Low Alternator Output?
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5th Wheeling
Joined: Aug 2006
Posts: 35
Likes: 2
From: North Carolina
Low Alternator Output?
Just returned from a 400 mile round trip and plugged in a digital voltmeter to the power port in the dash for the trip. At interstate speeds in 10th gear, for the majority (90%+) of the trip, the vm read 12.5-12.7v. . It would rarely read 13.5-14.4 intermittently with no rhyme or reason. To my uneducated mind, it's no wonder that I can't keep my battery charged! I thought the alternator was supposed to be putting out a minimum of 13.5v. unless at idle. Or is there something about the electrical systems in this model that I'm unaware?
Fleet Mechanic
Joined: Jan 2023
Posts: 1,304
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You need to identify your exact vehicle. Your truck may have the battery monitoring system (BMS). The PCM will limit the charge for fuel efficiency and you may not see 13-14v if the battery is fully charged. BMS must be reset when new battery is installed.
I don't know all the Fords that implement BMS so hopefully the experts on your truck can help.
I don't know all the Fords that implement BMS so hopefully the experts on your truck can help.
Thread Starter
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5th Wheeling
Joined: Aug 2006
Posts: 35
Likes: 2
From: North Carolina
My vehicle is identified below my post. Yes, it has BMS and I have registered the battery when I installed it. I just tested the battery and it reads 12.63v. and 722 CCA on a 2-1/2 year old NAPA 92 ah battery rated at 850 CCA when new.
Laughing Gas
Joined: Jan 2013
Posts: 1,052
Likes: 57
I'm probably wasting my time answering this question.
You are your biggest problem when it comes to battery issues.
AGM batteries do not tolerate overcharging very well.
When you do charge one, you need to use a charger that is meant for AGM batteries.
They have a specific charging algorithm for charging one correctly.
In fact, Ford is only trying to charge the battery to 85%
If you do see one charging at higher voltage that would be more of an indication of issues than the lower voltage.
The best indicator for battery health in your truck is the start/stop system.
If the start/stop is not working, there is likely a problem with the battery.
I suggest you buy a charger/maintainer meant for AGM batteries.
I also suggest you read up on AGM batteries and correct charging of them.
You are your biggest problem when it comes to battery issues.
AGM batteries do not tolerate overcharging very well.
When you do charge one, you need to use a charger that is meant for AGM batteries.
They have a specific charging algorithm for charging one correctly.
In fact, Ford is only trying to charge the battery to 85%
If you do see one charging at higher voltage that would be more of an indication of issues than the lower voltage.
The best indicator for battery health in your truck is the start/stop system.
If the start/stop is not working, there is likely a problem with the battery.
I suggest you buy a charger/maintainer meant for AGM batteries.
I also suggest you read up on AGM batteries and correct charging of them.
Thread Starter
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5th Wheeling
Joined: Aug 2006
Posts: 35
Likes: 2
From: North Carolina
Mr. Smith-
Thank you for your overbearing and superior attitude. I'm here to gain knowledge, not to be berated. I happen to have over 70 years of experience working, restoring and racing a variety of high performance vehicles and currently own 7 other vehicles beside the F150, some of which are highly collectible and I perform maintenance on all of them. They are all on chargers and those with AGM batteries use highly rated CTEK models specifically selected for use on AGM. Now, go find someone else to insult.
Thank you for your overbearing and superior attitude. I'm here to gain knowledge, not to be berated. I happen to have over 70 years of experience working, restoring and racing a variety of high performance vehicles and currently own 7 other vehicles beside the F150, some of which are highly collectible and I perform maintenance on all of them. They are all on chargers and those with AGM batteries use highly rated CTEK models specifically selected for use on AGM. Now, go find someone else to insult.
Laughing Gas
Joined: Jan 2013
Posts: 1,052
Likes: 57
If you understood how it worked, you wouldn't need to ask the questions you asked.
Read up on how the BMS system works and how AGM batteries are correctly charged.
Read up on how the BMS system works and how AGM batteries are correctly charged.
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5th Wheeling
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From: North Carolina
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Lead Driver
Joined: Jun 2005
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From: Calgary Canada
The BMS does reuce voltage output from the alternator once it determines that the battery is "fully charged". Your definition of fully charged may not match the Ford BMS. On a longer highway trip, it's not a sign of a fault if the voltage output is lower. You want to see the voltage output first thing in the morning right after you start the truck. it should be 14+ volts for at least some short amount of time. Or just sit in your truck and listen to the radio with the engine off for 10-15 minutes one day while the battery runs down and then see how it behaves after you start back up.
I installed one of those USB power outlets with a voltage display on the 12V power point on my desh for this exact reason. It's fun to watch it go up and down through the drive, and when the auto-start-stop engages at traffic lights. The voltage is rarely constant.
