The "design capacity" is not terribly useful with batteries, since your
battery may have either more or less capacity.

The battery controllers I've worked with have "design capacity", and
"maximum capacity", where maximum capacity is determined by the on-board
micro.  I'm not sure exactly how they track the maximum, but I believe
it is pretty much the "last full charge".

Note that with new batteries, the battery often can charge beyond the
design capacity.

In any UI, I'd provide the design and actual numbers as real numbers in
an "advanced info" dialog, but I would show the relative charge to
"maximum capacity" (last charge) in any simpler GUI.

    -- Garrett

Daniel Carosone wrote:
> On Tue, Jun 20, 2006 at 01:57:12PM -0400, Steven M. Bellovin wrote:
>   
>> 	This does pose an interesting question about what battery
>> 	status tools should display.  sysutils/asapm, which has
>> 	been patched to use the envstat interface if available,
>> 	gives battery percentage relative to the design capacity.
>> 	apm (and Windows) seem to give it relative to the last full
>> 	charge.  It isn't clear to me which is correct.  
>>     
>
> (This is very much a UI design issue, of course). 
>
> Of the two you describe, I think the Windows behaviour is likely more
> useful.  It does hide the battery degradation over time, but users
> would be likely be confused if they saw the battery was failing to
> fully charge no matter how long they left it, without some other
> indication.
>
> What might be best is a battery bar display that shows the design
> capacity on the full scale, and marks the difference between that and
> the last full as "inaccessible" (eg, in some other colour), and then
> the normal bar in the remaining space.  Perhaps clicking on it or
> hitting some other 'zoom'-like widget could toggle between showing
> this, and narrowing to just the last full range.
>
> All that said, though, I have my doubts about the accuracy (or, at
> least the practical interpretation) of these figures for real run
> time, even at constant draw, especially once batteries are older.
>
> --
> Dan.
>   


-- 
Garrett D'Amore, Principal Software Engineer
Tadpole Computer / Computing Technologies Division,
General Dynamics C4 Systems
http://www.tadpolecomputer.com/
Phone: 951 325-2134  Fax: 951 325-2191