AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Graham Davies" <YahooGroups@e...> 
wrote:
> --- In AVR-Chat@yahoogroups.com, "C Sizer" <aussiecol39@y...> wrote:
> 
> > ... I want to use 4 x 2300mAh AA
> > batteries in serial/parallel to
> > provide me with 3V.
> 
> Sounds like alkaline cells at 1.5 volts each.
> 
> > The printer runs on 5V but consumes
> > over 3.5A peak, so I require a DC-
> > DC converter circuit... to provide
> > 3V at 5A peak.
> 
> You mean to provide 5V/3.5A, which will suck six amps or more from 
> your 3V battery. I don't think this will work. The internal 
> resistance of alkaline cells will be too high and your 3 volts will 
> sag something terrible.
> 
> Like everyone else, I suggest you switch batteries.  Use four NiMH 
AA 
> cells to get 5V and regulate down to 3V. I don't think you'll need 
to 
> buck down, just use a linear regulator, unless you're taking a lot 
of 
> current at 3V. Although the capacity of the NiMH cells won't be 
2300 
> mA.h (I use 1800 mA.h cells but I think now you can do a bit 
better) 
> I think they will actually last longer. This is because the 
alkaline 
> cells will never give you full capacity at such a high discharge 
rate 
> and you're avoiding power loss in the boost converter. High quality 
> cells will be able to keep the voltage up because NiMH cells have a 
> low internal resistance. For example, the 7.2 volt 6.5 A.h modules 
in 
> my car have an internal resistance just a bit over 10 milliohms. I 
> can get 60 amps or more from the 300 volt battery and the voltage 
> stays up at 275 volts.
> 
> Graham.

Thanks Graham 

yes, you are right, I did mean for the converter to output 5V.

As this is to be a hand held device I was going for light weight, and 
reasonable power consumption. The thought was to connect the 
batteries in 2 pairs of 2 batteries in series (to give 3V) then 
connect these in parallel to increase capacity and reduce the drain. 
I had not considered the internal resistance issue, but of course 
NiMH will only give me around 2.4V in this configuation. Now that I 
think it out, there isn't any way I can get the sort of life out of 
the batteries I was after.

Looks like it's back to the drawing board for me :-)

Cheers
Col