Battery is fully charged but car is completely dead

[Doug V]

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.

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[Postal Bob]

Let's start with the basics. When did you last replace the batteries in the key fob? They may be completely dead which is why you get no response from the key fob.
And do you have the maintainer set for AGM battery, not regular battery?

Putting the battery maintainer on may increase the charge in the battery, but it doesn't completely charge the battery to full. But just because the battery may be charged, it doesn't automatically come out of sleep mode, until you turn on the car. Which is why no lights come on when you open the door. But if the battery in the key fob is dead, you won't be able to start it. Replace the key fob batteries. Or first try starting it by inserting the key fob in the special slot in the center console.

 

[Doug V]

Let's start with the basics. When did you last replace the batteries in the key fob? They may be completely dead which is why you get no response from the key fob.
And do you have the maintainer set for AGM battery, not regular battery?

Putting the battery maintainer on may increase the charge in the battery, but it doesn't completely charge the battery to full. But just because the battery may be charged, it doesn't automatically come out of sleep mode, until you turn on the car. Which is why no lights come on when you open the door. But if the battery in the key fob is dead, you won't be able to start it. Replace the key fob batteries. Or first try starting it by inserting the key fob in the special slot in the center console.

Key fob batteries have been replaced about 3-4 months ago. Battery tender is automatic and is compatible with AGM batteries. The battery voltage is good for a fully charged battery. It has been removed and tested. Key fob is always in the center console cup slot where these sensor is located. The car is completely dead. I cannot use any electronics and cannot press the start button to put it in accessory mode.

 

[Skye]

If you stand outside the car, fob in-hand, touch the inside of the handle, will the car unlock?

It reads like the answer will be no, nothing happens. But I did want to clarify. The answer will help tell me if the car really is dead, or we are dealing with a start button, brake pedal switch or other issue inside the cabin.

Given the conditions, I wonder what would happen if...the negative cable were disconnected from the battery, the car left to sit for an hour or more, then reconnected. If that would clear the car's logic and help it realize, "hey, i do have a battery here".

Edit,

Seeing the car enter sleep mode at the two-week mark is not necessarily an issue, and, could explain how the tender topped-out so quickly.

In my experience, Sleep Mode operates in two ways, simultaneously.

- Voltage. If the car determines the voltage has dropped to a certain point, it will go to sleep mode, to keep the battery at the highest voltage possible

- Time. Tender attached, myself and others have seen the car will enter Sleep Mode about the 13-day mark, no matter what. The battery is fine. But the car doesn't know when someone will be back. It then defaults to sleep mode

 

[BlkMach10510]

from google ai -
"2. Bad Ground Connection
Electricity travels in a circle. If the thick black ground cable that connects the negative battery terminal to the car's metal body or engine block is corroded or broken, the circuit is broken and the car will be completely dead.

• The Fix: Follow the negative cable from the battery and check where it bolts to the frame. Ensure this connection is tight, clean, and rust-free."

 

[TonyNJ]

My bet is bad ground connection.

 

[OLD8BA]

i thought same as above remarks ground {neg} lead if you have jumper leads use neg one on battery and onto a ground

as an aside i put 2 new duracel ? batteries into my fob and the died in 2 weeks it stumped me for a while i replaced the 2025 batteries with 2032, i had here they are same voltage very slightly larger but fit great

moral of story check shelf life of batteries when buying them i didnt hence problem

 

[ORRadtech]

If it's not a ground issue I'd check the "mega" fuse. I'm pretty sure there's like a 275A fuse bolted in the battery junction box that will cause the OPs symptoms. I've never heard of one blowing out before but maybe a bad connection or something.

 

[bKennedy]

Get a Multimeter and start checking if you have power at various locations. On my car the positive battery cable goes to what appears to be a junction block with cables going to the starter and various other places. I would start there and see if you get at 12+ volt reading with the positive on each of the cables and the negative on your battery and then on your engine block. If you have good voltage there you may want to start checking fuses the same way. At least you will know if voltage is getting to various spots and you do not have a connection issue.

 

[Cobra Jet]

Check these thread that are specific to non-start no power conditions:.

Go to Post # 4 in this one: https://www.mustang6g.com/forums/threads/bad-ground.202076/#post-4048998

There is also this thread where a GT350 owner had a no power non-start issue and could not get the car to start, it ended up being a 125A fuse:
https://www.mustang6g.com/forums/threads/car-is-dead-need-help.137125/

This M6G member had a non-start failure and after much diagnosis, it was found the cause was due to a wiring harness. The harness that was eventually found was under his S550 and it rubbed on the chassis enough that the wear chafed through the outer protective conduit and rubbed through the wire casings causing a short, which kept blowing a fuse. See post #34 of the below thread:
https://www.mustang6g.com/forums/th...-stalled-and-it-doesnt-start-or-crank.154984/

 

[Doug V]

If you stand outside the car, fob in-hand, touch the inside of the handle, will the car unlock?

It reads like the answer will be no, nothing happens. But I did want to clarify. The answer will help tell me if the car really is dead, or we are dealing with a start button, brake pedal switch or other issue inside the cabin.

Given the conditions, I wonder what would happen if...the negative cable were disconnected from the battery, the car left to sit for an hour or more, then reconnected. If that would clear the car's logic and help it realize, "hey, i do have a battery here".

Edit,

Seeing the car enter sleep mode at the two-week mark is not necessarily an issue, and, could explain how the tender topped-out so quickly.

In my experience, Sleep Mode operates in two ways, simultaneously.

- Voltage. If the car determines the voltage has dropped to a certain point, it will go to sleep mode, to keep the battery at the highest voltage possible

- Time. Tender attached, myself and others have seen the car will enter Sleep Mode about the 13-day mark, no matter what. The battery is fine. But the car doesn't know when someone will be back. It then defaults to sleep mode

Yes, completely dead.

I'll try disconnecting the negative cable for a while. If that doesn't work, I will try tracing the power cables to see if I can find a loose or damaged cable.

 

[Skye]

Yes, completely dead.

Check. Thanks for the clarification.

As others have mentioned, check the chassis ground near the strut tower. Ya never know.

Good Luck.

 

[bhulku]

Yes, completely dead.

I'll try disconnecting the negative cable for a while. If that doesn't work, I will try tracing the power cables to see if I can find a loose or damaged cable.

you probably did this but did you check all the fuses?

 

[ORRadtech]

you probably did this but did you check all the fuses?

Including the bigazzed mega one?

 

[TonyNJ]

I've had issues with the ground at the starter on an older S550. Starter was rusty. I'd get intermittent no starts. However, I don't think it was completely powerless like you've described. Regardless, trace the cables from the battery out, you'll probably find some corrosion somewhere.

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