I'm using a BM2S, i.e. Battery Manager board for 2 18650 li-ion batteries in series. This tiny board uses 2 FS8205A FETs for current/voltage upper/lower limits as a protection (board: http://www.ebay.com/itm/2S-3A-Li-ion-Lithium-Battery-18650-Charger-Protection-Board-7-4V-8-4V-HM-/401019234956 datasheet: http://www.ic-fortune.com/upload/Download/FS8205A-DS-12_EN.pdf).

So I connected this board to my 2 18650 batteries, the output is connected to a voltage reg. LT1529CT set to have an output of 6.2V.

The output of the LT1529CT is connected to a LED driver with 3 modes which draw 30mA, 110mA and 250mA.

The thing is that when I remove one of the 18650 from the battery holder and put it back the LEDs remain off until I (accidentally) create a temporary continuity between pins D12 and S1 of one of the FETs, then LEDs come on as expected.

enter image description here Can someone explain that to me and give me an advice on how to make the LEDs come on event after replacing the batteries?

  • 1
    \$\begingroup\$ Schematics missing \$\endgroup\$ – User323693 Feb 15 '17 at 5:19
  • \$\begingroup\$ I forgot to mention several things: the batteries are fully charged, the voltage at the input of the BM2S is 8.2V and at the output is 0V until I create the temporary continuity. \$\endgroup\$ – ClemRz Feb 15 '17 at 5:19
  • \$\begingroup\$ Here is the circuit @Umar (sorry for the ugliness): dropbox.com/s/pgv8kcfu09bj1ey/circuit.jpg?dl=0 \$\endgroup\$ – ClemRz Feb 15 '17 at 5:31

Lithium protection modules are usually designed to be permanently wired into a battery pack. When you remove one cell the protection circuit thinks the cell has become over-discharged, so it disconnects the output and waits for a charging voltage to be applied to the pack. Putting a charged cell back into the pack won't make the protection circuit 'wake up' because it thinks the cell is just recovering some voltage after being disconnected from the load.

To make the protection circuit wake up you have to apply an external voltage, or (as you did 'accidentally') force the battery voltage to appear on the output terminal.

  • \$\begingroup\$ Hi, thank you for your answer, it makes sense! Would it work if I add a 2 ways button on B+ and BM lines to "reset" the board when changing the batteries? \$\endgroup\$ – ClemRz Feb 15 '17 at 14:54
  • \$\begingroup\$ A momentary pushbutton wired between B- and P- might do it. But if you are removing the cells and charging them externally then the Charger Protection Board is largely redundant. And this unit's over-discharge protection seems very poor (2.3-3.0v±0.05v???). For long lifespan the cutoff voltage should be set to 3.2V per cell or higher. \$\endgroup\$ – Bruce Abbott Feb 15 '17 at 17:56
  • \$\begingroup\$ What 3.2V-cutoff circuit would you suggest @Bruce Abbott please? \$\endgroup\$ – ClemRz Feb 16 '17 at 4:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.