Aux battery disconnect

[FF5Cobra]

New jeep owner and member here
has anyone installed a battery disconnect( perk switch) on negative cable going to aux battery?

doing this would allow One to connect or disconnect Aux battery to use as needed.
of course aux battery would need to be occasionally charged.
but at least having a good aux battery available should be a plus if needed
or am I missing something reading thru these delete/ not delete battery post.

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[Flip]

I don't see why you couldn't do something like that. I think it might be difficult to maintain a charge on the auxiliary battery. Obviously if you don't keep it charged, rest assure it will be dead when you need it. I think a small battery jumper is probably a safer option IMO.

Best of luck my friend.

 

[WranglerMan]

If you wanted to that I’m sure it would work but you would also have to put in a charge point on the isolated side so you could disconnect the main and not have it bleed over the charge plus those little batteries don’t have great life and sort of a PITA to get to.

 

[THAW]

You could switch the AUX (ESS) battery ground cable with a 250 Amp relay (~$20) operated by an ignition-powered source or factory Aux Switch. Then, the AUX (ESS) battery would/could be disconnected when the ignition is off, but receive alternator charging when the engine is running. [Note: significant external discharge of the AUX (ESS) battery while disconnected from the vehicle could cause electrical issues upon reconnection.]

 

[Mguy]

New jeep owner and member here
has anyone installed a battery disconnect( perk switch) on negative cable going to aux battery?

doing this would allow One to connect or disconnect Aux battery to use as needed.
of course aux battery would need to be occasionally charged.
but at least having a good aux battery available should be a plus if needed
or am I missing something reading thru these delete/ not delete battery post.

This is what I did/do, using disconnected aux battery as starter/substitute for main battery. Includes some discussion of an aftermarket battery switch.

 

[LanceMagnum]

Yep, did exactly that - nothing fancy since I leave it connected most of the time and just disconnect when I'm camping off grid but have it for emergency start if needed

I upgraded the aux battery to a Braille
https://www.jegs.com/i/Braille-Battery/147/B14115/10002/-1

 

[AndySpill]

I have a quick disconnect switch like @LanceMagnum (mine happens to be of the knife variety but I think either is fine) at the main battery's negative terminal.

Where I may differ in approach from others here is in purpose of that switch, it remaining closed.

My thoughts were that if the Aux battery dies, and Murphy's law will have it that when it does, I'll be "in a tuxedo on my way to some formal event," that I can, without grabbing tools, open the switch, pull Fuse 42 (pulling this fuse optional for all dual AGM JLs but early 2018's without TSB 18-092-19), and apply my portable jump starter the main battery's terminals and know that the main battery alone, rather than the Aux battery as well, was receiving the jump starter's limited power.

I am leery of people using the Aux battery to power appliances with the engine off, if that's what was intended by some in this thread. It's such a small and relatively unreliable power source that many owners have reported it dying even when always connected to the alternator and never running ESS events, that they idea of taxing it more, and then at some point reconnecting it to the main battery (even if externally charged first) leaves me concerned.

Processes that will faster consume that Aux battery, and that at some point reconnect it to the main battery risk it taking out that main battery as well given the opportunity in such parallel connections of the Aux cannibalizing the main battery. Add to that the fact that lead acid batteries (of which the AGM batteries of our JLs are a variant) should not be taxed below 50% state of charge before recharged, and I think such overlanding power needs best be handled by beefier systems that better monitor power usage, particularly when the alternator isn't running, like those of Genesis Offroad.

I get that some contributors here are keeping the Aux as a backup starter. I thought about that and decided to go the portable power pack/disconnect the Aux route to address such situations.

 

[THAW]

Add to that the fact that lead acid batteries (of which the AGM batteries of our JLs are a variant) should not be taxed below 50% state of charge before recharged, ...

That's an AGM myth.

 

[Terrymo]

That's an AGM myth.

I had a peripheral question and hoping for a short simple answer. The figures I’ve highlighted regarding Odyssey AGM batteries seem counterintuitive to me. Are these figures accurate and why does it appear you get more discharge cycles at 50% DOD than at 80% DOD?

 

[Jebiruph]

From Grok, the Depth of Discharge refers to the amount of discharge - 80% means 20% remaining, 50% means 50% remaining.


Depth of Discharge (DoD) for AGM (Absorbent Glass Mat) batteries refers to the percentage of the battery's total capacity that has been used before recharging. It directly impacts cycle life—the number of charge/discharge cycles the battery can endure before significant capacity loss.

Key Recommendations

• Maximum safe DoD: Up to 80%. AGM batteries are designed for deeper discharges than traditional flooded lead-acid batteries (typically limited to 50% DoD for similar longevity). This is supported by manufacturers and experts, as AGM construction (electrolyte absorbed in glass mats) reduces sulfation and allows better deep-cycle performance.
• Recommended for optimal longevity: 50% DoD or less. Regularly discharging only to 50% can double or more the cycle life compared to 80% DoD, providing the best balance of usable capacity and lifespan in most applications (e.g., solar, RV, marine).

 

[THAW]

I had a peripheral question and hoping for a short simple answer. The figures I’ve highlighted regarding Odyssey AGM batteries seem counterintuitive to me. Are these figures accurate and why does it appear you get more discharge cycles at 50% DOD than at 80% DOD?

@Jebiruph beat me to it: the counterintuitive part may be that 80% DoD refers to using 80% of capacity, not 20%?

Anyway, the 50% DoD "rule" represents an entirely arbitrary trade-off, and was intended for sizing batteries to known daily power requirements to create emergency capacity and multi-year replacement intervals. Sadly, it has been widely misinterpreted as a battery chemistry "no-no", as it was earlier in this thread.

 

[Terrymo]

@Jebiruph beat me to it: the counterintuitive part may be that 80% DoD refers to using 80% of capacity, not 20%?

Anyway, the 50% DoD "rule" represents an entirely arbitrary trade-off, and was intended for sizing batteries to known daily power requirements to create emergency capacity and multi-year replacement intervals. Sadly, it has been widely misinterpreted as a battery chemistry "no-no", as it was earlier in this thread.

So 80% depth of discharge equals a 20% state of charge or have I confused terminology further?

 

[THAW]

So 80% depth of discharge equals a 20% state of charge or have I confused terminology further?

Precisely, 80% DoD = 20% SoC.

 

[Terrymo]

Yes!!! LOL

 

[THAW]

• Recommended for optimal longevity: 50% DoD or less. Regularly discharging only to 50% can double or more the cycle life compared to 80% DoD, providing the best balance of usable capacity and lifespan in most applications (e.g., solar, RV, marine).

Note that "doubling" the "cycle life" only preserves 20% of lifetime output.

For example, a 100 Ah battery cycled once to 80% DoD provides 80 Ah, and a 100 Ah battery cycled twice to 50% DoD provides 100 Ah.

The "or less" in the recommendation is notable because a lot of published charts/figures show a greater cycle life difference between 20% DoD and 50% DoD than between 50% DoD and 80% DoD. Generally, the lower the DoD, the lower the lifetime output, [rhetorical question incoming] so why stop at 50%?

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