Re: [A/S] '64 Globetrotter 19' electricals

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Chris,
 I made a test light many years ago, using a tail-light socket with a
large alligator clip soldered to it. There's about 36" of test prod wire
with a smaller clip on the end, and a turn signal bulb in the socket.
The latter draws over 1 amp, much more than a typical "test light". I
typically clip the socket to a ground someplace, test the connection by
touching the lead to a known "hot" terminal, then clip it to the
terminal under test. The light is bright enough to see from quite a ways
away.

Leave your fridge so it can be powered by 120VAC or gas, don't change
the heater to 12VDC or bother adding that heater. Here's why:

The 12VDC heater on a fridge will draw several amps, probably 10, maybe
more. This makes it totally impractical to operate on 12VDC except when
on the road, and it might not work well there either because of the long
wiring. Besides, why load your vehicle's alternator? And, you don't want
the load on your converter while connected to shore power, it's better
off just using the 120VAC directly. If you feel you must run your fridge
while travelling, use the gas.

I got that isolator relay from the local NAPA store. Dunno what it cost,
but I'm sure it was less than a typical diode-style isolator. Also,
you've proved my point about voltage drop in the latter, if it's getting
hot, it's dropping voltage. No voltage drop = no heat; my relay doesn't
get hot.

Your diagram looks good. It does get a lot more complex in the details,
but you have to start someplace.

Apparently, you're planning on doing it right, so I'd like to suggest
you install an eMeter. They're sold under several different names
("Link" being one) by several different companies. Although the eMeter
is a bit of overkill, it's really nice if you're into serious
boondocking and are relying on your battery(s). The thing is a small
computer that tracks the voltage, current, and amp-hours. The latter is
tracked "out" and "in" from/to your battery. It also takes into account
the rate of usage when calculating how much capacity is left: A 200 AH
battery bank will not supply 200 amps for one hour, if you're drawing at
that rate, the life will be quite short. OTOH:, if you're drawing 5 amps
of current, you'll get close to the 40 hours of usage calculated. The
eMeter takes this into account; I think it's called "Puekert's
Equation".

The eMeter requires a half-dozen or so light-guage wires between it and
the battery. A 500-amp shunt is installed near the battery and all
current goes through it. You can see my control panel at
http://www.oldengine.org/members/jdunmyer/avion/sep01_07.jpg; I have a
piece of 8-conductor 22 guage cable running from the meter to the
batteries, a distance of about 25 feet. There's a DB-9 connector pair
inline so I can easily remove the panel from the cabinet above the
fridge.

                                            <<Jim>>