Yesterday I've disassembled the vacuum cleaner and tried to connect the new battery pack.

My first try was a combination of 2 18650 cells in a cheap battery holder with a 10A battery protection board ( https://www.aliexpress.com/item/32920842556.html ).

It was not possible to start the motor, the short circuit protection immediately kicks in and cuts of the power.

Next try:

20A board. There are 30A boards as well, but only for 3S, which would be too much for the motor (original voltage: 9.6V; 2S: 7.2V; 3S: 10.8V).

https://www.aliexpress.com/item/32863604994.html

with a balancing circuit (~2$).