[maemo-developers] [maemo-developers] Re: Measuring power consumption of 770

[From: Heike C. Zimmerer]

Frantisek Dufka <dufkaf at seznam.cz> writes:

> Hello Igor,
>
> can you comment on idea mentioned in this thread about charging the 
> battery fully and then measuring DC current from charger? Is this 
> nonsense or not? Is the battery used when already charged and device is 
> still connected to the charger? I thought most laptops and PDAs work 
> this way i.e. disconnect battery and use only power from AC adapter.

It's not as simple as you may think to detect a fully charged battery.
The usual approach to do this reliably is to detect temperature rise
at the end of the charge cycle.  It cannot be done from voltage and
current alone.  The effort required barely justifies what can be
gained from, at least from a marketing perspective.  Environmentalists
my feel very different about that.

The extra circuitry simply isn't there (the 770's documentation
recommends taking it off the charger when battery is full, which
wouldn't make sense otherwise) and I don't know of any mobile or
laptop which does some real detection.

Even if it were, it wouldn't make much sense to measure at the mains
plug if you can do at the battery.  It's the Nokia's power consumption
itself the original poster asked for, not its charger's.  The charger
itself won't be optimized for minimum power drain.  You can't even
expect a roughly linear correlation between the charger's input and
output power.

To put things straight: If you measure "charger plus 770", you measure
exactly that, nothing else.  All assumptions you draw from it about
the Nokia's power consumption alone are worthless without knowing the
charger's part.

It's a simple rule for any measurement (in whatever domain) to measure
as near at the object as fesible (any dressmaker knows about that).
After all, it's just any cheap multimeter and a pair of wires what's
needed to get useful results.


Heike