Is it possible to connect batteries in series with BMS?

Question

Rechargeable batteries

There are 4 batteries, each 12V with its own internal BMS. Is it possible to connect such batteries in series (to get 48V)?

Answer 1

I assume that you mean lithium batteries - it is for them that BMS is primarily used. So, you can do this, but it is undesirable. If the BMS was only engaged in balancing the elements, then there would be no problem, but usually it also has protection against exceeding the charging / discharging current, and the settings for this protection in different batteries may differ slightly.
Imagine the situation that the protection worked in one of the batteries (the one that turned out to be set to a slightly lower current value) - it turned off, and the rest remained energized. This means that the open key (usually a field effect transistor) is energized by the other three - the circuit is purely serial. For a 12-volt BMS, the key is unlikely to be high-voltage - there is most likely a transistor with a maximum allowable voltage in the region of 25 ... 30 volts. But the rest give him a total of 36, and this can be fatal.
For this reason, it is highly desirable to have a single BMS for the entire battery - then at least there are at least some guarantees.