LiFePO4 cell “races” up towards the end of charging

If you are keeping a close eye on the BMS app during the final stages of charging, you may notice that one or more cells tend to go up faster than the rest. Seeing this can appear alarming, however, it’s perfectly normal.

Having the advantage of peeking inside the battery with Bluetooth raises a lot of questions and this is a common concern.

First, realize that 3.4V per cell is considered to be the “full” voltage of a battery cell or 13.6V for a 4 cell pack. There is very little capacity above 3.4V. In fact, there is less than 1% of additional capacity to be gained between 3.5V and 4.2V per cell.

There is a misconception that 14.2V must be reached to be considered full. 14.2 to 14.6V is what most charger manufacturers will set their chargers to. This number comes from the max that 4 cells could potentially reach if they all went to the accepted absolute max voltage of a LiFePO4 cell of 3.65V. (3.65 x 4 = 14.6). The technical max is 4.2V, beyond that excessive heating and damage will occur.

So then, why does one cell “race” in the App?

Because, at some point over 3.4V, at least one of the cells is going to be at 99%. And that last 1 percent of capacity exists from around 3.4 to 4.2V, as that last bit is pushed in, it appears to be “racing”. This can also be seen on a graphical representation of the LiFePO4 charge profile, which is generally referred to as a “flat” charge profile. Imagine watching that last portion, beyond the “knee” as it was charging, it would appear to be “racing” on a meter or in the app.

So, are the LiFePO4 cells matched?

Yes, the cells are matched for internal resistance. You’ll notice as soon as the batteries begin to discharge the cells will typically come back to within .005V of each other and stay there until you get close to 3.0V (empty). Towards the bottom of the charge profile, one cell will again “race” through the voltage range as it loses its last 1-2% of capacity.

Will the top-balance get better over time?

Yes. the top balance will get slightly better over time but probably never be perfect.

So, what is acceptable?

You’re paying for watts when purchasing a battery, so as long as you are getting the rated capacity, you should consider that acceptable. Conducting a simple LiFePO4 Capacity Test with a $20-40 device would show you if your battery is meeting its rated capacity.