How to make a cheap isolated dual-battery setup for $50

[dwh]

one question is if I hook up a winch to the starting battery is there something I should do while winching to protect the second battery in some way -

Well, as pointed out by Karma's post, you can add a switch to manually disconnect the solenoid and isolate the batteries so that the winch is running only from the starting battery and alternator.

[jjstahl3]

one last question it that I have a 70 amp output alternator and wonder if that will be enough to charge 2 batteries?

[dwh]

Actually, it would be too much if it actually pumped 70a into the batteries. Luckily - it won't.

Depending on the batteries' construction and their internal resistance, they might never allow more than 10a or 15a peak to flow through them at the voltage that your voltage regulator is set at.

[Southwest Chuck]

[QUOTE=evldave;1106209] ..............

• I've upgraded the 'jump start' solenoid to 200A. This gives us plenty of extra capacity to flow current if we want to use the solenoid to jump start our truck instead of using jump cables (lazy bastard). Better yet, order the way good isolator and get 500A!
• I also upgraded the fuses to 200A. You know, because if I didn't do that I might blow a fuse or two (depending on the voltage difference of the batteries, you were paying attention earlier, right?)
• I also upgraded my wiring to 2GA. Remember, always overcompensate for the size of your...wire! We could have probably survived with 4GA, but what if we want to put in a winch later? hmmm, 2GA would likely run the winch, 4GA would definitely NOT run a winch. hmmm...500A isolator...2GA wire...winch, 2 fully engaged batteries...this might need it's very own 'how to wire up your winch really cheap' addendum...
• I put 'in dash' switches for each isolator. This is a little complicated...each switch needs 'power' from somewhere in your cab. I'm sure you can find it. Just wire the switches with power from within your cab and then run the wire to the solenoids. By doing this you can 'turn on' your solenoids without having to start the truck!
• The trailer circuit is 80A. This is likely the most you will ever need. Remember overkill!

You upgraded the fuses and relay to 200A. If I up-grade to the 500A relay, would I up-grade the size of the fuse to match, also? I'm thinking about a winch tie-in situation...

[jjstahl3]

ok my 70 amp alternator that only put out 35 amps at idle seems to be sending only 12-12.6 volts to either battery - shouldn't it be sending 13-14 to actually charge the batteries? what am I missing? this reminds me why I believe electrical work is a mystery to me

[LandCruiserPhil]

If the vehicle is at idle and both batteries are low 12-12.6 would not be uncommon.

[teotwaki]

If the vehicle is at idle and both batteries are low 12-12.6 would not be uncommon.

That is the disadvantage with alternators: they are limited by RPMs and however their regulator is designed. That is one reason that they are labeled as being good for maintaining charge but not necessarily as good at being chargers.

[dwh]

ok my 70 amp alternator that only put out 35 amps at idle seems to be sending only 12-12.6 volts to either battery - shouldn't it be sending 13-14 to actually charge the batteries? what am I missing? this reminds me why I believe electrical work is a mystery to me

A) Many (most?) alternators don't put out all that much, if anything at idle. You can check it again at a higher RPM, but 35a is respectable output at idle.

B) Is that 35a what is going through the batteries? Or is that everything the alternator is putting out to power the whole truck?

C) The voltage of the "12v bus" is going to be determined by the batteries.

The voltage regulator will keep the alternator turned on until the voltage of the bus rises to whatever the set point is. The voltage of the bus won't rise as long as something - like the batteries - is drawing it down. As the batteries fill up, their voltage will rise, and the voltage of the entire 12v bus will rise along with them.

So it's not like you are going to see the alternator putting out 13.8v, when the batteries are at 12.6v - when you are measuring the voltage, you are measuring the voltage of the entire 12v bus.

[Southwest Chuck]

........

The fastest way to figure out where to run your wire to the fuse box is a 12V detector thingamajig. Turn your key to 'ON' (don't start truck) and then start probing empty holes in her fusebox. At some point your thingamajig will light up. Write down all her locations that lit up your thingamajig and turn her OFF. Then start probing those same empty holes in her fusebox that lit up your thingamajig again. Find any empty hole in her fusebox that lit up your thingamajig when she was turned ON but got no light from your thingamajig when she was turned OFF - that's the circuit you want to use (ALL THE TIME, if you get what I'm sayin )

I'll add a suggestion here, that you take one more step. You want to find a Ignition "on" hot wire, that's true. However, I would try and find one that that is "NOT" energized when the ignition is switched to the "Accessory" position. That way your batteries are still isolated when your engine is off and the ignition is in the accessory position (to run your radio or other accessories, etc.) If you use a circuit that is hot in both the "on" and "accessory", the relay/isolator will be engaged in both positions, tying the batteries together without isolation,(and eating up power) even when the engine is off and you're kick'in back with a beer at your favorite overlook listening to some tunes.

[SLO_F-250]

Quick question guys.... In the picture above he calls out a 12ga wire going to the fuse box to a fuse that is hot when the ignition is turned on. Do you simply take out the fuse, put the wire in on side then push the fuse back in?? Also does this wire have any significant power running to it? Was just wondering if I can clip it on to a hot wire under the dash? 12ga wire is fairly big so I wasn't sure. Please educate me as I have never run a wire to the fuse box.

I have everything mounted up and working but am concerned that where I attached the "ON" wire wont work due to current draw. I did test everything and it is working properly but just don't want to cause trouble down the road. Ill post a pict this evening.
THANKS GUYS!