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My daughter owns a 2018 Compass Sport WG. Yesterday it did not start with the dashboard button. She checked Settings and the Battery Voltage was at 11.4v. We hooked it to a Battery Charger, but still no Start.
Tried Remote Start and it started, however as soon as I step on the brake, it shuts off (does not shut off if I press the Gas Pedal).
After letting it idle for about 10 minutes, stepping on the brake did NOT kill the engine and she drove it for the rest of the day.
Problem is, it will only Start remotely now and every time it’s a crap shoot as to whether the brake thing happens.
Any help appreciated. TIA.
 

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My daughter owns a 2018 Compass Sport WG
What is a WG??? You said 2018 so you could only be talking about a second gen MP.

She checked Settings and the Battery Voltage was at 11.4v
Very bad. How long had the car been sitting without driving anywhere?

We hooked it to a Battery Charger, but still no Start.
How long was it on the charger? How many amps does your charger produce? Car batteries store HUGE quantities of power, and there is a limit to how fast they can charge even if you have a great big powerful charger, but most people have a little baby charger 10 amps or less and it takes a loooooong time for those to bring a battery that dead back up to strength.

Tried Remote Start and it started, however as soon as I step on the brake, it shuts off (does not shut off if I press the Gas Pedal).
Well, that is normal in the sense that if you use remote start you have to press the start button BEFORE you can press the brake and shift into gear. So nothing actually that weird about that unless you were leaving something out.

Problem is, it will only Start remotely now and every time it’s a crap shoot as to whether the brake thing happens.
Check that you are following the correct steps as mentioned above. If you use remote start, you MUST press the start button to bring the vehicle fully online before pressing the brake. Read the owner's manual.


In general, these cars HATE having low battery power, but your description is really odd to me because from what I have noticed these cars typically will not allow a remote start once the battery gets down to that 11.4 volt range. But I guess if you had your battery charger on it that might have made a difference. Lack of accurate details might be killing us here.

But anyways, low voltage tends to make the computers angry. You may have some other issues going on besides. Is the start button just straight up broken? Is there any reaction from the vehicle when you press it? Did you put a fresh battery in the keyfob or try the other keyfob? Did you try pressing the start button WITH the keyfob? (standard procedure for keyfob battery failure to fallback on passive RFID to start, again, read your user manual)

What you describe is pretty strange but all we know for sure is that you have an issue with a super dead battery. Figuring out why/how that happened is step 1. If the car simply sat without being driven for a week, that would typically do it (there is too much electricity used by the alarm and standby electronics, basically). If the car was being driven regularly and the battery still came up dead, then you likely have a defective battery that needs to be replaced.

The annoying thing is that these cars actually have TWO batteries so its a bit complicated figuring out which is bad. If it were me, I'd straight up replace them both, but it will cost a lot. You can take it to the dealership and roll the dice but believe me when I say they are being BOMBARDED with people complaining of dead batteries, and they will be reluctant to help because FCA is clamping down on reimbursing them for bad batteries.

Almost every modern car after 20016-ish seems to have this issue. The alarm and electronics have a significant draw when the car is parked, and no manufacture saw COVID-19 coming and it didn't matter that the alarm and standby stuff draws a lot of juice because before covid, we (as Americans) drove all. the. time. Thats all we do (did), was run the roads. Everyone had eleventeen places to be every day, we rack up billions of miles collectively, and cars never sat long enough for standby electronics to run the battery down. Then covid: boom, hundreds of thousands of cars parked. We've got almost no where to go. We work from home. Make one trip to a grocery store, maybe. And what are people finding when they go to make that grocery run? DEAD BATTERIES. or low batteries, anyways.

My mother's 2016 CRV is having the same issue ever since she started working from home, as well as both my Jeep and my wife's jeep. They never expected Americans to go from driving 100 places a day to a sudden full-stop.

Now to be clear, this is my personal theory and so far I can only offer a meager amount of evidence for it, but you asked on a public forum so you are getting my two-cents.

If its under warranty, let the dealership take a crack at it. If you are willing to start hucking some expensive hail-marys, I would start with replacing BOTH batteries. Its expensive, it sucks, but NOTHING computerized works properly if the computer doesn't get the voltage its expecting. You may have a computer problem (low voltage actually could potentially damage the car's computers permanently), but to narrow it down to that you MUST start with the batteries and work your way forward (or let the dealership try).

If you can, fully charge BOTH batteries with your charger and see 1) does one or both of them struggle to attain a full charge? and 2) Does the issue go away when BOTH batteries are fresh off the charger?

This will help you build evidence towards potentially bad batteries.

You may be tempted to stop into your local autozone and have them put their battery tester on it. Problem with that is many people, including yourself, don't even see or know about the second battery hidden right next to the big battery, so putting a tester on is a little trickier than cars of old. You have to unhook one as they are sometimes bridged together by a relay the computer controls. If you don't keep voltage on the lines you could loose radio settings or have to enter a code or some annoying thing (not personally tested on a compass but a common issue on other cars). So if you ask some neckbearded-napa cashier if they can test you battery... maybe check that they understand the car has two batteries and that they know how to properly separate and test each one.

Even if they do (know how to properly test), take it with a grain of salt because battery health is more complicated than what a 10-second test from their "magic box tester" can tell you. Batteries are rarely 100% blatantly bad. The biggest thing skewing results of a battery test in the parking lot of your parts store is the fact that you just drove it there, with your alternator charging it hard the entire time, so that battery might have some life in it after driving to the parts store, and then after sitting overnight back at your house could be a different story.

Takeaway: Parking lot battery tests are extremely unreliable.

If you can eliminate the batteries, either through good testing or straight up buying new ones, you can start to work through possible IBS (intelligent battery sensor) failure, and from there work back towards computer failure.

Its tricky, its messy, its time consuming, its potentially expensive and its not much fun. So if there is ANY warranty at all, let them waste their time first and see if you get a free battery out of the deal. Don't be surprised if the dealership doesn't actually resolve the problem on the first try, they are notoriously bad at just making some random guesses or throwing a battery charger on for a few hours, starting the car once, and then calling you to say its good to go. They are busy and they are drowning in warranty work because of Jeep/Dodge/Chrysler quality problems, and the parent company (FCA) is bleeding money from warranty claims and doing everything they can to weasel out of any warranty work they can.

Its a bad deal. Good luck. If charging the batteries works, then just start putting a trickle charger on when its sitting a long time. Don't forget to rotate the charger to both batteries. Let us know how it goes.
 
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