Electrical – FS8205A won’t kick in


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.

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

Best Answer

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.