Ford BMS Battery Monitoring System: How it Works, How to Properly Charge, How to Power Accessories

[scott011422]

Given the batteries are always connected together, their SOCs shouldn't deviate unless one is defective. The fan 5 trick is very much still alive and working.

Yes and no. Lets just say yes, After letting the vehicle sit for an hour the SOC's should be about the same. And being they are connected together they will receive the same charging rate. But the amount of charge required to fully charge each battery from that resting voltage is not the same.

For instance, 11.75v is the median voltage for an average AGM car battery. For my batteries, at that voltage, Its going to take 40ah of charge to fully charge the underhood battery and only 7ah to fully charge the AUX battery. So with both batteries tied together, how is ford going to continue charging the underhood battery when the AUX battery charges 6+ times faster and reaches fully charged? They cant. Battery hits full, Voltage rises and charging ends.

I'll check the fan again. I know last I checked after that BMS update running the fan above 5 was not having any real effect anymore.


***Edit***

Fan was set to 6 on our 2 1/2 hour trip last week and was hitting 12.75v. So definitely not working.

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[HammaMan]

Yes and no. Lets just say yes, After letting the vehicle sit for an hour the SOC's should be about the same. And being they are connected together they will receive the same charging rate. But the amount of charge required to fully charge each battery from that resting voltage is not the same.

For instance, 11.75v is the median voltage for an average AGM car battery. For my batteries, at that voltage, Its going to take 40ah of charge to fully charge the underhood battery and only 7ah to fully charge the AUX battery. So with both batteries tied together, how is ford going to continue charging the underhood battery when the AUX battery charges 6+ times faster and reaches fully charged? They cant. Battery hits full, Voltage rises and charging ends.

The DC/DC doesn't current limit. The resistance of the batteries by nature pulls the appropriate amps for charging. The SOC of the AGM limits the amps flowing into it. In the instance of a battery that's 6 times the size, it's eating 6/7 of the current while the smaller batt is eating 1/7th of it. When the key is off they will equalize any difference.

In the instance of a different chemistry like having an LFP aux batt, the resting voltage is 13.3v which keeps the under hood batt fully charged. My fan is always at setting 5. Only during remote start, when the driver's door is open, or when the key is not in the truck while running does the voltage sag.

Quite a bit of running around on this day. The remote starts should be obvious as well as times I left it running w/ key out of it.

When something current limited like a charger is connected, it runs in constant current until the voltage rises and then it switches to CV. It can't begin its CV top up until all batteries have stopped eating current and current can tapper.

I have an audio system that isn't uncommon for it to be eating significant amperage at times. Between heated seats/wheel/massage pump, and the audio spiking 80+ amps resulting in spikes of up to 170 amps of current, the voltage will fluctuate a bit between readings depending on system draw at that time so it can make running voltage readings a little bumpy. The Aux LFP can be eating 30 amps as well on charging depending where it's at in the curve so on a cold day where the truck hasn't been driven in a week I wouldn't be surprised to see a peak of 200 amps on the DC/DC at any given moment which will, on this graph at least, appear as a voltage fluctuations. When tesla bought maxwell, their ultra capacitors have pretty much vanished from the market and I've yet to find a chinese equivalent I trust to build a small cap array to further stiffen the voltage on the 12v rail.

I have the aug/sept BCM update that changed the charging behavior on most trucks resulting in the more aggressive charging curve. There's been another security-locked BMS update that appeared in mid/late Oct that requires the dealer credentials to implement as all future BCM updates will do. Not sure what further changes are in it.

 

[scott011422]

The DC/DC doesn't current limit. The resistance of the batteries by nature pulls the appropriate amps for charging. The SOC of the AGM limits the amps flowing into it. In the instance of a battery that's 6 times the size, it's eating 6/7 of the current while the smaller batt is eating 1/7th of it. When the key is off they will equalize any difference.

In the instance of a different chemistry like having an LFP aux batt, the resting voltage is 13.3v which keeps the under hood batt fully charged. My fan is always at setting 5. Only during remote start, when the driver's door is open, or when the key is not in the truck while running does the voltage sag.

When something current limited like a charger is connected, it runs in constant current until the voltage rises and then it switches to CV. It can't begin its CV top up until all batteries have stopped eating current and current can tapper.

I have the aug/sept BCM update that changed the charging behavior on most trucks resulting in the more aggressive charging curve. There's been another security-locked BMS update that appeared in mid/late Oct that requires the dealer credentials to implement as all future BCM updates will do. Not sure what further changes are in it.

Thats hard to say. The DC/DC would still have to make enough amps to run the truck and charge the battery. If it was charging as you say, where the batteries are taking the current they need, That can still be a lot of current. As you say, That's why dedicated chargers charge CC first and only CV when almost done.

My battery is currently at 12.43v. Since the BMS update, my high end is 15.12ish. Thats 2.69v in differential voltage. The IR of my battery is 5.7. So 2.69\.0057=472 amps. That is a lot of amps.

Ford is essentially charging the battery like an alternator would. 14.4v or higher and whatever is extra gets dumped into that battery like a buffer. They just are not or cant put out quite enough.



No, I still have not had any warnings or loss of functions or anything else that happens when the battery gets too low. But the battery still isn't fully charged which isn't great for the health of an AGM battery.

Like a dog with a bone...........................................

 

[HammaMan]

The IR of the underhood is going to favor it substantially at all points of the charging curve. The IR of the 7.5ah is less than the underhood batts whose IR increases as it charges. If you're seeing the ~12.8v, the truck is trying to consume the under hood batt's SOC. Not sure why they don't open the isolator as a charging strategy if they wanted to keep them about the same. There is a possibility that the isolator is opening. We'd need to monitor the isolator to see if that's part of the equation particularly on long drives.

As can be seen on my curves, the v dip/recovery shown in red is the LFP topping up the under hood batt. We still don't know if the reported SOC value is the sum/avg of both batts. Mine is always reporting 100%. We'd have to look for more PIDs to see if both batteries aren't listed some how. The current transformer on the aux batt does watch current but as of yet we don't have a separate PID.

My LFP will eat current all of the way up to 14.6v as it's balanced (though that top 5% of its capacity does taper current off quite a bit). I do have another LFP of the same, I could swap them and see if it's still balanced as it can drift given that the truck reaches a point and floats at 14.2v. LFP needs to be brought up to 14.6 for a full top balance to occur. Perhaps I'll top balance the one I have here and swap them out to see if the one in the truck is still balanced. As can be seen on the graph the first drive took longer to reach steady state after driving and that time lessened as subsequent drives occurred. The one unhelpful component of the graph is the that the bottom line to middle line represents 12-13v whereas the middle line to top line represents 13v-15v which isn't a very unhelpful visual representation of having a graph in the first place when an otherwise linear curve is interrupted at the 13-15v transition with each rising being a value of 2x vs values below the 13v line (the slope < 13v = 1:1, above 13v is 1:2). The representations are not linear despite it making it look like they are.

 

[Snakebitten]

Just an update for those wondering if Snakebitten is in fact installing an LFP Auxiliary, in the shadow cast by the mad scientist of the forum.

I'm currently "balancing" the cheap little 25AH LFP. Or attempting to follow the directions of HammaMan to do so. Once I get it as good as I know how, I'll pop it in Parallel with the ~1000 day old oem AGM.

I have so much history with the state of the LV storage on the truck, that it shouldn't be difficult to see any contrasts exhibited by the auxiliary swap.

Kitchen countertop (RV) draining off some charge so I can go at it again. (I'm really just pretending that I have a clue)

 

[mwf5888]

It's probably against the law for me to receive compensation from China?

I will say this though, the number of personal messages on 3 different forums regarding the device is substantial.
But I do wish there was an equivalent made by someone like Texas Instruments, even if it cost 10X's as much.

@Snakebitten unlike you I work from home with no daily commute, so my 5-month-old 2024 KR only has 3,200 miles on it. Do you have photos of the NOCO installation and how you use it? I am looking at the NOCO Genius 1-Bank 5 Amp. This one attaches to the batter posts and has a 6' cord to plug into an outlet (I have a plug 4' from the right fender.)

I was thinking of using velcro to attach to the top of the battery and snake the cord out of the front grill.

The image is x2 not the 1 but same wiring.

 

[Snakebitten]

I'll try to get a picture of it tomorrow, but I just have it velcro'd sitting on top of the fuse box between the AGM battery and the firewall. And I have the chord rolled up alongside the battery and the fender.

Nothing fancy and it can't go anywhere.

 

[Gros Ventre]

Yes and no. Lets just say yes, After letting the vehicle sit for an hour the SOC's should be about the same. And being they are connected together they will receive the same charging rate. But the amount of charge required to fully charge each battery from that resting voltage is not the same.

For instance, 11.75v is the median voltage for an average AGM car battery. For my batteries, at that voltage, Its going to take 40ah of charge to fully charge the underhood battery and only 7ah to fully charge the AUX battery. So with both batteries tied together, how is ford going to continue charging the underhood battery when the AUX battery charges 6+ times faster and reaches fully charged? They cant. Battery hits full, Voltage rises and charging ends.

I'll check the fan again. I know last I checked after that BMS update running the fan above 5 was not having any real effect anymore.


***Edit***

Fan was set to 6 on our 2 1/2 hour trip last week and was hitting 12.75v. So definitely not working.

