Robot automatic LiPo Charging

[sepl1977]

Hi!

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 .
For charging i  used one of this: https://hobbyking.com/en_us/turnigy-12v-2-3s-basic-balance-charger.html or this https://hobbyking.com/en_us/hobbykingr-dc-4s-balance-charger-cell-checker-30w-2s-4s.html (i killed them all : 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 .
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!

sepl

[sepl1977]

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

[OM222O]

simplest way to automatically switch from battery to external brick, or vice versa is to use "ideal diode" or "power OR ing" ICs.

if you want to disconnect the battery, you also don't need 3 / 4 mosfets. just disconnect the ground using an nmos. then the +11.1V wire will be "floating" and can't cause any issues.

if you use ideal diodes, then right after you disconnect the external brick, the battery takes over automatically.

Something like the LM5050 should do the trick for you:
http://www.ti.com/lit/ds/symlink/lm5050-2.pdf

the LM5050-2 even lets you test the fet in circuit with an MCU (or manually). just be careful which fet you're turning on and testing, because if both power sources are connected and you activate the fet on the lower voltage supply, you'll be shorting the high and low voltage supplies together, releasing the magic smoke!

To fix the charger board, just replace T1 with a SO-8 dual mosfet. I'm not sure if it's N channel or P channel from your picture, but you can check the burn part if it has markings, otherwise look on the seller's page or ask from others who have it.

[sepl1977]

thx so much OM2220! i never heard about an "ideal diode"... i will give it a shot...
and thank you for the repair hint and battery-isolation tip!
best regards!