[kennybobby]

Howdy Paul, welcome to the forum.

Do you have a handheld voltmeter with which you could measure the voltage of each cell? If so then make a list of the readings while at rest (with no recent charging or discharging event), and post it up here so we can see how they align.

Some TCCH/Elcon chargers seem to push the voltage too high, not sure if it is a calibration or software issue, but the result is chronic over-charging that in time causes cell imbalance such as you have experienced. Hopefully your BMS has limited the charging such that no permanent damage has occurred.

Depending upon your cell imbalance it may be possible to individually bleed off the high cells and resume operation with a modified procedure due to the defective module.

 

[kennybobby]

No need to worry about ruining cells by not charging to 100%, on the contrary, over-charging will definitely ruin cells.

The resting or open circuit voltage of LiFePO cells, when fully charged is about 3.33 V, possibly +/- some few mV depending upon slight chemistry differences, according to reports published by cell vendors, and measurements by users. So it appears that your cells are nearly full, i.e. 100% State of Charge.

To get to that state the charger must lead the voltage higher, e.g. 3.6 V, in order to create a potential difference for current to flow. The elcon procedure is usually CC/CV until current drops to some level, but that is where they tend to extend the final charge too long and over-charge.

Since you don't require an entire cell's energy in order to mow your yard, in the interest of cell longevity, it might be that you would want to limit your charging to 90-95% full SOC, e.g. 3.2 to 3.25 It will not ruin the cells, but extend their life and usefulness to not charge to 100%, plus there is always the danger of over-charging if something doesn't cut off properly due to a faulty resistor or capacitor.

Over-charging will ruin your cells--that has been discovered by many folks who have needlessly pushed to the upper limit. Give yourself some margin, both top and bottom, and they should last a long time. Obviously you have been doing something right to be on your 4th season.

Next time you mow, measure each cell OCV and write it down, and compare with the voltage reported by the BMS. Especially watch the problem-child cell to see if it is still higher than the rest. The BMS can be a good tool to help you monitor and care for your pack, but it needs supervision--trust but verify. The electronic components in the BMS are less reliable and more prone to failure than the cells, so you must be vigilant with a good voltmeter to ensure it is healthy.

In an ICE car you don't fill your tank to the brim-- trying to get those last few drops in can lead to sloshing out the spigot and wet pants. And you don't run your car to bare empty. Your cells are your energy tank, fill to 90%, run it down to 10%, and you will have long life from the cells.

If one or more cells is out of balance with the rest, then it can be either charged or discharged separately to make adjustment. Even a small phone charger can be used as a power supply to add charge, and a suitably sized power resistor can be used to bleed off excess charge. I'm not familiar with your BMS or Link 10 to know if they have such a feature built-in, but some do.

 

[kennybobby]

... Do the individual cells need to be disconnected from series connections to get an accurate volt reading on each cell?

No they can be measured without disconnect. To get an accurate reading, it is better to have no significant current load drawing on the pack when making the measurement. BMS systems measure individual cell voltages without disconnection, and you can too.