[s_tec]

This isn't the charging circuit - that goes in the charger. This circuit is responsible monitoring the state-of-charge (for that little LED bar graph on the front), disconnecting the cells if something goes wrong, and negotiating available current with the tool. It should also be responsible for cell balancing, but it looks like Milwaukee forgot to implement that feature (oops).

The videos at the bottom of the article have most of the details, since those dive into the communications protocols as opposed to the raw schematics.

[Saris]

Their M12 batteries don't have balancing (or a BMS inside), so they go out of balance and 'bad' very quickly. I've just added a balance plug to the outside of mine that I plug into my hobby Li-ion charger.

[zippergz]

I believe you on the technical details, but as an anecdote, I have M12 batteries that are 10 years old and still working fine. At least, good enough that I have noticed no issues with them and I don't even know off the top of my head which of my batteries are newer and which are older. I also have a bunch of M18 tools and batteries, and I've noticed no particular difference in how they age compared to the M12 ones. But I'm just a DIY homeowner, so my usage is relatively light.

[nicolaslem]

Fun fact, DeWalt 18/20V batteries also don't have balancing. The batteries have pins going to each cell for it but no chargers actually use them.

[glitchc]

There are always two charging circuits for Li batteries, one in the battery and the other in the charger. They pair together to negotiate the voltage and current. There's a dedicated protocol to do this.

[teraflop]

This is absolutely not "always" true. There are tons of 18650 cells that have no electronics whatsoever, and there are tons of dedicated charging ICs that connect directly to cells/batteries.