>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 01 July 2005 10:58
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE]: How to efficiently run a PIC from 12V battery?
>
>
>
>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

Not really a fair comparison though, i.e. switcher vs. linear.

>From the Energizer web site, and ignoring voltage drop over life:

1x Alkaline PP3 625mAh  Total energy = 14.4 Wh
2x Alkaline AA  2850mAh Total energy = 8.55 Wh

A PP3 has a significantly higher energy density than 2AA's, so with a suitable buck regulator it may last quite a bit longer than AA's dependant on current draw.  The downside is cost, 9volt "block" batteries such as the PP3 are significantly more expensive than AA's.

Regards

Mike

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