Date: Sun, 7 Jul 2013 18:33:22 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Vitrifrigo temp data
In-Reply-To: <FA7EE82BC4E24292BF807A4E94EB4660@troya3a1e48888>
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At 05:56 PM 7/7/2013, Troy wrote:
>Knowing that my 125 amp hour battery is capable
>of delivering about 65 or so AH's,(50% battery
>depletion) I know I can run my fridge for about
>three days before the battery needs charged, and
>that's the most useful information for me. I
>realize this is a very simplistic approach, and
>other variables apply, but I think it's good
>enough for general calculations. Do you agree?
Yes, I do.**
**With the caveat that as soon as you start
talking about batteries everything gets
complicated, and how much you depend on your
batteries will to some extent control how deeply
you have to get into the complications. For
example, a compressor fridge has a startup pulse,
and also very likely has current pulses that
occur several times per motor
revolution. Because of these peak currents such
a device will deplete your battery faster than a
straight resistive load with the same duty cycle,
and either one will deplete the battery faster
than a full-time load that averages the same
consumption. In general the faster you take the
juice out the less the battery can supply.
These things happen because of the way the
battery chemistry reacts to the magnitude of the
instantaneous load, and your Watts Up meter may
or may not (probably does not, unless it has an
input for the Peukert coefficient for the battery
you're using) take these things into account, or
may not do so quickly enough to account for
millisecond peaks. This is what you pay the big
bucks for in monitoring equipment.
>The meter also reads out in watt hours, and a
>few other things as well. For anyone interested
>in the watt hours, it's reading 275 Wh's. over almost 24 hrs at this point.
Watts are amps times volts and watt-hours are
amp-hours times volts. So assuming 12.0 volts,
that would be ~23 amp-hours, or just under one amp-hour per hour.
> I guess that amp hours is something I
> understand a little bit better, as the battery
> is listed in amp hr capacity. The deep cycle
> marine battery I have for example is a 125 amp
> hour battery. Using 20 amp hours of that is
> something that I can relate to. I don't know
> how many watt hours the 125 amp hour battery
> contains, and I guess that depends on voltage according to your description.
In theory, at 12.0 volts, about 1500 watt-hours.
> Fridge is now at 26°, so guess I need to
> change the settings. Amazing little unit I must say...
All they need to compete with LP absorption
fridges is an energy source as dense as propane...
Yrs,
d
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