Of course it can be charged separately. In fact, this circuit of using (discharging) in series and charging separately is not uncommon. Almost all series-connected lithium battery circuits will use this method. The reason is that lithium batteries are very sensitive to charging voltage. Slightly higher voltage will damage the battery. Once one section of a series-connected battery is damaged, the rest will also be affected. Eventually the whole thing could be scrapped.
It can be charged, but it is not good for you to use it.
Although lithium battery charging specifications do not prohibit series charging of batteries of different capacities.
However, the smallest ones with actual capacity will be fully charged first. Once the smallest ones are filled, the charge will automatically cut off. The older ones will never be completely charged. , so the total capacity will decrease.
Example:
If one 1000 mA and six 2000 mA are connected in series, then the total capacity is 7000 mA, which wastes 6000 mA.
The battery protection plate is necessary, otherwise the battery may explode