I'm putting in a second yellow-top from Optima this weekend and I'm only an amateur electrician, so I could use some help with the setup I need. I would like to have two batteries charging whenever my XJ is running, and use ONLY the secondary battery when the ignition is off. That way, I can completely deplete the secondary battery and then fire it up on an undrained primary battery. I want to take advantage of the 750 CCA... do I need a 750 amp solenoid for starting in cold weather?

 

you would want a relay, probably 250 amp i would say. the starter doesnt draw more than 300 amps or so. the relay would be connected in between the two battery positives, and the other side of the relay would go to a hot wire thats ignition switched.

also if you only want the secondary battery to be hooked up with the ignition off, youll have to put the factory battery terminals on the secondary one.

 

I was confused about that relay... the relays I'm used to (like those that come with light kits, fan kits, etc) have an external switch that causes the relay to switch on. What is the... activator(?) in a 250 amp relay for switching between batteries? What makes it switch?
If the starter only draws ~300 amps, why am I getting a battery with 750 CCA?

 

don't know if this is what you're looking for, but maybe it'll give some ideas...

https://www.cherokeeforum.com/f2/dua...-wiring-27292/

 

I found that while searching the forums earlier, I think it is pretty similar to what I want to do. Is the relay on the left in that diagram one that is already on the XJ? I seem to think this would be the starter relay next to my positive distribution block (4 relays on pass side engine bay). I'm also gathering materials to do the fitchva.com switch box and relay box, so the relay on the right would connect to my main master power switch (which will provide power to other switches to my HID overhead lights, backup lights, and winch).

This would have to be a heavy duty relay as well, right?

Fitchva box in progress...

 

Here is the wiring diagram for what you want.



Solenoid is a 250 amp continuous duty. That is important. Most are not continuous duty. It takes some current to close it, so use a relay to operate it off the ignition.

 

Where did u get the Fitchva box buttons/switches?

 

That is my set up. No, it is a relay that I also use to power up some other things like my OBA when the ignition is on. Your starter relay only works when you activate the starter. Otherwise it is only a connection point.

 

Where can I find a 250 amp (or more) continuous duty solenoid, and is my stock alternator going to be enough for these accessories?

 

ottratw.com

 

i dont know if you figured it out, but the ignition (key) is your switch.

 

I got mine at a NAPA auto parts store. You might have to order it. Yes your stock alternator will run it fine.

Finding a 130 amp ZJ alternator in a JY would be a good deal also.

 

On that, is pushing the key forward to start the car considered a momentary pushbutton switch?

Does anyone know if the stock battery tray will mount to the driver's side by adding a few base bolts?

I'll add a full writeup when I begin this project, just gathering parts for now. I'm putting a Cobra 3000 Watt 12V -> 120vac power inverter in as well!

 

Starter draw for a 242ci in good running order is 160-180A. Most "continuous duty" solenoids also have a "surge" rating - which means it can support (typically) ratedx3 amount of current for a short time (typically thirty seconds to a minute.)

As an example, I carry an 85A solenoid that has a "surge" rating of 255A for thirty seconds. If your Jeep doesn't start after thirty seconds of cranking, you have bigger problems. My 200A unit is good for ~600A@30sec - I don't have any of these on hand (I think,) but they can be had.

Solenoid closing current is usually 500-750mA - while you can run that off of the switched ignition circuit, a relay may be helpful (in comparison, the typical DIN relay - the little 1" cube - uses 100-125mA to close.)

If you decide to use a solid-state box instead (I'm still looking for one I'd be willing to carry and use myself - if I won't use it, you won't get it from me!) you can do a "self-jump" by simply using booster cables from one battery to the other. A solenoid isolator can be wired to cut in with the ignition off by the addition of a switch in a convenient location - see the instruction sheet for my solenoids (I just moved sites and I'm still reorganising. If you don't see the instruction sheet there, email me and I'll send you the .pdf.)

 

I'm starting to get the hang of relays and solenoids with a little research, and your help is much appreciated. Ideally, with an Optima Battery claim of 750 CCA, I would want a solenoid rated to 250 continuous amps, with a surge capable of 750 amps (250 x 3) as 4.3L XJ mentioned earlier. As you said, 5-90, it seems that's the continuous-surge ratio for most solenoids. Where does the alternator come in? I need to be able to charge both batteries, but doesn't that make them "compete" with each other?