12V Battery Charging Facts

[guinn]

I've seen a tremendous amount of disinformation here about how the 12V battery charging system works. Here are the "facts" that I determined when my 12V battery ran down!

I hadn't driven my 2021 ME for 3 or 4 days. I had been getting warnings about the battery being low with a message to plug it into the charger. Since it was already plugged in, I ignored these messages. When I tried to use the car, it was dead. No response to anything. I started figuring out what to do and opened the frunk using the wires in the bumper. I then charged the 12V battery using a portable 12V charger until I could get things going (overnight).

I decided since I had access to the 12V battery, that I would monitor its state of charge for a while. I used a couple of small jumpers and a voltmeter. I also downloaded a chart showing voltage vs SOC for a sealed lead acid 12V battery. Then I noted the battery voltage over a couple of days.

The results were a little erratic, possibly due to a damaged battery (which the dealer later replaced) and the fact that the software bug had not been fixed, but generally the voltage dropped from 12.89 volts (~100% SOC) to about 12.36 volts (~55% SOC), and then it jumped back up to 100% SOC. The behavior was essentially the same, regardless of whether or not I had the L2 charger plugged in. And it usually took about 12 to 13 hours to drop from 100% to 55%.

This makes perfect sense. The HVB is not used to consistently keep the 12V battery at 100%. It drops down to about half-charge, and then the charging circuitry uses energy from the HVB to recharge the 12V battery. Since the HVB can keep the 12V battery charged for a very long time, there is no reason that leaving the car unused would ever run the 12V battery completely down. (Ford is saying to do something if you are going to leave the car for more than 30 days, but I suspect that is just being very cautious.) And remember, my results were the same with or without having the L2 charger plugged in. So, energy from the L2 charger is not being used directly to charge the 12V battery.

My problem turned out to be a software bug, and I had to jump through hoops to get the dealer to apply the fix, but that's another story!

Sponsored

 

[volarium]

My understanding was that the HCB never charges the LVB, unless the car is running, or the L2 charge is actually charging the car. Your post seems to indicate that the HVB DOES charge the LVB, even when not running or charging, it just doesn’t maintain it to 100%. So it sounds like the previous information I understood was not accurate, is that correct?

 

[zvez]

I've seen a tremendous amount of disinformation here about how the 12V battery charging system works. Here are the "facts" that I determined when my 12V battery ran down!

I hadn't driven my 2021 ME for 3 or 4 days. I had been getting warnings about the battery being low with a message to plug it into the charger. Since it was already plugged in, I ignored these messages. When I tried to use the car, it was dead. No response to anything. I started figuring out what to do and opened the frunk using the wires in the bumper. I then charged the 12V battery using a portable 12V charger until I could get things going (overnight).

I decided since I had access to the 12V battery, that I would monitor its state of charge for a while. I used a couple of small jumpers and a voltmeter. I also downloaded a chart showing voltage vs SOC for a sealed lead acid 12V battery. Then I noted the battery voltage over a couple of days.

The results were a little erratic, possibly due to a damaged battery (which the dealer later replaced) and the fact that the software bug had not been fixed, but generally the voltage dropped from 12.89 volts (~100% SOC) to about 12.36 volts (~55% SOC), and then it jumped back up to 100% SOC. The behavior was essentially the same, regardless of whether or not I had the L2 charger plugged in. And it usually took about 12 to 13 hours to drop from 100% to 55%.

This makes perfect sense. The HVB is not used to consistently keep the 12V battery at 100%. It drops down to about half-charge, and then the charging circuitry uses energy from the HVB to recharge the 12V battery. Since the HVB can keep the 12V battery charged for a very long time, there is no reason that leaving the car unused would ever run the 12V battery completely down. (Ford is saying to do something if you are going to leave the car for more than 30 days, but I suspect that is just being very cautious.) And remember, my results were the same with or without having the L2 charger plugged in. So, energy from the L2 charger is not being used directly to charge the 12V battery.

My problem turned out to be a software bug, and I had to jump through hoops to get the dealer to apply the fix, but that's another story!

I'll be curious to hear what Mach-Lee says about this. I don't think I've read anywhere online that the car charges unless it's running or plugged in and charging.

 

[Maquis]

Dropping from 100% to 55% in 12 hours is not normal.

 

[ToOldForThis]

From my Owners Manual -
CHARGING THE 12V BATTERY
Your vehicle has a high voltage to low voltage energy transfer feature that keeps the 12 volt battery charged by the high voltage battery. If the 12 volt battery level is low, the high voltage battery transfers energy to the 12 volt battery when the vehicle is off.

 

[alexgorod]

Dropping from 100% to 55% in 12 hours is not normal.

What is normal? Please reference the source of your information. Thank you.

 

[DonkeyFarmer]

Normal: LV car battery should not lose more than 10% charge per day. Not even close to that.

Source: It's possible to start a car after it has sat for a week. I've done it. Most people have. If it was losing 45% in 12 hours, the battery would be flat dead after two days of sitting.

