Date: Tue, 26 Aug 2014 05:51:58 -0700
Reply-To: Brett Ne <brettn777@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Brett Ne <brettn777@GMAIL.COM>
Subject: Re: battery question
In-Reply-To: <20140826065520.2XEPL.46095.root@eastrmwml105>
Content-Type: text/plain; charset=UTF-8
I just finished testing my Dometic 215B yesterday after chasing down some
gremlins in my shore power 12v transformer. The fridge started at 82
degrees in the afternoon and gradually cooled down. By morning, it had
dropped down to 36. By the following afternoon, the fridge had raised to
46 degrees while the outside temp was 90 and the cabin temp was 97. This
was all done running off 12v with shore power plugged in. For monitoring
fridge temperature, you really need a remote thermometer installed; the
human hand is poorly suited for measuring temperatures(other than a child's
fevered forehead) and every time you open the door, the cold air flows out
and is replaced by warm air, further taxing the meager cooling capacity of
the fridge.
These fridges take a while to cool down, so if your trip was only 2-3
hours, you won't see much cooling. I usually try to run the fridge off
propane or shore power for several hours before a trip to pre-cool it. A
lot of people think that the 12v mode can't effectively cool the fridge
because they don't understand that the ammonia cycle fridge behaves
differently than a compressor fridge. It takes at least 30 minutes of
heating before the complex cycle reaches a state where it will start
cooling the fins, and if heating is interrupted for more than a few
minutes, it has to start all over again. Most systems will cut power to the
fridge when the engine is shut off. So, if you drive for 8 hours and stop
for gas, stop for lunch, stop at the store for a six-pack of camping
supplies, and stop for a quick tour of the string cheese museum, your
fridge may only be cooling things for 3-4 hours because of all the
interruptions in the ammonia cycle. There is a nice write-up of this
available on the interwebs called "Reefer Madness" that is a must read. I
have my fridge wired so that it will run off battery power even with the
engine off to avoid cooling interruptions. If I'm going to stop for more
than 45 minutes or so, I switch it over to propane, to avoid deeply
discharging the aux battery.
Since you're getting 13+ volts on both batteries with engine on, and a
difference in battery voltages with the engine off, we can deduce that the
alternator is working and that whatever battery isolation system you have
is also working. If your aux battery is dropping .26 volts in only 1
minute with a 10 amp draw that is typical of these fridges, that is too
much of a drop and suggests that your battery is getting old and tired.
Standard automotive batteries are not designed for discharge cycles, so I
wouldn't expect one to last much more than 3-4 years when used as an aux
battery.
Brett in Portland, OR
"Albert" '82 VanaFox I4 Riviera
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