> >> ... =A0what is being proposed here in that with 4 AAs
> >> there is no need for the boost converter, ...

> > For some values of "no need".
> > Lithium Ion cells with have ~=3D 4.2V terminal voltage.

> The OP has mentioned several times that he is using a specific model of N=
iMH
> cells. =A04 of those in series does work out just about right over the va=
lid
> range of USB voltages.

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.


             Russell

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