On Mon, Dec 6, 2010 at 10:07 AM, RussellMc wrote: > > His wishful thinking nor vain repetition doth not a summer make :-) - > to badly mix my quotes. > > He has said that he wishes to use Eneloop AA NimH cells to charge an > iPhone, that he expects them to provide about 1.2V/cell at 1 A load > and that his desired endpoint is 1.1 V/cell. > > He may have arrived at the 1.2V/cell at 1 A figure from datasheets et > al - and maybe not. Sanyo's webpages suggest > > http://www.eneloop.info/home/performance-details/voltage.html > > that at 2A he can expect 70% of capacity at over 1.2V/cell and that > the cells will drop under 1.1 V/cell at about 90% of rated capacity. > SO the batteries meet his spec reasonably well. > > If he's careful and somewhat magic he MAY be able to achieve 0.2V drop > outside the cells (wiring, battery connectors, electronics, output > connector, ...) and as IPhone is made by Apple and so must be magic it > can probably manage 0.2V drop as well. > > So at 4.8V battery voltage you get about 4.4V available for LiIon > charging (enough) but you hit 4.2V for LiIon at about 4.6V battery =3D > 1.15 cell, which the eneloop graph says is well up the discharge curve > - perhsp 80% - 90% of total capacity, which is highly commendable. > > If he or Apple blink and you get more than 0.4V total drop at 1A from > his battery pack to the LiIon pack then "there will be problems" > [tm][probably]. > > I feel (rightly or wrongly) that my original analysis stands re 4 x AA > NimH cells (even Eneloops) as being marginal but usable for USB > charging apps. I'd personally use 5+ cells, or a boost converter, if I > wanted a fully designable result. I suspect that you would too, on > reflection. > I was actually looking at this: http://www.eneloop.info/home/performance-details/discharge-current.html Specifically, the 1 Amp discharge curve. Anywhere between 1.1 and 1.18 volts/cell seem to be a good value according to that graph. I want to keep it *just* before the sharp drop in voltage. I'll probably stick with 1.15 volts/cell for now and adjust it later if I'm not satisfied. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .