Of course, you can already get ICs that will do all this for you ...
IIRC Linear Technology has them, but I am sure I have also seen them mentioned by another company, possibly TI or Analog Devices.
> > In the R-Pi E-14 forum someone wanted to know if there is such a
> > as a series connected lithium battery charger.
> > I suggested a module made up of a lithium charger IC, a 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 ~= 2A for a string of 18650 cells.
> So Vmax = 4.2 x 10 = 42 V. I max = 2A.
> Assume cells are all about fully discharged with Vcell ~~= 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 = 4.2V x 2A = 8.4W = 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
> It would work :-).
> Cost of the smarter system is two MOSFETS plus simple electronics per
> Simpler system = 1 MOSFET, TL431, few R's per cell.
> Russell McMahon
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