series connection of 18650 Li-Ion batteries with integrated protection

Thread Starter


Joined Apr 3, 2016
Dear all,
I recently bought five Li-ion 18650 batteries (
My idea was to power a 18V DC motor, so I simply connected the 5 batteries in series (5x4.2V=21V) without any additional circuitry.
The issue is the following: If I connect the motor to a workbench power supply, everything works (current consumption ~3A)
If I connect my Li-ion pack, the motor does not move at all and the batteries just get hot (one in particular VERY hot)
Afterwards, I checked if the batteries got damaged. The individual voltage of the batteries was 4.1V and the voltage of the pack was 20.5V. I then removed the battery from the pack and I recharged each battery individually. All the batteries seem to be ok.
I'm still puzzled why I was not able to power the motor. The description of the battery says that an overcharge/discharge protection circuit is integrated in each battery, but I was not able to find any datasheet about it. Maybe protected Li-ion batteries cannot be put in series? Or I am missing something else?


Joined Jun 22, 2012
I would say the battery current is at its limit, the motor will take more current at initial starting, yes you can series them up no problem.

Ps, is this the 18650 weekend, this is the 3 posting of these