Balancing Lithium Cells

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PzlPete
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Balancing Lithium Cells

Can someone please help me with this???? I put 14,500 miles on my XM-4000li with 20 40ah cells, with absolutely no problems. Plug and ride. Simple. No BMS. No problems. 47 mile range when new. Down to around 30 miles when I finally took her apart to replace the cells under warranty.

I am on my 3rd try at replacing the cells, after finding you cannot mix brand new cells with 14k mile cells. I now have all brand new 40ah cells, but some are thundersky (yellow) and some are winston (black). I wired them up in parallel for 3 days to completely balance them, and all cells were at 3.30 when I put them back in the cycle.

After charging for an hour, 2 of the thunderskys were at 4.25 volts !!!!! And the 8 winstons had barely budged, at 3.31 volts.

Does BMS actually regulate the charge to each cell?? Or does it simply shut down the charge if any cell goes over 3.8 (or a preset limit) and shut down the cycle if a cell goes under 2.5 (or a preset limit) ???

I cannot undestand why my original cells remained so perfectly balanced through 550 charges.

reikiman
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Re: Balancing Lithium Cells

There's a large range of state of charge where you can't tell much by looking at the voltage. At 3.3 volts resting, these cells could be 80% discharged or 80% charged and you cannot tell. It's not clear what the state of charge is until you reach near-complete charge or near-complete discharge.

It's possible the cells that went to 4.25 volts were simply nearly completely charged while the others weren't. But by going to 4.25 volts these cells may have become damaged.

Winston is the successor company to Thundersky - well, absent the claims by Sinopoly otherwise. It's plausible that mixing Winston and Thundersky cells is safe, maybe, but unlikely.

As for the behavior of BMS's .. it does depend on the exact kind of BMS.

Usually, BMS's contain a bank of components that bleed off energy. Think of these components as resisters, but they're actually something else. The BMS logic detects when a cell (or cells) is at too-high a voltage, and turns on the bleeding resistor until the voltage drops down. It'll also probably turn off the charger at that point.

This is known by some people as "top balancing".

PzlPete
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Re: Balancing Lithium Cells

Thank you Reikiman! I feared that mixing cells was a bad idea. My original 20 cells must have been balanced perfectly as each time I tested them at half charge they were all 3.28 to 3.29. At full charge they did vary from 3.52 to 4.08, but after running the cycle for 15 miles, they were all exact.

I am afraid that charging these cells to 4.25 will destroy them. I am going to do some more bench testing and cycle the cells several times to see if my balance improves over time. Is that worthwhile? Or should I go get a BMS that actually regulates the charge sent to each individual cell?

IBScootn
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Re: Balancing Lithium Cells

PzlPete,

As David mentioned, you should have fully charged each cell prior to building the pack. For these large cell LiFePO4 normally that is 3.65V but check the manufacturer recommended full charge value. When I saw you paralleled them at 3.3V, that sent a flag up. Had you mentioned that you paralleled them and then charged them all up to 3.65 slowly, the results might have been better.

Anyways, it might be helpful to add balance leads to each cell and then to Cell-Logs as I mentioned here:
http://visforvoltage.org/forum/13362-xm5000-reduced-range

A slightly more expensive Cell-Log (extra $10) allows you to log data on each cell and then upload to a PC for analysis as to how your pack is performing in real-time during a ride.

Good luck,

Motorcycles: 2011 ZEV Trail 7100, 84V, 60AH, 60+mph, Cycle Analyst, TNC throttle, modified charger. 2013 Kymco GT300i
Bicycles: 2017 Sondors Thin
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antiscab
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Re: Balancing Lithium Cells

Can someone please help me with this???? I put 14,500 miles on my XM-4000li with 20 40ah cells, with absolutely no problems. Plug and ride. Simple. No BMS. No problems. 47 mile range when new. Down to around 30 miles when I finally took her apart to replace the cells under warranty.

*snip*

I now have all brand new 40ah cells, but some are thundersky (yellow) and some are winston (black). I wired them up in parallel for 3 days to completely balance them, and all cells were at 3.30 when I put them back in the cycle.

If there were no problems, why were you replacing cells under warranty?

I experienced similar capacity loss when I had a 20 cell 40Ah battery in a scooter that did 70kmh continuous.

when putting new cells into an old pack, you just have to balance first, I mix and match cells of wildly different ages all the time, no problem
Just make sure you either take every cell to 3.65v (top balanced) or 2.5v (bottom balanced).

Both Thunkersky and Winston cells are yellow

Sinopoly cells are black

It's curious that you got hold of both types of cells from the same person.

