> In the R-Pi E-14 forum someone wanted to know if there is such a thing as= a > series connected lithium battery charger. > > I suggested a module made up of a lithium charger IC, =A0a switching > mechanism that could isolate those batteries in the chain (envisaging no > more than 3) once they've charged and some form of electronic load > (probably a FET) ... > Does this seem workable in theory? Yes. Almost anything sounds workable in theory :-) I've thought of simple ways of doing this One such follows. This can be complexicated as desired Assume all cells are roughly similar in performance and that gross mismatch does not occur. In particular. no cell should ever need to be below Vmin_usual so that it does NOT require slow trickling up beforenormal charging can start. This condition can be accomodated but is not covered below. Each cell has a simple shunt regulator placed across it. This can be as simple as eg a TL431, a transistor and a few resistors. Regulator is set to Vmax - say 4.2V. Cells are connected in series. Let's have 10 cells to make numbers easier. Provide 4.2 x N Volts current limited to desired charge rate. If charge rate is C then I is Ichg is ~=3D 2A for a string of 18650 cells. So Vmax =3D 4.2 x 10 =3D 42 V. I max =3D 2A. Assume cells are all about fully discharged with Vcell ~~=3D 3V Apply above supply to string of cells. Initially cells will charge at 2A. When any cell reaches 4.2V its shunt regulator will hold the cell voltage at 4.2V, the cell will take what it wants in constant voltage current tail mode and the balance will be passed by the shunt element. Dissipation per shunt =3D 4.2V x 2A =3D 8.4W =3D say about 10W/cell. Individual electronics modules COULD signal that they are in CV mode but some deduction and a little playing would allow the all-done state to be detected. Current tail off and terminate could be added by connecting a series element between cells and a shunt as before from top of cell to below series element. Shunt reg works as before but when shunting current rises to say 75% of max then series element is opened - disconnecting cell and shunt goes short. This removes power dissipation in charged cells. Shunted current is sent to next cells and smart smps charger backs off voltage to suit demand. When all cells are charged the needed voltage is N x short circuit shunt voltages and charger auto terminates. It would work :-). Cost of the smarter system is two MOSFETS plus simple electronics per cell. Simpler system =3D 1 MOSFET, TL431, few R's per cell. Russell McMahon --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .