How to adjust PCM alternator charge voltage

My Odyssey-made Diehard Platinum and Northstar AGM batteries have done perfectly well using the stock TJ charging system. The previous Odyssey lasted over ten years and my current Northstar is over 5 years old.
I think a lot comes down to how well the batteries are made as well. Both of these are really good batteries and can tolerate a lot. Dont have as much faith in the Johnson controls Advance battery :) Mine might even be made by Clarios. Need to pull it and see what I can find on the case and see what specs I can get.

Here is a decent read on the topic. https://workshoppist.com/car-alternator-charge-agm-battery/

Can't ask for much better than 10 years but one has to wonder what would change if the battery was to get proper absorption charging for a true 100% charge. AGM batteries need an absorption phase to reach 100% otherwise sulfation sets in and capacity is reduced. Bulk charging is done around 13.6v but that only gets the battery to a partial state of charge. Jerry, do you also throw your batteries on a line fed charger from time to time?

another good read https://www.practical-sailor.com/systems-propulsion/fighting-sulfation-in-agms
 
An an electrical guy who doesn't mind getting into circuits, I have never bothered checking since mine works well as the factory designed it. I never had a problem with how it charged my batteries in either of my two TJs. I believe this worry causing much hand-wringing for some to be completely unnecessary. If I had a problem with battery life I would have looked into that.
The reason I ask is that if you have a battery temperature sensor, that may be the reason why you get such good life out of your batteries, because it charges at a safer (higher) voltage for AGM batteries when allowed by the temp sensor. Meanwhile a lot of TJs (mine included) do not do this, and the PCM chooses an inappropriately low voltage to charge an AGM battery (acceptable for lead acid, not so much for AGM).

This isn’t chasing a systemic problem facing all TJs, just particularly a select few later model year TJs not equipped with a temperature sensor and also retrofitted with an AGM battery.
 
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So interestingly enough, if you look at the charge temperatures in this table for an AGM battery, you will notice that it calls for around 13.7v at 150 degrees. Right about in line with the factory voltage setting for the jeep. This could explain why Jerry and others have had good luck with their AGM's under the hood at the lower voltage. I am going to measure the temps starting today and see what they are when driving around town. It could be that it works out to be spot on.



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So interestingly enough, if you look at the charge temperatures in this table for an AGM battery, you will notice that it calls for around 13.7v at 150 degrees. Right about in line with the factory voltage setting for the jeep. This could explain why Jerry and others have had good luck with their AGM's under the hood at the lower voltage. I am going to measure the temps starting today and see what they are when driving around town. It could be that it works out to be spot on.



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This is basically along my line of thinking as well.

Jeeps that have the battery temperature sensor effectively set the voltage based on that temperature, and have no problems keeping the battery alive.

But for the random TJs produced without one, it defaults to the hottest temperature range possible, meaning it always charges at 13.7. Perfectly fine if you could keep the battery hot enough, but at colder temperatures than 150, you risk either undercharging or sulfating the battery, severely shortening its life.

Most AGM manufacturers would rather risk running a higher charge voltage at all temperatures than a lower one if only one voltage can be set. (Better to overheat a battery than sulfate it.)

This is opposite to standard lead-acid batteries, which could off-gas and cause an explosion if overcharged at a high temperature. (Better to kill all batteries every 5 years than blow up 1 in 10,000 batteries).

So if you don’t have a battery temperature sensor and want to run an AGM, it is worth your time and (limited) money to adjust the voltage to a safer level to allow your battery to last longer.

If you do have a battery temperature sensor, this whole thread is pointless and I wouldn’t change anything.
 
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It seems to me that concerns about charging voltage are more important (and totally valid) when using a battery in a deep cycle scenario which is not typically applicable to the starting battery in your TJ .

In a deep-cycle scenario, a battery is typically discharged to as low as 50% of capacity or more (>50% is not recommended) such as you might see in an off-grid camper with solar. When discharged to this extent, you need to cycle through bulk and absorption phases before going into float mode. The higher charging voltage is only needed for bulk and absorption modes.

So unless you run your battery down to an abnormally low state of charge (for a starting battery), you should never need bulk or absorption charging modes.

All that said, I keep my Jeep on a smart charger when parked for long periods. It has 4 charging modes, bulk, absorption, float, and storage. And it has selectable battery types, including AGM. The one I use currently is a CTEK 7002. If I was buying one today, I would probably get a Victron BlueSmart charger. I installed 3 different types of Victron chargers on my offroad teardrop and really like them. IMO, they are a better value than some others, very full-featured, and I really like the BlueTooth apps they come with. The app shows not only the current charging situation but also the history over time, not important maybe for a maintenance charger, but pretty valuable for a solar installation.

So IMO, if you are using your TJ battery primarily as a starting battery and not for example running some sort of huge sound system while parked with the engine off, you should not need to mess with your charging system to keep an AGM battery properly charged. If, on the other hand, you use it more like a deep cycle battery, you may need to do something if you want to get the longest life from your AGM battery.
 
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