New BMS fitted to a 12s LiFePo4 18650 ebike pack

[Peter Brigg]

Immediately after I charge this pack there is a fairly wide variation in cell voltages-the highest being 3.7v and the lowest 3.3v. I then attach a load consisting of 3 only 12v car taillight bulbs in series drawing approx 2amps and I believe this takes off the surface charge and gives a more reliable voltage reading.
After a few minutes with the load on the cell voltages come closer together but even if I repeat the charge /discharge cycle a few times i cannot get the cell voltages to match really closely.

The latest voltage readings I have are in the range of 3.4v highest down to 3.3v lowest with only 2 cells at the 3.3v level.

Am I expecting too much here-is this variation just an indication of some cells being in better condition than others?

Thanks

Peter

 

[999zip999]

Put it on the charger after fully charged make a chart of the cell voltage starting for the negative end as number one. Do put it down on a piece of paper just like.
1. 3.44v
2. 3.35v
3. 3.65v

12. Xxx volt
Please state here your state of change.
Make sure you have a good battery your meter.
What voltage is your charger set to ? Write that down .

 

[fechter]

It could just be the cells need to be balanced a bit better initially. If you have a single cell charger, you can charge each cell group individually to get them all to the same level (3.65v). Imbalance will be much more noticeable at the top end of the charge curve.

You should be able to get all the cells to the same voltage by either charging or discharging individual cells.

If you have cells that charge to 3.7v but immediately drop after the charger is disconnected, it's a bad sign.

 

[Peter Brigg]

I have some further information to add about this topic. I left the charged battery on the bench overnight (not connected to a charger)and to my surprise the cells had balanced themselves to within a very tight range.
They were all within the range of 3.34v to 3.3v. I checked the voltages with 2 different multimeters- same result.

Can I assume that the BMS is doing it's job and has balanced the cells by bleeding current from the high cells?

Regards
Peter

 

[Peter Brigg]

The charger is for a 36v LiFePo4 battery and the voltage measured with the charger on but not connected to the battery is 43.8v

Peter

 

[EbikeAus]

I have some further information to add about this topic. I left the charged battery on the bench overnight (not connected to a charger)and to my surprise the cells had balanced themselves to within a very tight range.
They were all within the range of 3.34v to 3.3v. I checked the voltages with 2 different multimeters- same result.

Can I assume that the BMS is doing it's job and has balanced the cells by bleeding current from the high cells?

Regards
Peter

3.33 -3.34 is a typical resting voltage for LiFePO4. I don't think you should assume this is balancing done by bms.
LiFe are different in this respect to other lithium chemistries, where the voltage doesn't necessarily show the state of charge

 

[999zip999]

How old is this battery and how hard have you been using it over this time ?

 

[Peter Brigg]

Some more information-I supplied the wrong details about the type of cell in the battery- they are Headway 38120 cells-so reasonably good quality.
I made the mistake when i made the battery of using a cheap BMS bought on eBay-after some use I was getting only a short ride out of the battery before it cut out.
When I pulled it out of the box and checked the voltages one cell had died (under 1 volt) so I replaced it and bought a better quality BMS again from eBay China. I was from Supower 111 and cost around $60 US and that seller specialises in electric bike components and has a big range of BMS so I am hoping that the quality is there.

I should have realised that a cheap BMS is going to cause problems-it looks as though this better quality BMS is doing it's job.

There is some small variation in the voltages of cells but not much-perhaps some of them were damaged with the cheaper BMS and their capacity is just not as good
as some of the others.
A question-how do BMS balance cells? Do they bleed current from high cells and bring them down to the level of the lower ones? The sensor wires are quite light so the bleed current if it exists must be quite small. I have read about how BMS work on this forum but not understood this point.

 

[fechter]

The BMS will only try to balance once the cells have reached the level where the shunts come on. For LiFePO4, this will be somewhere around 3.6v-3.7v.

 

[999zip999]

Get every cell to 3.60v or 3.65v before plugging in bms. Fetcher recommends starting at the negative or lowest voltage in add an angle plugging to the highest voltage in. A bms is meant to keep a balance pack balanced.

 

[John in CR]

Like said above, you gotta get your pack fully charge first. 3.3v or so isn't fully charged. Abused and old cells will still have 3.5v or higher after full charge and sitting overnight. Due to the significant surface charge LiFePo4 chargers need to repeatedly turn back on during the top balancing process to get the whole pack fully charged and balanced.

Invest in a 6s iCharger. I find it an invaluable tool in pack building, maintenance, and evaluation. Using mine I don't even need to use a bms on my 10 year old A123 4s packs that I have to balance once or twice a year. Your 12s would just require a 2 step process instead.

 

[Peter Brigg]

Yes-I have one of those chargers for a 12v battery I built as a standby battery for camping-I used 4 of the same cells I used for the bike (Headway 38120).
I notice that the charger has a limit of 6 cells so yes-you can use it on half the pack them do the other half
The charger also shows the voltages of each cell as you charge them if you push the right buttons which is also a help in knowing what is going on.

On something else-when I originally looked at the battery I saw one cell was very low-under 1.5v and today when I was about to bin it I thought of giving it a really fast charge from a 12v lead acid battery-just holding the leads on the single cell for a few seconds then giving it a rest- then repeating it later on. I have done this with NiMh cells to "rejuvenate" them. The cell became just warm not hot with me doing this.The cell now sits at about 3.3v and seems to be holding it's charge. Perhaps it is OK after all?
Thanks for all that info

Peter