 

[guinn]

My understanding was that the HCB never charges the LVB, unless the car is running, or the L2 charge is actually charging the car. Your post seems to indicate that the HVB DOES charge the LVB, even when not running or charging, it just doesn’t maintain it to 100%. So it sounds like the previous information I understood was not accurate, is that correct?

That is correct. The HVB was definitely charging the 12V battery despite the fact that my car was not running and the L2 charger was not plugged in.

 

[Maquis]

What is normal? Please reference the source of your information. Thank you.

I don’t know exactly what’s normal, I’m just using common sense. 45% loss of charge is about 16AH, meaning a quiescent amp draw of about 1.5A. That would be a lot.

 

[guinn]

Dropping from 100% to 55% in 12 hours is not normal.

I don't know if it's "normal", but that's why my car was doing. Once it hit about 55%, energy from the HVB was used to charge it back to 100%.

 

[guinn]

From my Owners Manual -
CHARGING THE 12V BATTERY
Your vehicle has a high voltage to low voltage energy transfer feature that keeps the 12 volt battery charged by the high voltage battery. If the 12 volt battery level is low, the high voltage battery transfers energy to the 12 volt battery when the vehicle is off.

Exactly. But it doesn't keep the 12V battery at 100% all the time. It allows it to drop to about 50% and then charges it back to 100%. At least that's what was happening on my car.

 

[guinn]

Normal: LV car battery should not lose more than 10% charge per day. Not even close to that.

Source: It's possible to start a car after it has sat for a week. I've done it. Most people have. If it was losing 45% in 12 hours, the battery would be flat dead after two days of sitting.

Read my post again. The LV battery should not lose anything per day. The battery management system charges it back to 100% after it drops to around 50%. Only when the HVB starts getting low does this become an issue.

 

[Maquis]

I don't know if it's "normal", but that's why my car was doing. Once it hit about 55%, energy from the HVB was used to charge it back to 100%.

If that was normal, you‘d lose about 1/2% every 2 days from the HVB. Mine has set for two weeks without losing any.

ETA - those are ER numbers. You’d lose even more on your SR.

 

[Mach-Lee]

What’s the disinformation here? I think I covered the auto recharge in the FAQ, yes the car can automatically recharge the 12V battery when it gets critically low (<45%) while the car is off, but normally that shouldn’t be happening very often. Recharging twice a day is a significant drain that indicates a major problem. Those constant cycles on the battery will wear it out in only a year or two then you’ll eventually come out to a dead battery. The normal drain is 10% after the first two hours and about 5% per day thereafter. Which means the automatic recharge should really only be happening if the car is parked for a whole week.

Things are different if you’re trying to install an OTA and your car doesn’t have a major 12V drain. Sitting at 70% won’t trigger a recharge or be enough for some OTAs to install. For most people, the 12V isn’t going to charge unless you have the car turned on or are driving it.

In short, if you get multiple messages in FordPass about drains or needing to plug in to charge, you have an issue that needs to be taken care of. If you have a failing OTA because of insufficient battery, then additional steps need to be taken.

There have also been cases where the automatic recharge does not get triggered properly because of a glitch, so I personally would not rely on it to occur. Which is why fixing 12V drains is important.

Also, the automatic recharge will not happen when the HVB is less than 15%. It will just let the 12V die. That’s the situation you should see the “Plug in to maintain 12V battery” message but they have it programmed wrong.

 

[guinn]

What’s the disinformation here? I think I covered the auto recharge in the FAQ, yes the car can automatically recharge the 12V battery when it gets critically low (<45%) while the car is off, but normally that shouldn’t be happening very often. Recharging twice a day is a significant drain that indicates a major problem. Those constant cycles on the battery will wear it out in only a year or two then you’ll eventually come out to a dead battery. The normal drain is 10% after the first two hours and about 5% per day thereafter. Which means the automatic recharge should really only be happening if the car is parked for a whole week.

Things are different if you’re trying to install an OTA and your car doesn’t have a major 12V drain. Sitting at 70% won’t trigger a recharge or be enough for some OTAs to install. For most people, the 12V isn’t going to charge unless you have the car turned on or are driving it.

In short, if you get multiple messages in FordPass about drains or needing to plug in to charge, you have an issue that needs to be taken care of. If you have a failing OTA because of insufficient battery, then additional steps need to be taken.

There have also been cases where the automatic recharge does not get triggered properly because of a glitch, so I personally would not rely on it to occur. Which is why fixing 12V drains is important.

Also, the automatic recharge will not happen when the HVB is less than 15%. It will just let the 12V die. That’s the situation you should see the “Plug in to maintain 12V battery” message but they have it programmed wrong.

I suspect that everyone hasn't read your FAQ on the subject, because I continue to see posts asking how long someone can leave their MME unplugged, and I see incorrect answers.

The drain I saw is what was actually happening with my car. Your mileage may be different. As I stated, my car needed a software patch, so it may have been running down the 12V battery more quickly than normal. I don't have any after-market items installed, so that was not a factor.

If I ever get in the mood, I will test my car again!

Sponsored