Many of the answers from the prior LiIon charging threads apply. Current needs to be limited, of course, to the maximum allowed by the batteries used. Zeners are nowhere precise enough for balancing. A reference voltage controlled clamp (super zener) would be better. eg a TL431, opamp sectopn, clamp transisto and "glue" per cell. Small % of battery cost. If running at 4.1V you MAY get close enough to allowable termination voltage at some temperatures that charging termination is needed. (Cells charged above a certain voltage need to have charging stopped as charge current falls to some fraction of Ichgmax. Lowering Vcellmax to 4.0V would probably [tm] eliminate the need for this. You can get multi-cell balancing ICs BUT the above is easy, cheapish and would work OK. An easy termination system would be when all cell clamps are set to say 4.1V and all are clamped. With only 1 or 2 clamped charging continues. Russell On 18 June 2011 01:04, V G wrote: > > Hi all, > > I want to build a small battery pack (self contained unit with charger an= d > variable output power supply). The charging will most likely be from an > input source of 5V (either 500mA USB source or 2A supply, but it shouldn'= t > really matter since it's all going through a switching supply). The batte= ry > configuration will be 3 x 18650 Li-ion in series. The 18650s have a built= in > "protection circuit". I want to float charge these at around 4.1V, so is = it > just safe to line the batteries up in series and apply a steady 12.3V? Wo= uld > I need 4.1V zener diodes across each of the batteries? > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .