Dual batteries not charging equally - JeepForum.com
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post #1 of 7 Old 08-23-2019, 10:27 PM Thread Starter
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Dual batteries not charging equally

Problem:
I setup my Jeep to run dual batteries through an isolator but am not able to get the larger capacity battery to fully charge via the alternator.

What I have:
Main battery is Duralast Gold (calcium type) - Full charge = 12.6v
Aux battery is Odyssey (AGM type) - Full charge = 12.86v
Alternator has been rebuilt for 200A output
Both batteries are less than 6 months old
Isolator is Stinger brand and connects batteries together when key is on and isolates them with the key off.

I am learning more and more about electrical as I go, and now see that I didn't design this very well. With that said, I don't know what the best solution is at this point. Should I upgrade to a better isolator system? Should I replace the Duralast battery for a second Odyssey? Is there another way? Getting tired of charging the aux battery manually every time I get home from Jeeping. Thanks in advance!


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post #2 of 7 Old 08-24-2019, 12:16 AM
kg6mov
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Different battery chemistry is gonna give you a headache on a system like that at the very least. The PCM is setting the alt votlage, and it's measuring it at the alt/pdc. Ideally you either need all batteries to be similar in type/capacity and electrical path, so the PCM can charge them like they're one, or have a secondary charge controller.

It's a lot easier of course if the batteries are mounted together. I would at least add a direct negative lead connecting the batteries. I'd also consider moving the aux fuse box to the primary side of the isolator, that load directly across the second battery isn't helping the system.
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post #3 of 7 Old 08-24-2019, 02:18 AM Thread Starter
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Quote:
Originally Posted by kg6mov View Post
Different battery chemistry is gonna give you a headache on a system like that at the very least. The PCM is setting the alt votlage, and it's measuring it at the alt/pdc. Ideally you either need all batteries to be similar in type/capacity and electrical path, so the PCM can charge them like they're one, or have a secondary charge controller.

It's a lot easier of course if the batteries are mounted together. I would at least add a direct negative lead connecting the batteries. I'd also consider moving the aux fuse box to the primary side of the isolator, that load directly across the second battery isn't helping the system.
Yeah, that's basically the conclusion I had come to. Never heard of a secondary charge controller before so I might look into that, but I think the best option is to replace the Duralast battery with another Odyssey. I'll stop by Autozone this weekend and see if they will work with me on that.

I have seen other ZJs with two batteries mounted together, but I don't understand how they fit like that. I don't think I could make that work in my engine bay - maybe it's a 4.0 thing?

When I was setting all this up I asked an electrical engineering friend about connecting the two batteries with a ground wire and he advised against that and said that connecting them both to the frame was basically the same thing. He had just graduated from college, so I'm sure there are things he hadn't considered.
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post #4 of 7 Old 08-24-2019, 04:46 PM
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Nah, the 4.0 is actually worse, but it's a tight fit. This is my setup:


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1984 F250, 460 BB, T19, work truck/future tow pig
1987 Mercedes 300D Turbo, the diesel daily, 30mpg in style

Quote:
Originally Posted by Texas ZJ1 View Post
I heard someone yell "mall crawler" from the back porch. I'll go see who said that. Be right back.
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post #5 of 7 Old 08-24-2019, 08:12 PM Thread Starter
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Quote:
Originally Posted by kg6mov View Post
Nah, the 4.0 is actually worse, but it's a tight fit. This is my setup:
That's the picture I've seen before. Do you have a thread on how you pulled that off? Custom battery tray? Battery sizes? I just got off the phone with AutoZone and they said I can swap my Duralast battery for another Odyssey tomorrow so I think I will do that, but I don't see them fitting like yours without some kind of modification.
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post #6 of 7 Old 08-24-2019, 10:19 PM
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Yeah, there's a bunch on it in the build: https://www.jeepforum.com/forum/f13/...l#post35742186


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1984 F250, 460 BB, T19, work truck/future tow pig
1987 Mercedes 300D Turbo, the diesel daily, 30mpg in style

Quote:
Originally Posted by Texas ZJ1 View Post
I heard someone yell "mall crawler" from the back porch. I'll go see who said that. Be right back.
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post #7 of 7 Old 08-25-2019, 12:01 AM Thread Starter
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Wow, great write up and photos. Thank you. Something like that will have to wait until I have my own welding setup, but I like it a lot. Hopefully having two of the same battery will solve the problem for now.
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