I installed one of those USB power outlets with a voltage display on the 12V power point on my desh for this exact reason. It's fun to watch it go up and down through the drive, and when the auto-start-stop engages at traffic lights. The voltage is rarely constant.
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5th Wheeling
Joined: Aug 2006
Posts: 35
Likes: 2
From: North Carolina
@seventyseven250, thanks for your informative and polite post. Very much appreciated. (BTW, wish I could teach my cat to do push-ups. All she does is eat, sleep pee and poop!)
Internet Brands
Joined: Jul 2014
Posts: 82
Likes: 59
There is a target State of Charge in the BCM that can vary across the vehicles, but somewhere around 80% is what the system targets for. After that, the alternator output may be reduced in order to save fuel economy. If there is a load on the system, it may revert back to a higher duty cycle and voltage. This is covered in a service bulletin for the BMS, alternator and batteries.
I don't believe it has anything to do with AGM batteries and improving the service life. I suspect that is not Ford's main interest, but fuel economy is. The bulletin also mentions this is done to increase fuel economy by reducing the torque requirements on the alternator.
I don't believe it has anything to do with AGM batteries and improving the service life. I suspect that is not Ford's main interest, but fuel economy is. The bulletin also mentions this is done to increase fuel economy by reducing the torque requirements on the alternator.
Laughing Gas
Joined: Jan 2013
Posts: 1,052
Likes: 57
AGM batteries accept a higher rate of charge until they are apx 80% charged.
After that, they lower the charge rate to keep from overcharging the battery.
Overcharging an AGM battery will shorten its life.
Honda has a very similar system and has been doing it much longer than Ford.
An Acura tech told me that BMS systems actually saves more fuel than start/stop systems.
Internet Brands
Joined: Jul 2014
Posts: 82
Likes: 59
Google is your friend.
AGM batteries accept a higher rate of charge until they are apx 80% charged.
After that, they lower the charge rate to keep from overcharging the battery.
Overcharging an AGM battery will shorten its life.
Honda has a very similar system and has been doing it much longer than Ford.
An Acura tech told me that BMS systems actually saves more fuel than start/stop systems.
AGM batteries accept a higher rate of charge until they are apx 80% charged.
After that, they lower the charge rate to keep from overcharging the battery.
Overcharging an AGM battery will shorten its life.
Honda has a very similar system and has been doing it much longer than Ford.
An Acura tech told me that BMS systems actually saves more fuel than start/stop systems.
Ford's bulletin clearly states this is to improve fuel economy, Ford would care less about maintenance/consumable than fuel economy ratings for their company and the target SoC is also programmed on non-AGM battery applications.
Laughing Gas
Joined: Jan 2013
Posts: 1,052
Likes: 57
I understand about AGM charging. What I'm saying is that I don't believe the alternator duty cycle getting down into the 12.x range is due to AGM for three reasons.
Ford's bulletin clearly states this is to improve fuel economy, Ford would care less about maintenance/consumable than fuel economy ratings for their company and the target SoC is also programmed on non-AGM battery applications.
Ford's bulletin clearly states this is to improve fuel economy, Ford would care less about maintenance/consumable than fuel economy ratings for their company and the target SoC is also programmed on non-AGM battery applications.
It does improve fuel milage....after it gets the battery above 80-85% of charge.
Ford warranties the batteries also...they do care about that.
Yes, Ford does care about long term cost.
Think about 10k OCI, 150k transmission OCI, 100k spark plugs.........good for the vehicle or good for their reported long-term cost?
Start/stop puts a lot of demand on that battery.
They specifically use an AGM because it will recover quickly....up to the 80-85% point.
Believe what you will.
It really doesn't matter to me.
Internet Brands
Joined: Jul 2014
Posts: 82
Likes: 59
Did you research or just answer???
It does improve fuel milage....after it gets the battery above 80-85% of charge.
Ford warranties the batteries also...they do care about that.
Yes, Ford does care about long term cost.
Think about 10k OCI, 150k transmission OCI, 100k spark plugs.........good for the vehicle or good for their reported long-term cost?
Start/stop puts a lot of demand on that battery.
They specifically use an AGM because it will recover quickly....up to the 80-85% point.
Believe what you will.
It really doesn't matter to me.
It does improve fuel milage....after it gets the battery above 80-85% of charge.
Ford warranties the batteries also...they do care about that.
Yes, Ford does care about long term cost.
Think about 10k OCI, 150k transmission OCI, 100k spark plugs.........good for the vehicle or good for their reported long-term cost?
Start/stop puts a lot of demand on that battery.
They specifically use an AGM because it will recover quickly....up to the 80-85% point.
Believe what you will.
It really doesn't matter to me.
A battery is not covered under a maintenance schedule. Corporate fuel economy rating is a bigger deal.