Automatic 3S LiPo charging for Robot-Vehicle

Thread Starter


Joined Aug 18, 2019

i am working on a robot-vehicle now for a while and all the "fun-stuff" (self-driving, face-recognition, control over wlan, motors, sensors, etc...) works fine but the biggest challenge (as it turns out for me) is the "Power/Charging/TurnON Management"...

These are the requirements:
After the robot has connected to the docking-station (a simple 12V 8A DC Power-supply) the robot detects a voltage on the external power rail and switch over from battery-supply (3 Cell 11.1V LiPo 5.1Ah) to external power.
If the battery is not totally full a charging device on the robot gets automatic activated and the battery gets loaded till the charging program is over.
At any time it should be possible to un-dock (charging gets interrupted and switch back to battery).
To turn the hole robot on, you have to push a push-button for a few seconds (this should work no matter which power supply is present: ext, bat or both)

This is my latest approach (see Schematics_01.jpg) (which dose not work):
I guess a solution with mos-fets instead of relays will be more elegant (and uses way less current) but for the 3(4) charging-lines from the LiPo i can't use them (at least no n-channel) and i head one of this cheap 8-Rleays-Boards lying around so i used this for all the switching (i know this sounds very amateurish -and i guess it is View attachment 184127.
For charging i used one of this: or this : it must be a device that needs no interaction with a human (buttons etc...) and can be run by DC.

The Problem:
Turning on, docking and un-docking (switching from one pwr-source to another) works fine but when the charger gets activated (R2 and R8 disconnect the main + battery pole from the circuit and R6 and R3-R5 gets closed to charge the battery [all delayed]) the charging starts normal but after aprox 2min magic smoke comes out of the charger and it is broken (i tested it 3 times View attachment 184128.
the charger is for me a black-box (i know how a lipo must be charged but i did not get in to the specific circuit of the charger) so maybe is the common ground the problem (?)...

Dose anybody has an idea why this problem appears or is there a "best practice" blueprint for this requirements out there?

Thx in advance for every hint!!!

best regards!



Thread Starter


Joined Aug 18, 2019
nobody any idea?
how dose a lawnmower-robot do this?
or dose anybody know some kind of battery-managmenet board/chip that can do this?
the last way i see for me is to use LiFePo4 Battery (more then two times the wight and volume) which makes charging much easyer