Disconnecting the 12V Battery to "fix" a problem - BMS Learned Values Reset (2024)

Mike G

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There have been discussions on the forum where the suggestion has been made to disconnect the 12V battery in order to "reset" some function of the vehicle that was not performing properly (TCU comes to mind for the '23 model owners). I realize that disconnecting the 12V battery is probably easier than finding the fuse for the offending module and just pulling that for a few minutes instead.

But it may be creating a different problem. There is a Battery Monitoring System (BMS) that keeps track of your 12V battery status and lets the BMS know when the battery is fully charged. Disconnecting the 12V battery can screw up the "learned values" in the BMS and then your 12V battery may not fully charge after that.

The fix for this issue is simple and techs are told to do this everytime they replace a battery or the FDRS program they use indicates that the battery state-of-charge is too low. It's a procedure that can be done in FDRS or with this simple routine that can be done before the next time you take a drive.

Press the start button without stepping on the brake (Accessory mode).

Flash the highbeams with the stalk 5 times, and then step on the brake pedal 3 times, and after that check the red battery icon on the instrument panel to see that it flashes three times.

And then press the start button again to turn the truck off.

And then you can turn the truck fully on and drive off. Your 12V battery should then fully recharge.

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Thanks! I got the three battery flashes.

Beats trying to rub my belly and pat my head while singing Yankee Doodle standing on my head.

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RickLightning

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I mentioned the battery disconnect to a Ford engineer while discussing my recent lack of GPS issues, i.e. I did the battery disconnect to try and fix the problem (didn't help, needed new coax splitter).

The comment I got back was as follows:

"Don't pull the negative. ...someone who did that and brick their car because it was doing an OTA."

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Like all things there are risks to this. Like Rick said if you kill 12v during an OTA you will brick the truck. It is fixable at a dealer but if you are pulling the 12v you are probably doing it to avoid the dealer to begin with.
The other risk is if you actually have a low 12v and reset your BMS it will have an incorrect baseline. You will then have an increased risk of the vehicle not keeping the 12v charged enough and might strand you. Granted the 12v shouldn't be that degraded on a truck so new that you have this issue, it's just one of those possible issues

MickeyAO

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Mike G said:

The fix for this issue is simple and techs are told to do this everytime they replace a battery or the FDRS program they use indicates that the battery state-of-charge is too low. It's a procedure that can be done in FDRS or with this simple routine that can be done before the next time you take a drive.

Press the start button without stepping on the brake (Accessory mode).

Flash the highbeams with the stalk 5 times, and then step on the brake pedal 3 times, and after that check the red battery icon on the instrument panel to see that it flashes three times.

And then press the start button again to turn the truck off.

And then you can turn the truck fully on and drive off. Your 12V battery should then fully recharge.

How do I do this through FDRS?


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carys98

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Gimme_my_MME said:

Like all things there are risks to this. Like Rick said if you kill 12v during an OTA you will brick the truck. It is fixable at a dealer but if you are pulling the 12v you are probably doing it to avoid the dealer to begin with.
The other risk is if you actually have a low 12v and reset your BMS it will have an incorrect baseline. You will then have an increased risk of the vehicle not keeping the 12v charged enough and might strand you. Granted the 12v shouldn't be that degraded on a truck so new that you have this issue, it's just one of those possible issues

I have previously disconnected the negative, charged the battery fully, and then reconnected and reset the BMS. Is this the recommended procedure? Right after I did that I was able to consistently get over 90% but now it stays in the 60s and maxes out around 83%. Should I repeat that procedure? I strongly suspect that my battery is faulty but the dealer tested it and claimed it is OK.

Gimme_my_MME

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carys98 said:

I have previously disconnected the negative, charged the battery fully, and then reconnected and reset the BMS. Is this the recommended procedure?

That is perfect.

carys98 said:

Right after I did that I was able to consistently get over 90% but now it stays in the 60s and maxes out around 83%. Should I repeat that procedure? I strongly suspect that my battery is faulty but the dealer tested it and claimed it is OK.

Your battery is ok, it is the BMS falsely causing these issues

azypather

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My OTA updates are failing, apparently due to a "Low 12V Battery charge" error per Ford. I don't see this message but that's what they tell me shows up on their end. They recommended me taking it in to the dealer. I will try this to see if it fixes the issue

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Mike G

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MickeyAO said:

How do I do this through FDRS?

It's a BCM tool. Fastest way to find it is to type "learned" in the search box on the top right of the toolbox page to filter for it. Below is where it is in the full list of tool routines filtered for just BCM related items.

Disconnecting the 12V Battery to "fix" a problem - BMS Learned Values Reset (5)

carys98

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Mike G said:

.

And then press the start button again to turn the truck off.

.

I think I may have solved my problem. The step highlighted above is very important (@Mike G or @Gimme_my_MME can you confirm?). I believe in the past I was immediately starting the truck after the battery icon flashed 3 times instead of powering off. I charged the battery overnight and followed the instructions to the letter this morning and I am now showing 100% SOC.

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Mike G

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Aug 3, 2023

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Another downside to completely disconnecting the 12V battery is (according to the service manual) before you do that you are supposed to first de-energize the high-voltage battery system....and then after you reconnect the 12V battery on a truck that has one-touch windows you're supposed to perform the window motor initialization.

So how many of you out there de-energized your HV battery pack before disconnecting the 12V battery?

...yeah that's what I thought... Disconnecting the 12V Battery to "fix" a problem - BMS Learned Values Reset (7)

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