# Is my BMS working or not?



## Eric (Sep 9, 2012)

I have this http://www.ev-power.eu/SBM-CBM-1-1/Simple-Battery-Management-Board-16-cells-48V-60A.html in my electric scooter, hooked up to 16 winston cells.
When i installed the cells they were all 3.1 - 3.2 volt, after charging they are now 3.74 - 3.84 - 3.85, did my bms do what it had to do or not?


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## kennybobby (Aug 10, 2012)

Did the cells get hot during charging? Are any cells swollen or puffy? Might want to do a capacity check to see if cells are damaged.


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## frodus (Apr 12, 2008)

Did you equalize the cells at all before you hooked them up? 

A Balancing BMS keeps things in balance. If they were unbalanced, it may take several charges depending on how unbalanced your cells started off.

I think that the biggest problem people THINK they have, is they don't fully charge each cell individually (or connect all in parallel) and then hook the pack up. They start with an unbalanced pack.


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## Eric (Sep 9, 2012)

@ frodus: As mentioned before all cells were at 3.2-3.1 volt, and yes, i was under the impression the cells would be completely balanced after the first charge and i made this post because i was surprised to see the werent balanced.
but from what you say it can take several charges to get them perfectly balanced?

@ kennybobby: all cels were new, not swollen, puffy or hot at all.
How can i perform a capacity check? thats interesting.


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## frodus (Apr 12, 2008)

Voltage is NOT a measure of state of charge

The cells between 3.1V and 3.2V will have varying states of charge.... so when you charge them, that cell at 3.2V will hit LVC first, charger will shut off, and you'll have all of those 3.1V cells undercharged.

Just give it 10-15 cycles and see how they are then.


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## Eric (Sep 9, 2012)

I see, ill give it a few more tries.
Ill keep checking voltage by hand, when should i be worried something is wrong? If the differences between the cells start ro rise?


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## frodus (Apr 12, 2008)

Yeah, if it gets worse, stop and check the wiring and make sure things are connected right. The voltages should get tighter and tighter as time goes on.

It may take a dozen or more charge/discharge cycles to really see it, depending on how far off the cells were from eachother, and how large the cells are. If the balance resistor shunts 100mA, and they're 100Ah cells, and some were at 70% and some were at 95%, then that 25Ah of difference is going to take quite a few cycles to start matching eachother.

Hopefully, you wired the BMS so it can shut off the charger, and also shut off the main contactor if a cell goes too high, or too low.


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## Eric (Sep 9, 2012)

I wired it like this: 










Not sure if that will work, it is the scheme that came with the bms.


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## kennybobby (Aug 10, 2012)

Really you shouldn't charge anymore until you determine what the upper voltage cutoff is for your charger and what is the cutoff voltage of the BMS. And the cutoff current.

Also what is the cell chemistry--Are those cells rated to be charged up to 3.75-3.85? What is the charging procedure for those cells, e.g. CC/CV ? 

You need to get a handle on where your limits should be before jumping in and throwing the amps to that thang...


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## dougingraham (Jul 26, 2011)

I read through the specs for this bms and it looks like it balances starting at 3.6 volts to a max of 72ma. The high voltage cutoff is 3.9 volts and it restores the connection when the voltage drops back to 3.8. It does not have a low voltage cutoff but it does have an over current protection that activates at about 200 amps.

It does not control the charger or the motor controller, instead it opens the negative lead to the battery when a fault condition is seen.

If the cells are much out of balance it will take a considerable number of cycles to balance the cells. You could set the charger current to less than 70ma and let it run and eventually the cells will be balanced.

You need to keep an eye on this as LiFePO4 cells will have their electrolyte break down if the voltage goes over 4.35 volts and permanent damage to the cell will result.

Voltage is a measure of the state of charge, just not a very useful one. Near the middle of the state of charge (around 3.3 volts resting for several hours) a voltage difference of only 0.0004 volts is about 1% difference in SOC. Near the top (above 3.4 volts) or bottom (below 2.7 volts) a difference of 0.1 volt means almost nothing as far as SOC is concerned. In other words it is terribly non-linear to measure voltage and expect it to tell you anything about SOC.


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## Eric (Sep 9, 2012)

Thanks doug, i will check the cells regularly to see if they balance.
They are LiFeYPo4 Winston cells and i dont want to damage anything, will finish the scooter this weekend and drive the first miles, i will measure the voltage before and after charging and make a little list on the cells voltage so i can see how things work out. And i will check the chargers specifications.


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## evmetro (Apr 9, 2012)

frodus said:


> Did you equalize the cells at all before you hooked them up?
> 
> A Balancing BMS keeps things in balance. If they were unbalanced, it may take several charges depending on how unbalanced your cells started off.
> 
> I think that the biggest problem people THINK they have, is they don't fully charge each cell individually (or connect all in parallel) and then hook the pack up. They start with an unbalanced pack.


Frodus, I think you hit the nail on the head. One needs to have a understanding of how cells charge to understand the upper end where voltage begins to go up very quickly or the bottom end where it drops very quickly. If cells that are going to be maintained with a top balancing BMS are not top balanced first, they could take a LONG time to balance.


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## Eric (Sep 9, 2012)

As mentioned before, i got the cells from my supplier, all at 3.1 or 3.2 volts. So with the knowledge i had/have i figured that would be allright to start with. As i understand now it would be better to balance them now, and then hook up the bms again?


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## frodus (Apr 12, 2008)

I would just wait for them to come into balance by cycling and using a BMS. If you don't want to wait, then yes, take the pack apart, parallel them or individually charge them, and then put the pack back together.


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## dougingraham (Jul 26, 2011)

I wasn't able to tell from looking at the specs of that BMS but it may be chugging away right now trying to get the voltage of all the cells to 3.6 volts even if you aren't charging.

When manufactured the cells undergo a special formation charge to build the SEM layer. They then discharge the cell and measure the capacity. Then they refill about half way. So when you received them they would have about half charge (if they were still new in box.) By about half they could be within a few percent of balanced. This is not bad and if you didn't have a BMS you could get by for years if your charger was set to undercharge a little and you never ran the vehicle until it was empty.

If you are going to use a top balancing BMS (which is what you have) you can help the BMS out by individually charging each cell. An inexpensive RC hobby charger with a setting for LiFePO4 cells works well for this. You might even know someone you could borrow one from for a few days. I would disconnect the BMS from that cell when you charge it or it could interfere with the operation of the hobby charger. You can also use a lab power supply as a charger for this. Set the voltage to 3.6 and the current to whatever you feel comfortable with up to 1C. For 40 AH cells this would be 40 amps or less. There are few lab supplies that can anything close to that. (I mention this only because I happen to have one that does 400 amps.) And you are done when the current is reduced to C/20 which would be 2 amps for a 40AH cells or 1 amp for a 20AH cell. This is not at all critical. If you start out with already charged cells the current will drop below this threshold almost immediatly. With a manually top balanced pack the BMS should do almost nothing.


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## steven4601 (Nov 11, 2010)

Sorry for asking,
The forming process which modifies the behaviour of the LiFePo4 battery cell when completed at the factory is known to me, but what is the abbreviation SEM for with relation to the forming process?


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## dougingraham (Jul 26, 2011)

steven4601 said:


> Sorry for asking,
> The forming process which modifies the behavior of the LiFePo4 battery cell when completed at the factory is known to me, but what is the abbreviation SEM for with relation to the forming process?


My mistake. It is supposed to be SEI (Solid Electrolyte Interphase) layer. Not sure why I typed SEM. When I see SEM I think Scanning Electron Microscope. This is why I need to proof read before I hit submit.

Sorry for the confusion.


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