When winston or sinopoly or just about every other large format lifepo4 manufacture sell cells, they first match them based on capacity and internal resistance. Then each cell is charged to 50% (ie a brand new batttery comes from the manufacturer already balanced).
If after being shipped off by the manufacturer, the cells are charged or discharged (by say a well meaning middleman) than they won't be at 50%, and a manual balance will be needed. This would seem to be the problem you have.

keeping the cells in a battery as close to identical as possible reduces the risk of overcharge or overdischarge if the battery as a whole is pushed close to it's charge or discharge limits.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km

PzlPete
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Re: Balancing Lithium Cells

Thanks for the reply Matt! My range had decreased from 47 miles to just under 30 miles, and the 2 year warranty on the batteries only had 1 month left. So I dismantled the cycle and tested them and sent them back to Xtreme Scooters. Xtreme said that any cell that fell under 3.00v after I ran the cycle until it wouldn't run anymore (ugh!) they would replace under warranty. So I ran the cycle until it wouldn't move (bad idea), and then shipped them the cells that were under 3.00v. Two cells were only 1.50v, so they were definitely bad. Of course running it to a dead stop could have killed them too.

Xtreme sent me back the only 11 Thundersky cells they had. So I had to buy 9 more Sinopoly (sorry, not Winston) 40ah cells from Evolve Electronics.

I now have 11 yellow and 9 black cells in my cycle. They all balanced to 3.30v prior to install. I had no way of top balancing or bottom balancing them. But now after first charge, two of the thunderskys shot up to 4.25v, and the black winstons are all still at 3.31v.

It doesnt appear to me that these cells are a match in terms of capacity and resistance !!

Of course my previous cells used to fully charge to 4.08v for the highest cell and 3.52v for the lowest cell. And they still ran for 14,5000 miles with a good range. And after running the cycle for a few miles after full charge, the cells were in perfect sync at 3.28v, so..... yes I am confused.

Do I just need to cycle my mix of thundersky and sinopoly cells to see if they harmonize ???

IBScootn
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Re: Balancing Lithium Cells

Unlike other battery types, lifepo4 aren't likely to "harmonize". Like Matt said, you need to top balance them. Just buy one of the 10A single cell Lifepo4 chargers off of fleabay. Or if you have tons of time on your hands, you can use a cell phone charger, just make sure to have a Cell-Log attached with HVC at 3.65V.

Motorcycles: 2011 ZEV Trail 7100, 84V, 60AH, 60+mph, Cycle Analyst, TNC throttle, modified charger. 2013 Kymco GT300i
Bicycles: 2017 Sondors Thin
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antiscab
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Re: Balancing Lithium Cells

Xtreme said that any cell that fell under 3.00v after I ran the cycle until it wouldn't run anymore (ugh!) they would replace under warranty. So I ran the cycle until it wouldn't move (bad idea), and then shipped them the cells that were under 3.00v. Two cells were only 1.50v, so they were definitely bad. Of course running it to a dead stop could have killed them too.

that's a quick and easy test to find the cells that are low on capacity.
It kills the low capacity cells, but you're replacing them anyway.

Xtreme sent me back the only 11 Thundersky cells they had. So I had to buy 9 more Sinopoly (sorry, not Winston) 40ah cells from Evolve Electronics.

I now have 11 yellow and 9 black cells in my cycle. They all balanced to 3.30v prior to install. I had no way of top balancing or bottom balancing them. But now after first charge, two of the thunderskys shot up to 4.25v, and the black winstons are all still at 3.31v.

It doesnt appear to me that these cells are a match in terms of capacity and resistance !!

That's not actually conclusive - the cells could have been all the same capacity, just at different states of charge.
without capacity testing, there's no way to know for sure.

were the other thundersky cells above 3.31v?

The tricky thing is, the two thundersky cells may have suffered capacity loss due to overcharge - have they started to swell?

Given the cells supplied by extreme are left overs (maybe not even new?), they probably weren't at the 50% charged they came from the factory.

Of course my previous cells used to fully charge to 4.08v for the highest cell and 3.52v for the lowest cell. And they still ran for 14,5000 miles with a good range. And after running the cycle for a few miles after full charge, the cells were in perfect sync at 3.28v, so..... yes I am confused.

Do I just need to cycle my mix of thundersky and sinopoly cells to see if they harmonize ???

your previous experience is what you get from a battery that was balanced at 50% state of charge (as they come from the factory)
to re-iterate, all the cells at the same voltage does not necessarily mean same state of charge.

Don't just cycle them as is - you already know they are out of balance, the two thundersky cells will overcharge and definitely fail.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km

PzlPete
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Re: Balancing Lithium Cells

Thank you for all of the advice!!

In absence of a BMS, I am actually using small airplane motors to bleed off the overcharge on the cells with high voltage. And then plugging the charger back in to bring up the low cells. Repeat. Until the cells are balanced at full charge. It seems to be working. And the lower voltage Sinopoly cells appear to be taking more and more charge each day. It appears from my experience that cycling brand new cells increases their capacity. True?

tigger19687
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Re: Balancing Lithium Cells

Just to be slightly off topic here ....

