Auxiliary Battery Jump

[pcrawfordpt]

Apparently you can jump your own auxiliary battery from your main battery if stuck out on the trail or away from services. I did see a tip from another JL owner who showed jumping from another vehicle to the first terminal/auxiliary, but didn’t know you could just jump it yourself from the main battery to the first terminal/auxiliary. But it does make sense, as long as your main battery isn’t dead as well.

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[Redruby]

Great tip thx

 

[ShadowsPapa]

Unless in ESS mode, shut down by ESS, both batteries are in parallel.
The only reason you'd ever have to do that is if your Jeep is broken - if there's another issue.
When you charge your battery with cables on the top, you are also charging the aux battery.
Only if there's a fuse blown or a relay bad would you ever need to do this.
So if you have to jump to that post - you'd better be checking it out.

I've jump started Jeeps with a bad aux battery by using cables on the top posts, or like with Grand Cherokee, there are posts on the right fender in the engine bay. Hook to that and you're good to go.

Check the wiring diagrams.
Ground for both batteries is on top of the IBS on the crank battery negative (that's the top or main battery)
The positive for both batteries is on the positive post of the main or crank battery.
The aux is connected there unless there's a problem. Disconnect that clamp off the + of the main or crank battery - it will still be hot! Touch it to ground and blow F3. Then your aux battery will no longer be connected HA.

Basically, if the AUX battery is dead, your main battery will be dead too unless you blew a fuse or have a bad relay. IF the aux battery goes, it will kill the main battery.
The jump being shown above is because that Jeep has a problem.
I just charged my batteries with one charger connected on top. The aux is 100%, 12.7 and the main or crank battery is 100% and 12.7 volts.
The aux handles system electronics, the main or top battery is connected to the starter - but the aux battery is also connected so when you crank it over to start it, you are drawing from both batteries, not just one.
If you leave it like he has it, you'd better disable your ESS system otherwise it will draw power from the main battery when in ESS engine stopped mode and may not restart.

 

[pcrawfordpt]

Apparently you can jump your own auxiliary battery from your main battery if stuck out on the trail or away from services. I did see a tip from another JL owner who showed jumping from another vehicle to the first terminal/auxiliary, but didn’t know you could just jump it yourself from the main battery to the first terminal/auxiliary. But it does make sense, as long as your main battery isn’t dead as well.

Video doesn't seem to be showing up?

 

[pcrawfordpt]

Unless in ESS mode, shut down by ESS, both batteries are in parallel.
The only reason you'd ever have to do that is if your Jeep is broken - if there's another issue.
When you charge your battery with cables on the top, you are also charging the aux battery.
Only if there's a fuse blown or a relay bad would you ever need to do this.
So if you have to jump to that post - you'd better be checking it out.

I've jump started Jeeps with a bad aux battery by using cables on the top posts, or like with Grand Cherokee, there are posts on the right fender in the engine bay. Hook to that and you're good to go.

Check the wiring diagrams.
Ground for both batteries is on top of the IBS on the crank battery negative (that's the top or main battery)
The positive for both batteries is on the positive post of the main or crank battery.
The aux is connected there unless there's a problem. Disconnect that clamp off the + of the main or crank battery - it will still be hot! Touch it to ground and blow F3. Then your aux battery will no longer be connected HA.

Basically, if the AUX battery is dead, your main battery will be dead too unless you blew a fuse or have a bad relay. IF the aux battery goes, it will kill the main battery.
The jump being shown above is because that Jeep has a problem.
I just charged my batteries with one charger connected on top. The aux is 100%, 12.7 and the main or crank battery is 100% and 12.7 volts.
The aux handles system electronics, the main or top battery is connected to the starter - but the aux battery is also connected so when you crank it over to start it, you are drawing from both batteries, not just one.
If you leave it like he has it, you'd better disable your ESS system otherwise it will draw power from the main battery when in ESS engine stopped mode and may not restart.

...great insight, but can you explain that in simpler terms, so the rest of us can understand? Are you saying what was shown on the video doesn't work? Their main battery was fine, but the Aux was dead. This jump got them through their trail over several days.

 

[ShadowsPapa]

When things are working normally, both batteries are connected together. You can't have one dead without pulling from or being a load on the other. That's why when the aux battery goes bad, it kills the main battery as well. And when you charge at the main battery, the aux battery also gets that power from the charger.

I stole this image from another post here - you can see the green is the path of power from and between the batteries.
The negative are tied together very directly - aux battery (which feeds the system electronics when the ESS, or electronic stop/start is active) is connected to the terminal that goes on the main or crank (runs the starter more directly)
So ground is really closely tied, negative are tied together at the main/crank battery (-) and to the body ground.

The positive are also tied together. The positive from the main or crank battery is connected to N2 and through N3 down to the PCR which is a relay, then to the aux battery positive. So you can't apply power to the main battery without also applying power to the AUX battery because it's connected - follow the green dashed line.

