BMS cut off , one unbalanced cell

maydayone

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Aug 16, 2021
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Yes, there should be a voltage he can set SCC to that doesn't cause disconnect. I think cells have diverged too far with other cells < 3.4V, ought to have remained balanced up to maybe 3.5V so SCC could be set for 3.5V per cell (14V). If not meeting published spec for charge voltage, should be replaced/serviced under warranty.

If warranty isn't an option, then break out the can opener and do the manufacturer's job for them.

I've dug into stuff and found manufacturer used components outside their specs. After warranty up. Boss though we should have them re-engineer the product and I though we should fix it ourselves. He opted for replacing failed components and continuing to use them out of spec. Cooling system for a laser. My solution would have been change a resistor so operated within spec RPM, and replace hose disconnects with lower backpressure ones.



Is that 800 British Pounds = $1096?
Unless those are superior quality cells, sounds to me like twice the price people here have been paying.
Double-check on that.
Also, make sure about trustworthiness of supplier, since with many there isn't a practical return option. (Maybe that's the price difference you see?)
You can get the cells cheaper from china but you have yo wait like 2 months. There are UK based companies who buy them test them and sell with a next day delivery, that's why you pay extra. How much would you expect for a EVE 280ah cell?
 

maydayone

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With 3.4v on other three cells you are very close to full charge. There is not much difference in state of charge when all cells are above 3.4v. The high cell should continue balance bleed after bms cutoff. It should come into balance. May take a few bms cut off cycles before it does.

Leave a little bit of time after bms reconnects to continue its high cell bleed before starting charge again. Usually balance bleed enable requires a cell to be above 3.4v.
The BMS I have balances with 35mA, how long would that take to balance 120ah cell? A LONG time! Apart from the fact that when the runner cell hits the balance start voltage (3.5V now but i tried with as low as 3.35V), then it only takes maybe couple of minutes before it gets to 3.75V. So 2 minute times 35mA, how many cycles would that be?:) Apart from the fact that it won't balance below 5A cause that's how the set it, which again i can't change because I would have to connect the PC to the BMS and it a sealed unit.
 

Hedges

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You can get the cells cheaper from china but you have yo wait like 2 months. There are UK based companies who buy them test them and sell with a next day delivery, that's why you pay extra. How much would you expect for a EVE 280ah cell?

I'm only a lurker, never bought any. But I thought I've read of people paying $80 to $125 (with commodity v.s. batched & matched at the extremes). People in the US have been buying usually from Amy if ordering direct from China, or from Michael as a local reseller.

At least knowing the China price, shipping cost, and local services, you may be able to request a price that is better but still profitable for your vendor. Unless you're competing with other buyers, in which case he gets to name his price. For a small purchase you probably don't have any bargaining power anyway, unless you offer to accept rejects like scratch & dent. Which is OK for appliances, but not for LiFePO4.

The BMS I have balances with 35mA, how long would that take to balance 120ah cell? A LONG time! Apart from the fact that when the runner cell hits the balance start voltage (3.5V now but i tried with as low as 3.35V), then it only takes maybe couple of minutes before it gets to 3.75V. So 2 minute times 35mA, how many cycles would that be?:) Apart from the fact that it won't balance below 5A cause that's how the set it, which again i can't change because I would have to connect the PC to the BMS and it a sealed unit.

Of course you're not balancing 100% SoC with 0% SoC.
35 mA would balance one Ah per 30 hours.
If 2 minutes is the time it takes a runner to get from 3.35 to 3.75V (at what, 20A?) that's < 1 Ah.
What you need to do is set SCC voltage low enough that runner stops at 3.65 or 3.70V and sits there. Then it can have days to keep bleeding high cell while the others slowly charge from 3.30V to 3.35V.
 

maydayone

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How do you know it isn't balancing when the charge current is below 5 amps? I haven't heard that before.
Ok so the Daily BMS has a icon which comes green when balancing takes place. The way the BMS is set on this battery is that it does not report any incoming or outgoing current if it's below 5A ( on the first battery it was 2 A). Simply put if you're charging or discharging battery with less than 5A the BMS shows current in/out as 0A. Because of that the SOC % is way off, i.e. battery was full with 21% indicated or empty with 74%. The balancing only takes place when charging, since th BMS is showing 0A if the current is below 5A the balancing does not take place - green icon is off - the BMS "thinks" there is no current going in. As far as i was able to find out that's one of the settings in the BMS but you can't access it via Bluetooth, only PC connection which i can't since it's sealed unit. As i mentioned before on the first battery this was 2A amps. And the same principle , if the current was less than 2A , BMS was showing 0A an no balancing. This was my first approach, lower the current to minimum so the BMS has more time to balance before the cell hits the top. If it was lower than 2A it was not balancing. Well even with 2A it was only few minutes before the BMS cut in.
 

maydayone

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I'm only a lurker, never bought any. But I thought I've read of people paying $80 to $125 (with commodity v.s. batched & matched at the extremes). People in the US have been buying usually from Amy if ordering direct from China, or from Michael as a local reseller.