The Aux battery does not recharge 6X faster. Two batteries in parallel will charge/discharge proportional to their capacity. In other words they will recharge at the same rate. The thing to pay attention to is the input voltage, its the same for each battery. The only way to recharge one battery at a different rate than the other would be to apply different voltages to each battery. If one battery is more discharged than the other its in parallel with, the battery with the higher state of charge will contribute to the charging current of the lower battery until they are equal in state of charge. Then they'll recharge at the same proportional rate.

 

[scott011422]

The Aux battery does not recharge 6X faster. Two batteries in parallel will charge/discharge proportional to their capacity. In other words they will recharge at the same rate. The thing to pay attention to is the input voltage, its the same for each battery. The only way to recharge one battery at a different rate than the other would be to apply different voltages to each battery. If one battery is more discharged than the other its in parallel with, the battery with the higher state of charge will contribute to the charging current of the lower battery until they are equal in state of charge. Then they'll recharge at the same proportional rate.

The smaller battery has 6 times less capacity, if both batteries are connected together and charging at the same rate, surely the smaller battery will fill up first?

If you have 2 batteries connected together with the same charge voltage and current, Why wouldn't the one with less capacity finish first? I mean, I understand that the larger battery will have the larger potential for current, and path of least resistance and all that. So I know that the larger battery will "take" more of the available current than the small battery, that its not a 50/50 split between the two, But i'm assuming it still charges and would finish first?

Are you implying the small battery will not charge really charge until their natural current absorptions are equalized? (I don't know at what point that would be since I don't know the IR of my AUX battery)

This is a logic puzzle for me, Help me see what im not understanding. I've never come across charging two different sized batteries at the same time. I've only dealt with matching batteries.

 

[Gros Ventre]

The smaller battery has 6 times less capacity, if both batteries are connected together and charging at the same rate, surely the smaller battery will fill up first?

If you have 2 batteries connected together with the same charge voltage and current, Why wouldn't the one with less capacity finish first? I mean, I understand that the larger battery will have the larger potential for current, and path of least resistance and all that. So I know that the larger battery will "take" more of the available current than the small battery, that its not a 50/50 split between the two, But i'm assuming it still charges and would finish first?

Are you implying the small battery will not charge really charge until their natural current absorptions are equalized? (I don't know at what point that would be since I don't know the IR of my AUX battery)

This is a logic puzzle for me, Help me see what im not understanding. I've never come across charging two different sized batteries at the same time. I've only dealt with matching batteries.

A battery charges at a rate proportional to the difference between the input voltage and the state of charge "voltage" not the size of the battery. For two batteries in parallel, they see the same input voltage thus they'll receive a charging current proportional to that. Neither battery will finish before the other because they see the same input voltage.

 

[Snakebitten]

It's time for the lab!,

I sometimes wish I didn't find such things so interesting, but I'm pretty sure I'm just came wired to? Lol

I'll grab some small cheap 12V storage (batteries) of various capacity and use this new toy (lab power supply) to experiment.

But I have a feeling this thread already has a few fellas that know what I'd find.

 

[Mszczewski]

@Snakebitten unlike you I work from home with no daily commute, so my 5-month-old 2024 KR only has 3,200 miles on it. Do you have photos of the NOCO installation and how you use it? I am looking at the NOCO Genius 1-Bank 5 Amp. This one attaches to the batter posts and has a 6' cord to plug into an outlet (I have a plug 4' from the right fender.)

I was thinking of using velcro to attach to the top of the battery and snake the cord out of the front grill.

The image is x2 not the 1 but same wiring.

I did this install a while back with the 10 Amp version.

Details here: https://www.f150gen14.com/forum/threads/battery-tender.11792/page-2#post-263506

 

[KTM753]

It's time for the lab!,

I sometimes wish I didn't find such things so interesting, but I'm pretty sure I'm just came wired to? Lol

I'll grab some small cheap 12V storage (batteries) of various capacity and use this new toy (lab power supply) to experiment.

But I have a feeling this thread already has a few fellas that know what I'd find.

Interesting means you're interested in learning something new everyday. That's a good thing! Thanks for sharing your journey/progress. Others of us are reading and learning along with you.

 

[mwf5888]

I'll try to get a picture of it tomorrow, but I just have it velcro'd sitting on top of the fuse box between the AGM battery and the firewall. And I have the chord rolled up alongside the battery and the fender.

Nothing fancy and it can't go anywhere.

@Snakebitten where did you connect the cables from the NOCO?

 

[Mszczewski]

@Snakebitten where did you connect the cables from the NOCO?

The positive to the bolt on the positive clamp. The negative to one of the frame grounds coming off the negative terminal. That way, you will be on the correct side of the BMS shunt.

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