AGM vs FLA Batteries and our Jeeps

[DaltonGang]

I've been doing some reading on the difference between the AGM and the Standard(FLA) batteries. FLA(Flood Lead Acid) . it seems the big weakness our AGM batteries have is that once they are fully discharged, by the crappy Aux Battery, their lifespan is now cut short. The AGM likes to be charged above 90%, and below 50% it's days are numbered. Our Aux battery seems to be taking out main batteries at a fast pace. This seems to be why many here change both batteries at the same ti.e, or end up replacing the main battery after the Aux is replaced.
Also, it seems the AGM deteriorates fast with high heat. Hmm, you think the Wrangler might be a little hot in the engine compartment?

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[BrierPatch]

This is interesting. My 2015 WK2, which doesn't have any form of ESS, is still on its original factory battery, it will be 9 years old in April and it's an AGM, but it sits in the floor just under the passenger seat, not in the engine compartment. The battery has never been run down or dead and I've never put it on a battery tender either. It's at 88K miles and is kept in a climate controlled garage.

 

[AndySpill]

I've been doing some reading on the difference between the AGM and the Standard(FLA) batteries. FLA(Flood Lead Acid) . it seems the big weakness our AGM batteries have is that once they are fully discharged, by the crappy Aux Battery, their lifespan is now cut short. The AGM likes to be charged above 90%, and below 50% it's days are numbered. Our Aux battery seems to be taking out main batteries our at a fast pace. This seems to be why many here change both batteries at the same ti.e, or end up replacing the main battery after the Aux is replaced.
Also, it seems the AGM deteriorates fast with high heat. Hmm, you think the Wrangler might be a little hot in the engine compartment?

I think some of what you are saying is factual, like Sealed Lead Acid batteries, all else equal, allowing higher discharge rates with less effect on battery lifespan that AGM (absorbent glass mat) batteries do, and that they do fair better at higher temperatures, but I also think that limited lifespan of our AGM batteries may have to do with overall lack of quality in their production and design of the dual AGM battery ESS system in the JL than differences in each battery's basic design.

IMHO what most contributes to early demise of our main batteries is the parallel connection of two dissimilar sized AGM batteries at the factory, not the battery chemistry itself.

AGM batteries delivery and acquire charge faster than sealed lead acid ones and as a result have been what manufacturers have, good, bad, or indifferent gravitating to when needing to design engine start stop systems into their vehicles to enjoy better CAFE fuel ratings.

 

[2nd 392]

This is interesting. My 2015 WK2, which doesn't have any form of ESS, is still on its original factory battery, it will be 9 years old in April and it's an AGM, but it sits in the floor just under the passenger seat, not in the engine compartment. The battery has never been run down or dead and I've never put it on a battery tender either. It's at 88K miles and is kept in a climate controlled garage.

You’re due- replaced my 15 GC’s last month, it hit 9 years a few days ago. No getting sluggish indication, it went from starting fine to just barely starting, and I mean barely.

 

[BrierPatch]

You’re due- replaced my 15 GC’s last month, it hit 9 years a few days ago. No getting sluggish indication, it went from starting fine to just barely starting, and I mean barely.

Thanks for the heads up. I've been meaning to replace it for a couple years. Time has just gone by faster than I realize. I know I've pushed it longer than I should have and am on borrowed time.

 

[CarbonSteel]

IMHO what most contributes to early demise of our main batteries is the parallel connection of two dissimilar sized AGM batteries at the factory, not the battery chemistry itself.

Heat will kill a battery far sooner than one operated in a moderate environment and ^^^ this ^^^ is 100% at the root of it.

The auxiliary battery is not even needed and Jeep would have been far better off with a single H7.

 

[Mguy]

AGM likes to be charged above 90%, and below 50% it's days are numbered.

Some elaboration. AGMs are designed for engine starting, deep cycling, or both. Manufacturers' stats for cranking amps (starting) and amp-hours (cycling) usually comports with the marketing. The bottom line is that starting batteries can produce very large but short discharges (which barely dent full capacity), while cycling batteries, if discharged more slowly, can go deeper (up to 50% capacity is generally accepted to be optimum for maximum cycle life/beneficial use). Both types should then be charged to 100% as soon as possible. Both types have a slow self-discharge rate, and both types will recover well from self-discharge if not permitted to go below 90% or so capacity. Some "walk down" in capacity is to be expected over time for both types, but not likely to be noticeable for starters that are otherwise well maintained.

 

[THAW]

(up to 50% capacity is generally accepted to be optimum for maximum cycle life/beneficial use)

Data I've seen suggests this cycle recommendation is based on misinterpretation of deep cycle AGM battery lifespan. While cycle life is inversely proportional to depth of discharge, comparing total lifetime Amp Hour output demonstrates minimal variation.

That is, a 100 Ah battery repeatedly discharged 50% may be capable of 1,000 discharge/charge cycles, a total of 50,000 lifetime Amp Hours output. Whereas a 100 Ah battery repeatedly discharged 80% might be capable of just over 600 cycles, but thereby produces roughly the same lifetime current output.

