Battery is fully charged but car is completely dead

[Mike Pfeifer]

A voltmeter will be your best friend in diagnosing this issue.

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[Skye]

I'm attaching references of the aforementioned mega fuse, the 125A unit.

This piece is not mentioned in the owners manual.

The fuse is located underneath the three hot leads within the under hood fuse block.

The item is akin to the main fuse in the service panel of your house, the link upstream to the power plant.

Losing this fuse severs the car's connection to the battery.

 

[2021CSGTJH12]

Battery is fully charged but car is completely dead

I have searched for topics and have not found anything that matches this issue. If this has already been answered in another thread, please reply with the thread subject.

Background
My car is a 2020. I purchased it new and have had it for about 6 years going on 7. I have aftermarket amps and speakers which have been in the car for about 4 years. The original battery died and was replaced at 5 years by an Odyssey AGM. The BMS was reset at that time. The AGM battery has been in the car for about 1 1/2 years. I have always used a battery maintainer from the beginning and have never had an issue with a dead battery or issues with the electrical system.

I had a weird thing happen. My car has been completely dead and unresponsive for about 2 weeks. No lights, chimes, noises, etc. It does not start or respond to the key FOB. About a week before my card died, I saw a notification on my Sync display saying my car is going into deep sleep mode, so I put a battery tender on it as I always do. Within a few hours, the battery tender goes into maintenance mode because that battery is fully charged.

I have checked the battery terminals (+ / -) for bad connections and corrosion and they look good. I have checked the fuses and there are no visible issues. I checked the Voltage quality module fuse in position 24 (30A) and that is good.

Has anyone heard or experienced this? Is there a larger fuse that I am missing? Do the new Mustangs have a fusible link between the battery and the fuse box? I will get a voltage tester to see what I can find, but I am not sure where to start. If the alternator were bad, I am sure I would still have working electronics. Same thing if the starter were bad.

So what happened? Was this fixed? I noticed you mentioned leaving key fob in center console which is inside the car. The car never goes to sleep this way.

 

[Doug V]

So what happened? Was this fixed? I noticed you mentioned leaving key fob in center console which is inside the car. The car never goes to sleep this way.

I am going

So what happened? Was this fixed? I noticed you mentioned leaving key fob in center console which is inside the car. The car never goes to sleep this way.

The key fob in the cupholder is only when driving.

I found the charger might be the culprit. Never consider this because it appears in working order. Got a new one installed so I am waiting. If the battery holds a full charge and there is no power to the fuse block, I am going to try the recommendation above from Skye. Removing the block that feeds the fuse panel to see if there is a larger fuse below.

 

[Skye]

I am going to try the recommendation above from Skye. Removing the block that feeds the fuse panel to see if there is a larger fuse below.

While I posted some photos, the credit for the fuse insight goes to @Cobra Jet 's reference. In that thread, while the owner of the vehicle confirmed the fuse had been tripped, how and the sequence of events that led up to it remained something of a mystery.

Best Wishes in whatever comes about.

 

[2021CSGTJH12]

The reason I asked is if you looked into my edit and one of my post about battery dying just 2 months with key fob leaving 1-2 away from my 2021 Mustang as key fob hangs on the wall. I did this to prevent car locking and alarming itself as my kids walk by the car often.

since then I have placed a new battery in there and after 40 hours of sitting. Voltage showed 12.93 at 4 hour mark and 12.73 at 40th hour. So seems OK. This is done with M
key FOB far away from the garage and
Inside my house.
It was hard for me to believe KEY FOB can do this!!!

 

[Doug V]

The reason I asked is if you looked into my edit and one of my post about battery dying just 2 months with key fob leaving 1-2 away from my 2021 Mustang as key fob hangs on the wall. I did this to prevent car locking and alarming itself as my kids walk by the car often.

since then I have placed a new battery in there and after 40 hours of sitting. Voltage showed 12.93 at 4 hour mark and 12.73 at 40th hour. So seems OK. This is done with M
key FOB far away from the garage and
Inside my house.
It was hard for me to believe KEY FOB can do this!!!

I will keep that in mind. My drawer where the both key fobs sit is directly above the garage where the car is parked.

Once I figure this thing out I will definitely look into this, and move the key fobs to a different area.

 

[Gen 6 Mach1]

I would not want to think the key fobs are the problem. I did buy faraday boxes for my fobs , so the fob doesn't communicate to the Car . I also use when I'm out ,put the fob in after I lock up the car .

 

[Cobra Jet]

My key fob is less than 1 car length from my 2018; have not had one issue with any “communication” issues or the fob causing any such battery drain or non-start condition - ever. That’s in 43k of ownership since new and still on the original Ford battery that came with it.

I think the “fob communication” is a myth, unless someone can actually provide actual metrics documentation showing the communication between the car and the fob with time stamps.

—

If the car is completely dead even with a new battery, meaning no lights no start, no radio, no nothing it has to be either ground issue OR the 125a max fuse is blown…. Or worse case, the PCM somehow got fried… but even if it was a bad PCM, I don’t think all power would be nixed to the rest of the components in the car.

You’re going to have to do methodical diagnosis to determine the issue and a multi-meter will be your best tool as mentioned above in some of the responses.

 

[Garfy]

When OP mentioned "completely dead" I'm presuming that not even the interior light comes on when he opens the door. When I open my door, not only the interior light goes on but you can hear relay clicks and the park lights go on as well. As for key fobs, mine is about 20 feet away in the laundry room adjacent to the garage. I've never had an issue and I finally replaced my fob battery after 5-1/2 years when I got a "low battery" message on my dash. My other fob has not been used in a long time but I suspect the battery in time will need replacing just from age; I'll probably swap it with the one I've been using all this time to see how long it'll be before I get a low battery warning on it.

Getting back on topic, as someone advised, get a good digital voltmeter and check the voltage at the battery terminals to ensure the battery is indeed fully charged, then check for power at the fuse box. If any of the 3 cables on the box are dead, start there. From there I'd probe fuses to be sure there are no burned fuses which would be indicated by having power on one side, but not the other.

 

[Doug V]

Turns out the key FOB reply was correct. I never would have guessed.

My spare key sits in a nightstand about 10 feet above the car when parked in the garage. Almost right over the key fob sensor in the console. It explains the chimes and flashing lights when i walk past the car.

I pulled the AGM battery and was able to get it to take a charge. I picked up a faraday box for my key fobs. The car no longer chimes and flashes when walking by. The battery is staying charged. Its been 3 weeks and haven't had any issues.

Thanks for all the suggestions.

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