No, no, not safe. Overcharged battery may explode because of overheating. why would you want to use batteries with different capacities? If your BMS is completely trustworthy, it will cut off charging when the smaller cell is full and discharging when it is empty, restricting the larger cell to the smaller capacity.
Can I make a 2-cell 18650 battery pack connected in series with different mAh capacities and charge it without risk (with a balancer, of course), or does it carry the same risks as a different-voltages battery pack (connected in series too)? No, no, not safe. Overcharged battery may explode because of overheating.
Sometimes multiple cells have their protection combined on a single associated board but per-cell circuit is provided as it is not safe or advisable (maybe that's "neither advisable nor safe") to use multi-cell protectors alone. The overall battery of cells may have an added layer of protection above the individual protectors.
I've found them for 1, 2, and 4 batteries only. My (limited) understanding of lithium batteries is that all batteries that will be used in a single device should be charged together so that the cells can be balanced, and that if this isn't done, it poses a fire risk.
For most li-ion cells, the standard maximum charging voltage is 4.2 volts per cell. As charging progresses, the voltage gradually increases until it reaches this maximum limit. At this point, charging should stop to prevent overcharging, which can severely damage the battery and pose safety risks. Part 2. Understanding discharging li-ion cells 1.
As we mentioned before, you must use a proper lithium ion/polymer battery charger. The good news is that nearly all batteries you will encounter are going to be 4.2V. And you can use a 4.2V charger for both lithium ion and lithium ion polymer.