From Full River:

 

[alphawolff]

This is interesting. My 2015 WK2, which doesn't have any form of ESS, is still on its original factory battery, it will be 9 years old in April and it's an AGM, but it sits in the floor just under the passenger seat, not in the engine compartment. The battery has never been run down or dead and I've never put it on a battery tender either. It's at 88K miles and is kept in a climate controlled garage.

My 2011 grand cherokee still had its original AGM battery when we sold it in 2020.

Stop start is the main reason our batteries don't last as long anymore. I still find 2011-2017 or so grand cherokees/durangos coming in with the factory battery. Those lasted one hell of a long time.

The stop start system drains the tiny aux battery until it no longer holds a charge, then the aux drains the primary until it also no longer holds a charge. As the OP said AGM batteries *hate* being discharged and is the primary reason the battery life is so shit these days.

Heat is also a contributing factor, but the batteries in our newer grand cherokees/durangos go bad just as much as our wranglers do. And those are under the seat, totally isolated from the outside heat/engine heat.

If you go out one day and find your battery voltage under 10v or so you should just replace them and not even waste your time trying to recharge them.

 

[AndySpill]

Data I've seen suggests this cycle recommendation is based on misinterpretation of deep cycle AGM battery lifespan.

This is at best ambiguous and at worst wrong. Before taking even the tiniest of steps forward, lets first nail down what's meant by "lifespan."

If you define lifespan, as is done in the world of batteries, by the number of discharge and charge cycles, whose best surrogate is the passage of time, (i.e. how long a battery will last) then as you point out, the less you discharge a lead acid battery (and all batteries discussed here are lead acid based) before recharging--a cycle if you will--the more cycles you will get--or stated in different terms, all else equal, the battery will degrade in performance slower over time and last longer as measured by a calendar.

If you are defining lifespan as the total number of amps that battery will put out over its life--not BTW the normal definition of lifespan here but a definition just the same--then depth of discharge matters less and perhaps people do misbelieve that more overall power over the battery's useful life of charging and discharging is to be had by extracting that power in smaller amounts before recharging.

In this second definition, as if the battery could talk, it might say, "roughly speaking I can give you just so many amps of power over my life of charging and discharging. You decide how fast or slow you want it."

While cycle life is inversely proportional to depth of discharge, comparing total lifetime Amp Hour output demonstrates minimal variation.

That is, a 100 Ah battery repeatedly discharged 50% may be capable of 1,000 discharge/charge cycles, a total of 50,000 lifetime Amp Hours output. Whereas a 100 Ah battery repeatedly discharged 80% might be capable of just over 600 cycles, but thereby produces the same lifetime current output.

From Full River:

 

[Mguy]

Let's nip this thread drift in the bud. JL owners with battery and starting concerns are rarely interested in the subjective nuances of deep cycling. Perhaps Jeep overlanders see the benefits of such discussion, or maybe on a forum concerned with RVs, boats, solar systems, and wind farms.

 

[alphawolff]

The JL design employs its Aux/ESS battery for applications other than starting, doesn't that fact ostensibly qualify it as a deep cycled battery?

I have zero issue with your suggestion to avoid delving further into battery cycle life, I did so only to dispel mythology around a commonly referenced arbitrary 50% DoD recommendation mentioned twice.

Incase people are wondering why we have two batteries I actually have the answer to that

Essentially during development they found that during the start part of a stop/start event the voltage spike was simply too great and would cause the radio to restart, among other gremlins. In order to combat this they decided to split the loads in order to keep the interior at a steady voltage, hence the dual batteries.

A few years of the jeep cherokee only used a single battery for their stop/start system, as well does both of the alfa romeo's. They accomplish this by using a set of in-rush current relays and capacitors to allow the voltage to stay stable during the starting event. From what I understand this system doesn't work as well for extended stop/start events, so any vehicle designed/re-designed after 2017.5 or so was given two batteries.

Honestly if they used two full sized batteries for the stop/start system I highly doubt we would have as many battery issues as we do. The aux battery is just too damn small.

 

[DaltonGang]

I've fixed it already, a long time ago. Pulled one wire, taped up the end, and pulled one fuse. Never an issue since.

 

[DaltonGang]

Incase people are wondering why we have two batteries I actually have the answer to that

Essentially during development they found that during the start part of a stop/start event the voltage spike was simply too great and would cause the radio to restart, among other gremlins. In order to combat this they decided to split the loads in order to keep the interior at a steady voltage, hence the dual batteries.

A few years of the jeep cherokee only used a single battery for their stop/start system, as well does both of the alfa romeo's. They accomplish this by using a set of in-rush current relays and capacitors to allow the voltage to stay stable during the starting event. From what I understand this system doesn't work as well for extended stop/start events, so any vehicle designed/re-designed after 2017.5 or so was given two batteries.

Honestly if they used two full sized batteries for the stop/start system I highly doubt we would have as many battery issues as we do. The aux battery is just too damn small.

Your explanation of why they did this makes no sense. Every other car manufacturer uses only one battery with their Stop/Start system, without the radio getting buggy. Admit it, the design engineers screwed the pooch with the JL. Old saying: "If it isn't broken, don't fix it". The JL ESS is a prime example if this is ignored.

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