Duff/weak cell?

[hohisilver]

Being relatively new to the world of LiFePO4, I'm wondering if I've got a duff Headway cell or whether it's just "weak". It's always the first one to get up to voltage on charge, and this evening I did a bit of a range test and this same cell nearly tripped my controller cutoff when I saw it momentarily dip to 2.5V under heavy acceleration. Is this amount of variation normal or is it duff?

[GregsGarage]

Make sure your cell connections are good, that can also cause voltage drops. When it drops to 2.5 volts, how does that compare to the best cell? Your pack will always be limited by the weakest cell in the pack. As long as this cell performs well enough to suit your range requirements, then its not a problem.

[retepsnikrep]

Double check those cell voltage readings with an accurate voltmeter as well to confirm an accurate voltage is being reported by the BMS, if you had a bad voltage ref zener diode for that cell then it could skew the results. unlikely though.

As Greg says every battery has one cell that is the weakest, one has to be

[hohisilver]

It did occur to me today that as it's 2 cells connected in parallel, if one of the cells in the pair was duff I might only be getting 12AH instead of the intended 24AH, explaining why it drains and charges quicker than all the others?

[GregsGarage]

It did occur to me today that as it's 2 cells connected in parallel, if one of the cells in the pair was duff I might only be getting 12AH instead of the intended 24AH, explaining why it drains and charges quicker than all the others?

Or one of the cell connections are loose/high resistance. I had this problem with one of my cells, I run 3x 40ah cells in parallel. A loose connection caused the same symptoms that you have.

[hohisilver]

Still haven't had a chance to take the pack out to check that connection, but I haven't used the scooter for over a week now because it was dropping badly (I had it cut the controller after only 2 miles ). So I thought this evening I'd just give the pack a quick boost charge, and it was certainly quick! That cell (which had been showing the lowest voltage) shot straight up to cell maximum within a few seconds and finished the charge! Does that suggest a badly loose connection, or a duff cell? I'll be taking the pack out to check it in a couple of weeks when I've got some time off, so I'm just wondering if I might need to get hold of another cell or two as replacements

[GregsGarage]

You definitely have a high resistance either internal in the cell or at the connections. At a guess I would say faulty cell but you are right to take the vehicle off the road and investigate. At least the bms told you of the problem.

[Beemer]

It doesn't suggest a loose connection to me or it would not charge and discharge so fast.

If the cells have screw terminals I'd suggest buying a couple of replacement cells.
If you have a single cell operating BMS, first swap the daughter boards to make sure your BMS is not awry or you will repeat the problem elsewhere.

Hoping that helps

[hohisilver]

Thanks guys, I didn't think a loose connection would cause such rapid charge/discharge, no doubt I will find out in a couple of weeks. At least they are screw tab Headways so (subject to sourcing a new cell or two) replacing won't be too difficult. The BMS is 25 on a board so no chance of swapping them around

[GregsGarage]

It doesn't suggest a loose connection to me or it would not charge and discharge so fast.

Problem is that the bms isn't actually measuring cell voltage, but rather the voltage at the cell terminals. This is not necessarily the same. I will explain. When charging the voltage from the charger is split between all the cells. If the resistance across each cell (including its connections) is the same then all the cells will split the voltage equally. However if at any one cell it's internal and/or external resistance is higher, then it will hog more voltage. If you don't believe me sit down and draw yourself a circuit consisting of a voltage source (your charger) and 10 identical resistors connected to it in series (this represents a battery pack). Choose any values you like for this exercise, for example a voltage of 10 volts and a resistor value of 10 ohms. Now work out the voltage drop across each resistor. In this case it will be 1 volt because all the resistors are the same. Now start increasing the value of only 1 of the resistors and see what happens to your voltage readings.

My point is that you have a high resistance at the problem cell causing the high voltage reading, but if the resistance is external to the cell than you are not actually measuring the cell voltage but the voltage across the connection. So it is impossible for the bms to tell you if the problem is internal or external to the cell, it can only tell you which cell to check.

Of course as Beemer pointed out the problem could be a false reading from the bms. Use a volt meter to confirm the cell readings, being careful of course as working on pack voltages can be lethal.

So do you order a couple of cells now? I probably would at least to have some spares.