At least knowing the China price, shipping cost, and local services, you may be able to request a price that is better but still profitable for your vendor. Unless you're competing with other buyers, in which case he gets to name his price. For a small purchase you probably don't have any bargaining power anyway, unless you offer to accept rejects like scratch & dent. Which is OK for appliances, but not for LiFePO4.



Of course you're not balancing 100% SoC with 0% SoC.
35 mA would balance one Ah per 30 hours.
If 2 minutes is the time it takes a runner to get from 3.35 to 3.75V (at what, 20A?) that's < 1 Ah.
What you need to do is set SCC voltage low enough that runner stops at 3.65 or 3.70V and sits there. Then it can have days to keep bleeding high cell while the others slowly charge from 3.30V to 3.35V.
The battery is in the campervan so it's charged with solar, victron motto, 30A, but give the weather it's usually below 10A. So when the cell started running away(above 3.4V) i lowered current to 2A to give time to the BMS to balance. I couldn't lowered the balance start voltage much more because below 3.4v cells are quite even. So when the cell starts running above 3.4V IT only takes few minutes with that 2A to get to the cut off voltage. This was my point that 35mA for few minutes per cycle would take forever. I now i have to put it at least 5A so the BMS to balance so there is that.

As i mentioned earlier I've contacted the supplier and I'm returning the battery
 

HRTKD

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My Overkill Solar (aka JBD) 4s 12v BMS is definitely showing sub 2 amp charge/discharge numbers. Is it balancing when the charge is less than 2 amps? It's not something I have watched, but my cells are in balance within .010v.
 

maydayone

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No, if it's not reporting any incoming current, which it isn't below 5A on this battery (2A on the previous) then it's not balancing
 

maydayone

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What is your absorption voltage, and what's your highest cell when you hit the absorption voltage ?
My Overkill Solar (aka JBD) 4s 12v BMS is definitely showing sub 2 amp charge/discharge numbers. Is it balancing when the charge is less than 2 amps? It's not something I have watched, but my cells are in balance within .010v.
 

MisterSandals

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So when the cell starts running above 3.4V IT only takes few minutes with that 2A to get to the cut off voltage.
You seem mighty infatuated with charging the 3 cells above 3.4V where there is maybe .5% of the total capacity. If you set your charge voltage to a voltage BEFORE any of the cells run away, you will get 99% of you battery capacity. (this is not the first time this has been suggested so it won't be the first time it gets ignored)
Who knows, maybe after successfully cycling a few times, the batteries will become better balanced with the aid of the BMS balancing.
And yea, i know you paid a LOT of money for a new battery that was supposed to work better, and that sucks. But you have options to try (conservative charge profile) to get 99% of your capacity with very little effort. Maybe you find it good enough to keep because returning batteries and waiting, or ordering cells and waiting.. is not on the fun list.

LFP Voltage Chart.jpg
 

maydayone

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Aug 16, 2021
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You seem mighty infatuated with charging the 3 cells above 3.4V where there is maybe .5% of the total capacity. If you set your charge voltage to a voltage BEFORE any of the cells run away, you will get 99% of you battery capacity. (this is not the first time this has been suggested so it won't be the first time it gets ignored)
Who knows, maybe after successfully cycling a few times, the batteries will become better balanced with the aid of the BMS balancing.
And yea, i know you paid a LOT of money for a new battery that was supposed to work better, and that sucks. But you have options to try (conservative charge profile) to get 99% of your capacity with very little effort. Maybe you find it good enough to keep because returning batteries and waiting, or ordering cells and waiting.. is not on the fun list.

View attachment 62552
Well, you're right. I might seem fixed on this but to me it's quite simple . I set it up as per instructions (14.4v) and it doesn't not work as advertised. The battery cuts out even as low as 13.8V, an last time I spoke with Sterling they told me not to lower it anymore than 14.2V - the new recommend setting. It seems like to them BMS cutting in on each charge is quite normal..
You've probably already seen me expressing my thoughts on getting the replacement unit which is completely misconfigured and essentially worse than the first one.

I think the whole topic should be concluded as the battery is going back.
 

maydayone

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Aug 16, 2021
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Thanks, so when your battery gets to 14.2V during charging, what are the voltages and delta on your individual cells then?
 
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