Aux Battery for Emergency Starting.

[Mguy]

If you have electrically isolated (deleted, bypassed) your aux battery, you've probably wondered what to do with it. Throwing it off a cliff is one idea, but here's another. The goals and parameters are 1) emergency start capability with the dead or drained main battery disconnected, 2) use of the negative/F42 disconnect for normal driving, 3) easy, reasonably accurate charging of both batteries using the Jeep's alternator and, when desired 4) quick and easy return of all wiring to stock.

Note, Jebiruph has a post that discusses reserve use of the aux battery by connecting it in parallel to the main battery. See https://www.jlwranglerforums.com/forum/threads/ess-dual-battery-management.60034/

A few notes on my experience. Testing of my new Mopar aux battery shows it's resting voltage is 12.8. After 3 months, it self discharges to 12.6v, and starts the engine with the main disconnected. My aux battery is now only 6 months old, but AGMs that are fully charged every 3 months with virtually no loads in between should easily last, at least for starting purposes, several years.

My aux battery is recharged after 3 month by reconnecting it's negative and running for four or so hours in about a six hour period. The main battery is fully charged before this process. Typically there are about four stops/starts for the two hour down time, but there's no ESS stopping or use of the accessory setting on the ignition switch. After six hours, with the batteries going into rest, the aux battery is promptly disconnected to prevent cross-draining. While not the best way to charge dissimilar AGM batteries, it's not terrible.

The actual setup is a variation on the negative/F42 disconnect. Unlike with Jebiruph's reserve setup, the aux battery is intentionally not paralleled as a starting "boost" because the poor or dead state of a failed starting battery often means attempts will not work. Instead, the aux battery is used as a complete substitute for the main battery. This emergency start setup could include a battery switch and wiring for paralleling/selecting/boosting, but that would make restoring to stock more involved. For those not concerned with added hardware, a switch is a good way to go.

So, switching for emergency starting is at the negative side of the system. This means the battery positives need to always be connected to each other, and to the system itself. The stock positive wiring, left in place after the negative/F42 disconnect, does this. However, for aux battery starting, current would go through the 150 amp N3 fuse. While an average aux battery start may not be an issue, it's not good to have the fuse in emergency situations.

I considered several alternatives for connecting the aux battery positive to the starter, which itself is connected to the positive bus on the main battery. The N3 lug from the aux battery could be moved to the main battery positive bus, but I found there was not enough slack in the wire without some sort of extension. Another solution would be fashioning a copper strap or bridge from N1 to N2, which already have stock connections to the aux and main battery positives. I went a third way, adding a 2 gauge wire from N1 to the main battery positive bus. For a good electrical connection, and to overcome the height of the N1 terminal guide edges, I made and a spacer out of 1/4 inch thick copper (photos to follow).

Negative "switching" for an emergency start is removal of the ground lug from the IBS and connecting it to the already disconnected aux negative lug. Returning to stock is simple. Remove the added positive cable (not necessary to do when charging the aux battery), reconnect the aux negative (and if necessary, the ground) to the IBS, and insert a fuse in F42 (debatably not necessary if a blown one is already in the socket).

There are a couple of lingering concerns I have with running the Jeep after an emergency start. The intent with the setup is to return to a safe/repair location with just the aux battery connected and receiving whatever charge the Jeep chooses to give it. The Jeep's loads won't care if, and I'm assuming that, the aux battery is a sufficient buffer. However, with this setup the IBS is not sensing anything, so error codes wouldn't surprise me. So far I have only idled the Jeep for a few minutes after test aux battery starts, during which everything, including system voltage (averaging about 14.5v), seems fine.

Hopefully some owners will find this setup useful. Comments welcome.

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[Reinen]

Well, it would work (if not a little complicated for the average Joe with a lot of manual operations). But when would this really be useful?

If you add an isolated house battery to use for camp power and strictly restrict the burden of the JL batteries to only the JL (with everything wired correctly and no severe parasitic drains), you should never have a dead JL battery unless something fails. Either the alternator, cabling or Main battery. The "standby Aux" would only help with an alternator failure for a very short time. It won't help at all in a cabling failure. It only helps in a Main battery failure and you should see that coming. They rarely fail quickly, instead just fading away until it can no longer meet your minimum requirements.

If you need boost functionality you should install a Genesis Dual Battery system. It gives you a real Group 25 boost battery instead of the rinky-dink AUX14, it has a much better automatic recharge schedule and it's much easier to use. The "standby Aux" would be a lot of work for 1/4 the power capacity.

