On Jul 1, 2005, at 2:34 AM, Jan-Erik Soderholm wrote:

> Now, Let's say we have a 12V/100mAh battery and one
> 4.5/200mAh battery. Now, the 4.5V battery *seems* to have
> double capacity (which most equals with "lifetime" under
> similar conditions). But, if using a well designed step-down
> converter, will that not draw *less* current from the higher
> voltage source, then the target is using ?

Yes.  For a suitably "well designed switching step-down converter",
which is more than trivial to design.  ACTUAL battery capacity is
in Watt-hours, or V*mAH.  But your circuit will always be most
efficient if you can operate directly off battery voltage, with
no regulators at all, and you need to consider the whole circuit
if you're aiming to maximize battery life.  I think it was linear
technology who pointed out some time ago that running 2 AA batteries
to a 5V step-up regulator provided MUCH better capacity in "about"
the same space as a 9V battery with a 5V linear regulator...

BillW

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