Many of us have had the aux battery go bad - and what it can do, and often does, is drain the main battery. They are connected together - + to + and - to -.
If you put a load on one, you drain both.
When you start the vehicle, yes most of the "power" to the starter comes from the main or crank battery, but some also from the aux battery.

If you put a volt meter on the aux battery, it would also drop when cranking.

Putting a jumper from your main or crank battery + means that if that aux battery has a problem, say it's shorted, it will draw down your main battery as well.

If it takes a jumper to make things work, I'd be getting it fixed pronto. You don't know what caused the aux battery to "die" and it's very possible it will take the main battery out with it - it did in the case of one of our Jeeps. I kept jumping it to get my wife to where she wanted to go and finally....... it just wouldn't start back up as the aux battery killed the main battery.
I'd be checking the high current fuses as well.

What they were doing "got them by" but I'd have been really concerned with that aux battery tied directly like that to the main during normal operation.

 

[Radio Guy]

...great insight, but can you explain that in simpler terms, so the rest of us can understand? Are you saying what was shown on the video doesn't work? Their main battery was fine, but the Aux was dead. This jump got them through their trail over several days.

The Shadow guys explanation is correct and something is fishy in the video like the fuse to the AUX battery is blown or the ESS relay is bad, etc. At rest both batteries should be in parallel and it would be impossible for one battery to be dead and the other charged. Since the big battery is the starting battery and the aux battery can power just the electronics in ESS mode, I suspect a blown fuse and the aux battery is dead in the video. In this case they can charge the aux battery via the method in the video but I don't see a way for the vehicle to charge it when the engine is running if there is no connection to the main battery.

 

[pcrawfordpt]

The Shadow guys explanation is correct and something is fishy in the video like the fuse to the AUX battery is blown or the ESS relay is bad, etc. At rest both batteries should be in parallel and it would be impossible for one battery to be dead and the other charged. Since the big battery is the starting battery and the aux battery can power just the electronics in ESS mode, I suspect a blown fuse and the aux battery is dead in the video. In this case they can charge the aux battery via the method in the video but I don't see a way for the vehicle to charge it when the engine is running if there is no connection to the main battery.

I don't think they claimed to be charging the Aux batter with their jump. They pretty much gave up on it being dead. This just allowed them to start their vehicle without having to jump it every time. I believe the plan was to change it when they got home. The little jumper at the end was to bypass the use of the Aux battery all together, again, until they got home. I do find it odd that you and Shadowspapa claim that both batteries should die if all is well with the system. I have heard of many many owners that have just the Aux battery die much sooner than the main battery, leaving them stranded. That said, what fuse exactly are we looking to check to see if there is a problem as you both suggest? Thank you.

 

[jebiruph]

What is described in the video does not work in a regular JL/JT. If it worked for them, then they have a modified battery system. It's not a big deal to run with the aux battery as a disconnected spare, that's what the fused bypass jumper facilitates. Here's the thread on the Wrangler forum where we discuss using the aux battery as a disconnected spare. https://www.jlwranglerforums.com/forum/threads/ess-dual-battery-management.60034/post-1287541

 

[ShadowsPapa]

There's a lot of posts even in this forum where people have said the aux battery went bad and took the main battery down.
I'm one of those with my wife's Jeep.
Of course there are ways a battery can be "dead" and be isolated - an open inside the battery, for example, renders it dead and doesn't touch the main. But if that's the way it went, then it should still run as normal and start as normal, just without the aid of the aux battery. The main battery is still connected to the vehicle electronics when starting and when you are driving down the highway. So if the aux is open, all that happens is if ESS kicks in, there's nothing powering the vehicle electronics.
If the aux dies due to a short- it's a load on the main battery, draining it, just like if you plugged in some other electrical load. It becomes a load on the main battery, dropping voltage at the end of the system where the aux is and eventually draining the main battery dead (and eventually killing it because it's deep-cycling it to empty)
There are other ways it can go south.

Simple versions of what could happen
If the aux is shorted, it drains the main and acts like an electrical load in that vicinity and causes low voltage to the electronics even though the main is still technically connected to the electronics. It loads the whole thing - possibly blowing fuse F3.
If it's open, it's dead and everything works except ESS and you don't have the advantage of both batteries supplying cranking and starting power.

Interesting thing about YT videos - sometimes people making them don't have any clue what they are doing or why, just that hey, look what we can do........ and sometimes they assume everything else is that way.
I watch most - ready with that grain of salt, sometimes for entertainment, and once in a rare while, actually learn something. But most make me cringe, laugh or even want to yell at the computer.
A friend used to say "just because it says professional grade on the box doesn't mean you are one" and with YT - just because someone made a video doesn't mean they understand what they are doing, it works, so they make a video.

