AVR-Chat

On Jul 31, 2004, at 9:13 AM, C Sizer wrote:

> Hi Cobb,
>
> I probably didn't explain myself too well, plus my typo where I said
> I wanted 3V from the converter, I really meant 5V.
>
> I have 4 batteries (this is the maximum room I have in my case). The
> printer runs from 5V and consumes 3.5A peak. The idea was to connect
> 2 pairs in series to give 3V, and then connect the 2 x 3V cells in
> parallel to double the capacity.
>
> As this is a hand held device I do not want to connect all 4 in
> series and use a regulator to bring it down to 5V, thats a waste of
> power as I want maximum operating time from the device

So a horse of a different color...

Using a DC-DC converter relieves you from worrying about how to connect 
the batteries.  Pick the right converter topology and go with it.  If 
you use NiMh or NiCd, you may not need the converter at all if you 
connect the batteries in series.   Charging the batteries in anything 
other than the series configuration will be harder too.

You still haven't said what your average current drain is when not in a 
standby condition.

> Others have raised the issue of internal resistance which I had not
> considered, and suggest NiMh batteries which would mean I would have
> only 2.4V max to boost to 5V.

Then don't connect them that way...   Add a boost converter if you must 
to cover the low voltage case of 4.4V when the batteries are drained, 
but it's probably not worth the bother.  NiCd batteries have less 
internal resistance if you can find them.

Mike