I have a 4-cell 18650 battery pack that currently has LG M26 INR (Li-ion) cells rated at 2600 mAh and I want to replace those with LG MJ1 INR cells rated at 3500 mAh and having the same voltage and discharge current specs (max. 10 A).
The battery pack is using an integrated BMS based on a BM3451 TJDC-T20B. I don't know how the rest of the charging system is designed.
Can I simply swap the new higher-capacity cells in? I don't want to be reckless, especially when it comes to Li-ion cells.
The new cells are electrically compatible with the old ones and will work just fine. In fact, they feature a lower permissible discharge voltage, which makes them even better suited for operation with the BMS you have.
You can go ahead and swap them right in. However, keep in mind that you need a spot welder and nickel strip to attach to the cells safely. Don't solder directly to the cells.
Also, it might be a good idea to characterize the cells beforehand and select ones with closely matched capacity.
The only problem you might run into is over-charge protection tripping since the charger might not expect the higher capacity, but this is not a safety issue. You'll just have to try it out. (Some chargers refuse to push more charge into a battery than they expect.)