What was your speed when it was new and what was it when you were getting less miles ?
I was just wondering if you could really notice the difference besides in the length of miles on a charge.

PzlPete
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Re: Balancing Lithium Cells

Speed new was 50mph+. After 14,300 miles, speed was down to under 40mph, and as I mentioned, range was almost half.

After some attempts at mixing Sinopoly and Thundersky cells (dont do it), I am finally installing 20 new GBS cells in my cycle with new charger and balancers on all cells. Will keep you updated.

PJD
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Re: Balancing Lithium Cells

PzlPete,

Getting a BMS system was good idea - although in terms of consistency and reliability, I prefer Winston Cells over GBS - although Winston Cells don't seem to be available anymore?

14,000 miles plus was pretty good for pack without a BMS.

All BMS's that I know of work by preventing any individual cell from going over a particular set-point (typically 3.65 to 3.75 for lifePO4's) by shunting the current past the individual cells when the cell reaches this set point. This allows the voltages of the other cells to "catch up". This is done in conjunction with some means to "throttle back" the charger current to less than an amp or so, since shunting multi-amps past a cell would generate too much heat for the BMS to survive.

Without a BMS, I can't see mixing new cells with old cells, and even with a BMS, mixing different manufacturers is not likely to work well. The issues is not just different states of charge, but more importantly, cells of widely different age and different manufacturer have widely different internal resistances, This causes the voltages of the higher internal resistance cells to go high well before the cells with lower internal resistance.

PzlPete
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Re: Balancing Lithium Cells

PJD, thank you for your input. You are right that mixing cells from different manufacturers, and of different ages, simply does not work. I tried it. My first attempt was replacing 12 of my 20 cells under warranty, but with a resulting 6 mile range (yes 6) I could see that would never work. So, since Sinopoly cells were the only ones I could find, I purchased 8 new Sinopoly to go with my 12 new Thundersky's, and no matter what I tried, those Sinopoloy's would not charge to more than 3.3 volts, while the Thunderskys would go off the charts at 4.20 and above. Insane. I now have 12 brand new Thundersky and 8 brand new Sinopoly's 40ah that I would make someone a great deal on!! (but dont try mixing them)

So, I purchased 20 GBS cells, with a new charger, and balancers on every battery, installed it on Friday, and rode 140 miles Saturday and Sunday. What a dream. 41+ mile range with no problem. 50+ mph top speed. The cells were selected with equal resistance and capacity, so they started out very balanced. The balancers come on at 3.55 volts and limit charge to those cells while the other ones come up to capacity. You can watch the led's come on indicating a cell has reached capacity. All 20 stay in perfect balance now. I can go over 40 mph while the voltage on all 20 goes from 3.55 down to 3.22, so the GBS cells are staying in the "sweet spot" and I think they will last even longer. Thank you for all of your input!!

TeamSlacker
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Re: Balancing Lithium Cells

I now have 12 brand new Thundersky and 8 brand new Sinopoly's 40ah that I would make someone a great deal on!! (but dont try mixing them)
!

PzlPete, I sent you an email or msg about this, did you by chance get it?

PzlPete
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Re: Balancing Lithium Cells

Yes I do have 20 brand new 40ah Lithium cells that I would sell, but shipping them is a pain due to the hazardous materials regulations here. I am going to try to sell them local pickup only. Phboenix AZ. I would take $300, and throw in the other 15 cells I have that still have plenty of life.

My cycle now has 20 GBS cells with balancers on them, and these cells are fantastic. They full charge at 3.65 volts and then the balancers kick in to keep every cell at exactly the same level when the charger shuts off at 71.6 volts. I can ride 44 miles before the cells are each around 3.2 volts, which is plenty of range for me, and the cells remain within the "sweet spot" of 3.2 to 3.65, so I am hoping these last even longer than the 14,500 miles the Thunderskys lasted with no balancers.

TeamSlacker
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Re: Balancing Lithium Cells

Just in case you have emails turned off (just sent you one). I possibly know someone headed to AZ today that could pick them up. If you didn't get the email, please let me know asap. thanks

Yes I do have 20 brand new 40ah Lithium cells that I would sell, but shipping them is a pain due to the hazardous materials regulations here. I am going to try to sell them local pickup only. Phboenix AZ. I would take $300, and throw in the other 15 cells I have that still have plenty of life.

My cycle now has 20 GBS cells with balancers on them, and these cells are fantastic. They full charge at 3.65 volts and then the balancers kick in to keep every cell at exactly the same level when the charger shuts off at 71.6 volts. I can ride 44 miles before the cells are each around 3.2 volts, which is plenty of range for me, and the cells remain within the "sweet spot" of 3.2 to 3.65, so I am hoping these last even longer than the 14,500 miles the Thunderskys lasted with no balancers.

TeamSlacker
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Re: Balancing Lithium Cells

ooops.. dupe

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