Personally I find the dual/boost battery design a bit old-school and prefer the isolated house lithium battery route. It handles low power long duration uses much better than lead-acid. It virtually eliminates accidental depletion of the JL batteries through human error. You just deplete the house battery which you don't need to get yourself out. In a pinch, the house battery can be used to recharge the JL batteries by keeping a small DC-to-DC charging cable. Also keep a print out of the procedure to delete Main or Aux on the trail in case one of them goes bad (which you'll probably never need to do if you pay attention to them), and keep a small cheap multimeter in your trail gear for troubleshooting.

Finally, ESS checks your batteries every time it engages which is more often than even an OCD human would. I do not understand why one would want to eliminate a more than daily battery check while on-road, especially if you actually do go into the middle of nowhere where roadside assistance doesn't exist. It gives you more then enough warning of battery issues if you pay attention.

 

[LazyJL]

Might be easier to carry a lithium booster for the few times the battery fails. https://www.amazon.com/gp/product/B016UG6PWE/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[AndySpill]

Mike @Mguy:

Thanks for taking to time to share your experiences.

Realizing that your objects go beyond mere emergency starting of the vehicle, but focusing on this one thing for a moment, in the case of a dead battery, I am curious to know the pros and cons of alternatively disconnecting the ESS battery's negative cable at the main battery, pulling Fuse 42, and acquiring, charging, and having with you one of those portable jump starters that you could connect to the main battery's terminals in the event that the main battery is unable to crank the engine.

Clearly cost of such an item would be one factor. Perhaps a blown N3 fuse, where the alternator is unable to provide power to the main battery, and the ESS battery cranks and provides current to get you home in your setup is another(?)

TIA

 

[AndySpill]

@LazyJL and I "simulposted."

 

[Mguy]

Thanks Reinen, Lazyjl, and AndySpill for the posts. So we're on the same page--

A dead battery is one that will not take a charge. Running a vehicle with a dead battery can create an abnormally large load that overheats and damages the alternator. Modern vehicle electronics can also be damaged by an unbuffered charging system that is working very hard and likely producing transients.

A drained battery is one that is discharged to a degree that precludes starting, but is otherwise healthy.

Now:

1) Carrying an external "booster" battery makes sense, but will only be useful in a drained battery situation. Even a very big booster won't do a start with a dead battery, and may not start with one that is deeply discharged.

2) While a vehicle with a dead battery can usually be jumped using another vehicle's battery and alternator, for the reasons above it's not the best solution when the vehicle is to be driven away.

So, I do carry a fairly big external AGM booster that is housed with a built in charger (my two previous lithiums blew out of their cases after a couple of years). If my Jeep fails to start I would try the booster, but if not successful my next move would likely not be jumper cables. Based upon current knowledge, I'd go with the aux battery and replace the main.

Also, I do agree that a lead-acid battery will usually give some warning before dying, and that ESS provides some predictive and diagnostic help. That's a big reason why I don't use software to shut down ESS and do sometimes deliberately not push the button. However, there are electrical abnormalities and human errors--I was already once caught by the premature death of an AGM bank due to a load failure/short. And I have on occasion "left the lights on."

So the question remains for all owners who are running with a disconnected aux battery. What's to be done with that hard to reach little devil. It's sitting there with accessible cables, and topping off every three months or so is fairly easy. The answer for me is an emergency start setup that cost almost nothing. But when the aux battery dies, I just might throw it off a cliff and not spend money on a replacement.

 

[THAW]

Have you tested the aux battery only start in cold/freezing conditions? If so, which motor?

 

[dsgrey]

Might be easier to carry a lithium booster for the few times the battery fails. https://www.amazon.com/gp/product/B016UG6PWE/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Yes, I carry a much less expensive version in both vehicles (Tacklife). Easy to store and no longer need to carry jumper cables. They've worked well on my vehicles and others. The spouses vehicle went completely dead with zero voltage, charger wouldn't charge unless completely disconnected, fully charged in 15 minutes with a whopping 54CCA so the battery was toast. I used the booster and it started up without any issue.

 

[Mguy]

Have you tested the aux battery only start in cold/freezing conditions? If so, which motor?

Good questions. The Mopar aux battery at 12.6 volts open circuit started my 3.6L gas engine at temperatures of 20F and 40F.

 

[AndySpill]

I don't know which motor, and it probably was not extremely cold out, but here is the Aux, alone, starting the vehicle:

 

[Mguy]

I don't know which motor, and it probably was not extremely cold out, but here is the Aux, alone, starting the vehicle:

Thanks for the post. There are questions about which engines, and under what conditions, can the aux battery be used for starting. I'd also like more info on operation after starting, and on charge time while running the Jeep after an extended self discharge. I think it's going to be tough to get these sort of responses.

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