I want to put a second battery (for my radios) in my 1994 F-150 pickup
truck and connect it to the original battery (for the starter and
engine) with a relay such that the batteries are in parallel when the
engine is running (and the generator is presumably generating) and are
separate when the engine is not running. I did this with my previous
'84 F-100 pickup, but I can't find the "on when engine is running" wire
in my F-150. Does this wire appear anywhere under the hood?

--W0PBV.

On 06/30/2011 12:14 AM, Myron A. Calhoun wrote:
I want to put a second battery (for my radios) in my 1994 F-150 pickup
truck and connect it to the original battery (for the starter and
engine) with a relay such that the batteries are in parallel when the
engine is running (and the generator is presumably generating) and are
separate when the engine is not running. I did this with my previous
'84 F-100 pickup, but I can't find the "on when engine is running" wire
in my F-150. Does this wire appear anywhere under the hood?

--W0PBV.

I found one for my fog lights relay power, in the fuse box, mounted on
the driver side inner fender liner. I used a connection going to a relay
that is only on when ignition.

When connecting a second battery, for accessories, that charges when the
engine is on, but can't discharge the main battery, I always use
a the heavy duty RV diode block, with center connector fed by heavy wire
(4 to 10 gauge) that connects to the alternator positive post,
after separating that charging wire from the alternator.


Alt Pos. charging wire Battery
+ -------------/ /----------- +
cut or disconnect Alternator charging wire,

reroute Alt + wire to Diode pack Center Bolt,
Alt+----------------------|
Orig. battery +------| Post |-----+(new battery)

The two 150 Amp to 300 Amp diodes usually are in a heat sink, sold by
most auto and RV stores and online dealers, audio stores, or, he
http://www.hellroaring.com/

who promotes very low voltage drops, less than .0005 VDC!
Also sells Solid State Relays...