm]

I know, and the point is not to balance them perfectly but to balance them enough for the BMS to be able to do the rest of the balancing without problems.

if you hook all the cells up in parallel YES for a few minutes you will see some current going from one battery to another but soon - VERY SOON -- that all stops ...

Actually it doesn't stops, it's just slower, a lot slower... that's why I recommended to let them like that for days.

 

[ghostwriter66]

I know, and the point is not to balance them perfectly but to balance them enough for the BMS to be able to do the rest of the balancing without problems.

Actually it doesn't stops, it's just slower, a lot slower... that's why I recommended to let them like that for days.

OK -- I will give you that BUT you are NOT talking DAYS -- you are talking weeks and months ... I have (and several others smarter than me) have tested this "slap them in parallel and they will equalize" theory and its WRONG ...

If you did take the OP's C1: 3.284v, C2: 3.336v, C3: 3.339v, C4: 2.640v setup and put them in parallel, you would see a current movement for the first 3-4 minutes -- after that it will quickly drop to virtually zero. Technically it would be like 0.05 so YES after a weeks/months it would have leveled out but this fallacy of slapping them together an overnight they are fixed is not sound advice. (NOT saying that you are saying that at all -but others have)

YES the difference between his C1-C3 may be enough to initial push some current into the C4 to get some of the voltage up - but I would be surprised if it changed very much at all ... heck the Resistance in the busbars will probably negate any self-generating current from the batteries ..

 

[BiduleOhm]

Again, it's not to achieve perfect balance, it's so they are balanced enough to make a pack and let the BMS handle the light work.

One thing I just tought about is that he can't charge them while in // because he doesn't have a charger that do one cell voltage but he can discharge them a bit with anything he has on hand that is resistive and handle at least 3.5V (headlight bulbs comes to mind). It should help to balance the cells a bit more.

 

[Capt Bill]

Ordered a 200AH LiFePO4 12v system from China... but due to the insanity in the world, one of the cells arrived 45 days after the other 3. Seller assures me they are all of the same manufacturer and chemistry (absolutely no markings on the battery whatsoever), but failed to disclose they were used and way out of balance with each other.

Voltages are currently:
C1: 3.284v
C2: 3.336v
C3: 3.339v
C4: 2.640v

Needless to say, not especially thrilled with this, outside of the dispute window and definitely won't be buying from him again and will get a rather unhappy review, but... I have these cells now.

C4 voltage is way under the other 3 cells. How can I balance these to at least get some use out of this battery?

What is that Supplier Name: Likely one I want to Not use !

 

[jholovacs]

SHUN BIN off of Amazon

 

[Craig]

ok I am going to test this theory I have 2 cells one at 2.61 volts and 2.06 volts.
These are LTO cells so the one is fully charged and the other is basically empty.

I hooked them up and put my clamp meter on one of the connections.\
first 0-5 minutes amps were at about 150 to 100 next 5 minutes amps 100- 50 next five minutes amps at 50-15 or there abouts.

After 15 minutes cells were 2.2V and 2.25v I used a 12 inch 6Awg welding cable to connect negative terminals and a piece of 2 inch flat aluminum on the positive ends.

So they balanced relatively fast and I coud only detect they heated up maybe 10 degrees F above ambient Temp

Video

 

[Craig]

What was most interesting to me is hand tightening the bolts got me to 100 amps putting a wrench on them got me over 150 amps at its peak.

 

[BiduleOhm]

What was most interesting to me is hand tightening the bolts got me to 100 amps putting a wrench on them got me over 150 amps at its peak.

Yep, contact resistance varies with contact pressure, that's how carbon piles work actually