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Ok... im needing to hook up a battery in the bed to run the hay spike, and winch.... im also planning to put a (small) inverter in the cab.... im considering soldering ends onto 2ga wire, and just bolting it onto the starter, on top the main power feed, and running back from there. That way the alt can tend that third battery, and i can set up a maintainer etc, on the battery in the bed, and service all three. Afaik, i can pass power through there, without causing harm, but, since ive been wrong before..... im asking ya'll to confirm or deny.
The thing that comes to mind is the possibility of it dragging down the main batteries if it isn't matched in capacity. I wonder if you could isolate it with diodes? That way it could receive power but nothing feeding back towards the system. That's kind of a double edged sword though being that if it had some sort of draw it could drain the entire system. Maybe a main disconnect for when not in use or even a relay timer that could be tripped and connect it to the system for a designated time length then disconnect automatically. That way it could get topped up but not be a constant drain and no need to remember to switch on or off.
No worries running off the starter. Perfectly fine.
I would isolate the bed battery so it doesn't draw down the starting batteries, but that would defeat the single maintainer theory. If you wired in a ignition switched solenoid it would still charge when running.
For maintaining the most simple and cheapest option would be 2x maintainers.
What im considering is a 100w solar panel on the cab roof. Its not the cheapest, but it would work anytime the sun is up, without needing external power. But if i do that i think it should all be connected.
I would avoid teeing off of the starter and just go to the battery post of your choice. Engine vibration is a factor, plus that lead is now in the way of anything you may wish to do later on. Six more feet of wire and your life is a lot easier in the future. Also, stay away from diode isolator. I've seen a lot of them eat a volt between charge source and load. (Third battery) Alternator will be charging 14.3 and the aux battery will only see 13.7 at best. Most cases, less than that. Extensive experience with VW camper vans has taught me to never use a diode. What works well is a continuous duty solenoid that energizes as soon as the battery light goes out and begins charging the battery.
If it were me, I would use a direct connection that was very secure off one or both of the batteries. I would likely use an Anderson style quick connector that locks in for a secure connection near the 12v source. Too many variables connecting to the starter in my opinion, but I am often wrong...
A continuous duty solenoid like Kwikkordead recommended is a great idea.
You can get a 200 amp continous duty relay for $15.00 on Ebay at this time. This will work a lot better than a diode. No voltage drop. Just make sure the wiring is more than adequite for 200 amps or the wiring itself will cause voltage drop. I used to use either 6 or 4 guage welding cable, depending on how long the run was. Don't use solder on the crimps. Just crimp, then weatherproof. https://www.ebay.com/sch/i.html?_fro...id&_sacat=6028
What you want to do is run a signal wire from terminal 86 to the signal wire for the alternator. The one that turn the battery light on, on the dashboard display. Terminal 85 goes to ground.
87 comes from the positive terminal of the main battery, and 87a goes to the load. The relay will not energize until the alternator is charging and power is coming out of the signal wire for the alternator.
One caveat: This relay MIGHT be too much of a load for the signal wire to be able to power without causing some issues. I don't know for sure on this point. But you could use a smaller load reduction relay to run the bigger relay.
Another option that hasn't been mentioned is a battery combiner. They combine the batteries when there's charging voltage and disconnect when there isn't. No extra 'trigger' wire to install as with a relay/solenoid set-up and no chance of the auxiliary battery drawing down the engine batteries accidentally. As long as the maintainer can bring the engine battery voltage above the threshold required to activate the combiner it'll also charge the auxiliary battery, I believe they combine at about 13.3v and disconnect around 12.8, but don't quote me on it.
