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.