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any suggestions for a ... 12V DC watt-hour meter?

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Posted by danny burstein on July 8, 2006, 5:45 am

I'd like to compare the power used by my laptop

   a: a 12v ->120VAC (cheap) inverter, and then with
     a regular wallwart

   b: an expensive direct DC->DC convertaplug thingee.

The meter would go inline between the cigarrete lighter
jack and the first power cord.

Any suggestions? Thanks.

Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

Posted by Loren Amelang on July 8, 2006, 5:53 pm

On Sat, 8 Jul 2006 05:45:51 +0000 (UTC), danny burstein

At the computer's power input jack, there should be essentially no
difference in power usage. I assume what you want to know is which
route is overall more efficient, so you'd need to measure DC voltage
and current at your main battery - ahead of the DC->DC or DC->AC->DC
conversion. There are DC watt-hour meters for whole-house use, but the
load of one notebook computer will be practically unmeasurable with

The hard part of what you want to do is that a computer is a very
inconstant load. How much power it uses at any moment depends on what
it is doing - disk activity, screen brightness, how hard it is
"thinking", and whether it is charging its internal battery or not -
so it is almost impossible to duplicate a test with a different power

If your goal is to compare power supplies, find a resistive load that
draws somewhat the same current as your computer, and use it as a
constant test load.

In my experience, a DC->DC "car charger" is always more efficient than
running an inverter and a wall wart, and newer models of all of those
are better (run cooler, waste less power) than older ones. In general,
a lighter physical weight converter will run cooler and waste less


Posted by danny burstein on July 8, 2006, 10:08 pm

 [ snip ]

eyup, that's why I mentioned the meter would be between
the cigarette power jack and the first power cord...

 [ see below ]

I figured on doing umptity hours wth it. I know, for example,
that my Kill-a-Watt shows a daily average draw of (roughly)
ten watts for my laptop, with figures as high as 40 when
the drive is unning and the battery is charging.

hmmm... so you're saying if I put a one amp load (comparable
to my averaged out laptop - see above) i'd get a usable
comparison figure? Sounds good..

The input to my laptop (well, technically the output
from the wall cord txformer yoyo) is listed as 24 V, 1.875 amps,
while the wall side claims 120 vAC, 1.3 amps. Fortunately
those aren't the steady draws...

Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

Posted by clare at snyder.on.ca on July 8, 2006, 9:17 pm

On Sat, 8 Jul 2006 05:45:51 +0000 (UTC), danny burstein

You don't need to know watt-hours, or even watts, to know which is
best. All you need to know is current - use a standard ammeter.
Connect it between the cig plug and the power device.

Posted via a free Usenet account from http://www.teranews.com

Posted by JoeSP on July 9, 2006, 7:45 pm

I used my laptop on a fairly cheap Xantrex inverter, and there was quite a
bit of variation in load, depending what the processor was doing.  When I
loaded Google Earth for example, the variable-speed cooling fan "screamed"
in response to each graphical challenge, and it was basically silent when
not much processing was going on. This is because it's speed was linked to
the load being used.  It was a fairly good indicator of the variable current
draws during laptop use.  When the battery was being charged, it was running
full speed all the time.

I think a fairly good inverter will be over 90% efficient at peak load, and
a cheap one might be 50% efficient or less.  For your laptop, the difference
between a straight DC source and an inverter converting 12V to 115V and back
to DC again will be a considerable amount of waste heat produced at every
stage.  I wouldn't be a bit surprised if the power consumption is double at
best, and possibly 4-5 times higher than a direct DC hookup.

A 12V to laptop DC converter is probably your best bet for efficiency.

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