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Battery Isolator - with dual alternators and dual starting batteries?

MCV

MCV

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UT/WY/CO drifter
I've been stranded a dozen times with dead batteries, and I've given up on getting Stellantis to replace both defective-upon-delivery factory batteries. After weeks of diagnostics at multiple dealers, the only erratic parasitic draws we've identified are associated with the batteries. I reach the same conclusions with my overnight amperage draw tests. So I'm installing two new AGMs out-of-pocket, with the anticipation that will solve everything. However, my question is whether it is a solid idea to also install a battery isolator. I have dual alternators and dual batteries. I am NOT asking about trailer or accessory batteries, I'm referring to the two under-hood batteries to start the diesel. I recognize on cold startup, our diesels need the amperage from both batteries to run the grid heater, etc. It is my understanding that are battery isolation systems to handle that, by only allowing cross-flow when ignition is on. However, I don't understand how the dual alternators work, and whether any modifications I do in terms of installing a battery isolator will confuse the computer control of that second alternator. So two questions: 1) Is it a great idea to install a battery isolator so I can be certain to have at least one fully charged battery (which is sufficient to start my truck on a warm day), and 2) if so, is there a particular system that will work better?

I'm entirely open to someone telling me that I am being stupid, just replace the batteries and all will be fine. I may be overworking the problem, but I am tired of having a 100% dead truck. (So dead that FOB won't work, Lithium jump starters won't work, etc).

Background: I have the dual alternators (440 amp) as part of the snowplow prep package. However, I have no high amp devices; no winch, no snowplow. Basic stock Tradesman.
 
You will not want to separate them, fix the issue.

How are the batteries drawing power? There is nothing in them to consume power.
 
Are you saying the batteries are draining themselves and there’s no draw from anything else?
 
Yea, I wouldn’t do anything other than swapping the batteries. If they still go dead on you then you weren’t dealing with just defective batteries, something is killing them and that needs to be fixed. I would put it back to the dealer at that point and / or start some lemon law process if that is an option.


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@AH64ID and @areuriding, I am unable to answer what exactly occurs within a failed battery that causes them to discharge. Typically, when I (or the dealer) conducts a parasitic draw test we measure something under 50 milliamps draw, which is as expected. However, periodically, one RAM dealer was able to document intermittent 5 amp draws! 100 times greater! It would be random - something documented on an overnight test. Reportedly, when they disconnected the second battery, they never observed that rogue problem, so they deemed the 2nd battery to be defective with some internal short. That is what was directed by the STAR engineer with Stellantis. So they replaced that confirmed defective battery, but left the other original battery in place, despite it being drawn entirely flat to 0% charge (no interior lights, wouldn't recognize FOB, reset all computer settings) at least a dozen times. With one new battery, and one original battery, I have had 100% success rate starting the truck the next day. But, if I let the truck sit for a week, the batteries will still be discharged to 0%. So flat that a lithium jump starter can't do anything. Still, anytime I do short-term parasitic draw tests, I measure somewhere 30-50 milliamps.
One dealer told me that was a symptom of UConnect - that it is making some random "call home" or trying to upload or download something. But 5 amp sounds ridiculously high for a radio. I have always had trouble with the radio - if I haven't driven the truck for a few hours, it always starts at max volume, won't let me mute it until I get past the safety warning pages, and changes to a different source (AM/FM, etc). But no dealer could ever diagnose anything that allowed him to replace the stereo. I've now been to 5 dealers without resolution on the radio or the periodic dead batteries. So it will be cheaper for me to just replace the batteries and hopefully resolve the issue.
 
It’s worth a shot on the batteries as there are some anomalies going on.

I’ve had the best luck with Costco batteries, and their warranty, in recent years.
 
Ok, I asked because I went through a very similar issue with my wife's new Jeep Wrangler Rubicon. I made numerous trips to the dealer because it would randomly have a no start issue and the dealer could never replicate it. Jeeps have 2 separate batteries; I won't bore you with the set up they have. In any case, I ended up replacing both batteries and took it in and asked to have Uconnect refreshed and updated. At first, they refused but after speaking to the service manager and him seeing I had been in 9 times for the same issue with no resolution he accommodated my request. It's been almost a year now and I've had no issue. I didn't replace the batteries with factory ones because after a lot of research I found several other Jeep owners with the same issue. I ended up with Interstate. I hope this helps, it sucks to have to eat the cost but in the end if it fixes the issue it's worth it.
 
Ya it sucks but I would go to Costco and throw a set of Johnson’s in it. Had a 16 Ram 1/2 ton with gremlins that would kill a battery in a week in the winter If the battery was more than 12 months old. Blamed it on the constantly frozen air suspension compressor but it also had the instrument cluster random waking up in the night problem but never figured it out and traded it in. Just bought a 23 Jeep Grand Cherokee and the dealer said ya don’t let it sit for a week without driving it …it might eat the batteries.:eek: So going to put a tender on it. I guess it’s the progress of the modern world.
 
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I have an 04 F250 that has the same problem. Nobody can find a draw on the batteries but, just like you, if I let it sit fr a few days they are near dead. Or on really cold days let it sit us-used for just 1 day.

I replaced both batteries with old Interstate AGMs that were in my tractors & all was good for about 6 months when it suddenly started happening again. I figured since they were old batteries that I would just swap them out. Since then those old batteries have been used in multiple projects without issue.

I put in 2 brand new batteries (Interstate AGM once again) and they also lasted about 6 months before the issues resurfaced.

Back in July I picked up a small battery tender that I leave connected to one battery & I plug in my block heater every night (even in nice weather) and even with it sitting for a couple weeks I have had no problem firing it up.

Trying to chase down the phantom draw & it's eventual return of problems seemed never ending. After deciding to use a battery tender every night, regardless of my plan to use the truck or not the following day (or even again on the same day), is a decision I wish I made years ago. t doesn't help when away from home & a power source but I carry a 100' extension cord just in case I can find access.

Just some food for thought based on my experience with a similar problem.
 
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