 

[Radio Guy]

So here is a question that may answer why the video jump start worked. When starting the vehicle at any time, does the ESS relay open every time using only the main battery for cranking while the AUX battery is only powering the electronics?

If this is true then if the AUX battery has failed as in it will not hold a charge, has no capacity but otherwise does not drag down the main battery, then opening the ESS relay would leave no power to the vehicle electronics during starter cranking. In this case doing a jump from another vehicle or battery to the N1 terminal would power the vehicle electronics during starting if the main battery is able to crank the engine over. Placing a jumper from N1 to N2 would also allow the vehicle to start and run if the main battery is able to crank the engine and that's why I carry an N1 to N2 jumper in my vehicle for emergency use.

In my opinion the ESS and AUX battery should be torn out of all vehicles and dumped in the trash.

 

[ShadowsPapa]

So here is a question that may answer why the video jump start worked. When starting the vehicle at any time, does the ESS relay open every time using only the main battery for cranking while the AUX battery is only powering the electronics?

If this is true then if the AUX battery has failed as in it will not hold a charge, has no capacity but otherwise does not drag down the main battery, then opening the ESS relay would leave no power to the vehicle electronics during starter cranking. In this case doing a jump from another vehicle or battery to the N1 terminal would power the vehicle electronics during starting if the main battery is able to crank the engine over. Placing a jumper from N1 to N2 would also allow the vehicle to start and run if the main battery is able to crank the engine and that's why I carry an N1 to N2 jumper in my vehicle for emergency use.

In my opinion the ESS and AUX battery should be torn out of all vehicles and dumped in the trash.

The aux battery aids in cranking - it's in the system except when ESS has stopped the system. Then it is isolated and the AUX battery runs the electronics, etc. and the main battery is left alone so it's not drained and there's power to crank.
They are paired in a cold start - I'll have to see what it says about ESS starts. I thought they were also paired for ESS starts for quicker starts (more amperage from the combined batteries)

 

[Radio Guy]

The aux battery aids in cranking - it's in the system except when ESS has stopped the system. Then it is isolated and the AUX battery runs the electronics, etc. and the main battery is left alone so it's not drained and there's power to crank.
They are paired in a cold start - I'll have to see what it says about ESS starts. I thought they were also paired for ESS starts for quicker starts (more amperage from the combined batteries)

If both batteries are paired during normal cranking then a dead (as in no capacity) AUX battery should not prevent the vehicle from starting. Several people have reported not being able to start the vehicle when the main battery is good but the AUX battery is dead. That would point to the ESS disconnecting the AUX battery from the main during cranking and a dead AUX would then prevent the vehicle from running due to dead computer and electronics.

Someone, maybe me, should run some tests and verify if both batteries are in parallel during normal cranking or if ESS separates them. I can't imagine the AUX battery aiding in cranking because if the main battery is toast then all cranking current would come from the AUX battery and flow through the ESS relay and fuse at N3. I doubt if those components and associated wiring are designed for cranking currents.

 

[ShadowsPapa]

If both batteries are paired during normal cranking then a dead (as in no capacity) AUX battery should not prevent the vehicle from starting. Several people have reported not being able to start the vehicle when the main battery is good but the AUX battery is dead. That would point to the ESS disconnecting the AUX battery from the main during cranking and a dead AUX would then prevent the vehicle from running due to dead computer and electronics.

Someone, maybe me, should run some tests and verify if both batteries are in parallel during normal cranking or if ESS separates them. I can't imagine the AUX battery aiding in cranking because if the main battery is toast then all cranking current would come from the AUX battery and flow through the ESS relay and fuse at N3. I doubt if those components and associated wiring are designed for cranking currents.

I can't argue with you (bad word, for lack of another) as I had the same initial thoughts - could it really crank the engine?
I've been going through my PDFs and other files, links and so on and can't find where I read what I read. So, I'd be very interested, even curious, of any testing to show once and for all when that relay is open, when it's closed.
I'll keep looking for my source.
Your logic seems fine and now it is bugging me as I spent a lot of time over days and weeks digging through things from FCA, Toyota and other resources on this.

 

[Radio Guy]

I can't argue with you (bad word, for lack of another) as I had the same initial thoughts - could it really crank the engine?
I've been going through my PDFs and other files, links and so on and can't find where I read what I read. So, I'd be very interested, even curious, of any testing to show once and for all when that relay is open, when it's closed.
I'll keep looking for my source.
Your logic seems fine and now it is bugging me as I spent a lot of time over days and weeks digging through things from FCA, Toyota and other resources on this.

A good way to prove or disprove all this would be to find an easily accessible point to measure coil voltage on the ESS relay. If it gets energized by 12v during cranking then the batteries are separated. I could measure voltage on the main batt and at N1 to get an idea like the main is pulled way down due to cranking current and N1 only dips a little but I want to specifically prove the ESS relay engages and the batteries are separated during cranking by design.

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