The Easiest and slickest dual battery installation is on Gen3 trucks.
on a 1993-94 it's literally "child's play" to install a 1990-92 Ranger or 1991-94
Explorer battery tray and mounting bracket in the big empty hole on the passenger side.
you can get away without an "isolator" if both batteries are identical
(same make, size and condition) if you want a backup battery that is
normally isolated you'd need an isolator, however the insertion of an
isolator that prevents the two batteries from "seeing" each other also
causes a voltage drop at the battery typically 1.2Volts
Unless you buy a Rather expensive one that uses Schottky diodes
(which cause far less voltage drop, typically 0.7Volts)
The other issue is that the isolator you get must be capable of handling
your maximum alternator output, the "little" isolators that you see everywhere
with 1/4" threaded studs are usually only rated for 70amps.
the isolator I have sitting on my lap (literally I went out into the shop to grab
it so I would spell "schottky" correctly) has BRASS 3/8" studa and is rated for
140amps, that's 140amps Continuous... this bugger is huge it's literally the size of a loaf of bread
It looks like it's made on the same aluminum extrusion that the little three stud isolators
are made on the difference is that the studs are 4" apart and the piece of extrusion is
long enough that if I sawed it up to make the "little" ones I could make four of them out
of this 12" long chunk of aluminum.
How expensive was it? I have no idea and I'm actually kinda scared to find out...
I got it out of an ambulance in the junkyard.
Now, you've got your second battery installed and isolated so it charges when the
alternator is spinning, but how do you get the power out of it?
For that you either need to open the hood and jump-start yourself with a couple of
jumper cables (actually only one as it would be presumed that you'd connect the
negative of both to ground)
OR the much more "elegant" solution is a big relay (a little starter relay won't cut it)
and a switch to turn it on, and lastly a BIG BRIGHT yellow, orange or red indicator lamp
to remind you that it's switched on to remind you to turn it off again.
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