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

[Snakebitten]

By the way Jersey Jim, I see that you have the Fluke 87, so you are discerning on such things.
Can I ask regarding the clamping meter, why the 21030T rather than the 21070T?

The question is based on reading DC amps.

I have a few meters, but have never invested in quality. But if this Powerboost continues to draw me into areas that it already has, I might be tempted to get some better tools.

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[Jersey Jim]

Paul, you are correct about the .0001 ohms, but please disregard the 4.7mV displayed on the junk meter, in the pic with the 2 meters side by side. Something went wrong with that meter and the decimal point didn't shift. As I mentioned several posts ago, 1 mV represents 10 amps. That reading should have been .47 mV, which that meter shouldn't even be able to display would have displayed .5mV. Take note of the 2 pics in the following post. 3.4mV on the Fluke represented the 34 amps that was charging the battery, and was displaying simultaneously on my Forscan app. along side it at the same time in that same post. .0034v/34A=.0001 ohms.

That drop-light test load of 4.7 amps should not even register on a meter that can only display 3 decimal places of voltage. .00047 volts is what needs to be measured, which is about half a mV. That is why a meter that cannot shift into a range of millivolts, is limited to 10 amps or more. Anything less than 10 amps is gueswork. The 4.7 amp test-load would be .00047v, or .47 mV.

Now if your meter can display .1 mV resolution, then you can discern every amp. Hence the 3.4 mV charging example (34 steps of .1 mV). Keep in mind, 3.4 mv is .0034 volts (34 amps).

 

[Jersey Jim]

Bruce, I only bought that junk meter (at Lowe's) because that's all they carried and I like to keep a meter in every vehicle, boat and camper for simple troubleshooting in a pinch. I can't afford that many GOOD meters. I apologize for the misleading value that was off by a factor of 10.

I don't have a vehicle like yours, but it sounds like a fun time digging in to.

I hear what you're saying about an intermittent parasitic load. Even 11 mA for 730 hours a month comes out to 8 amp hours. That's more than 10% of my battery's capacity, and it doesn't get much above 80% SOC to start with.

I suppose if I drove that infrequently and ran short trips, I would probably just schedule a periodic remote start for 15 minutes. My gas consumption at idle is .23 gal/hour, so a gallon would get me 17 starts, lasting 15 minutes. 2 per week may be all that is needed. Of course that low consumption I stated was at warmed-up slow idle I'm sure.

 

[Snakebitten]

Thanks for the reply Jim.

Yea, the Powerboost throws in a few DC voltage wrinkles indeed.
That High Voltage battery IS the alternator for the 12V batteries (auxiliary battery in the cab) via the DC/DC converter, at least when the crankshaft isn't turning.

Basically as the truck's draw on the 12V battery in turn (without the ICE running) lowers the SOC of the HV battery down to ~40%, the ICE lights and charges the HV battery back to ~63% SOC, and then the ICE shuts back down. Takes as little as 1 minute if the truck is in Park and "ECO Idle" is enabled, which is a 1300RPM idle. I think ECO means Eco in T I M E

I suspect that the normal parasitic draw of a Powerboost shut off completely is supposed to be similar to the truck you are testing. But on those random nights that I mentioned, there's definitely something going on that is drawing the HV battery down as much as 5-7%.
Seems considerable since most nights will be less than 1% loss.

The good news is that recovery is as simple as starting the truck!
Even if the SOC of the HV battery is down in the 30's %-wise, it's more than capable of providing 14.7V at 12V. The truck is not going to be 12V-dead unless the parasitic draw the previous night sapped the 12V battery without its ability to replenish from the HV battery.

Although some fellas have returned to their Powerboost and found it dead. Unable to start.

 

[PaulGrun]

Paul, you are correct about the .0001 ohms, but please disregard the 4.7mV displayed on the junk meter, in the pic with the 2 meters side by side. Something went wrong with that meter and the decimal point didn't shift. As I mentioned several posts ago, 1 mV represents 10 amps. That reading should have been .47 mV, which that meter shouldn't even be able to display. Take note of the 2 pics in the following post. 3.4mV on the Fluke represented the 34 amps that was charging the battery, and was displaying simultaneously on my Forscan app. along side it at the same time in that same post. .0034v/34A=.0001 ohms.

That drop-light test load of 4.7 amps should not even register on a meter that can only display 3 decimal places of voltage. .00047 volts is what needs to be measured, which is about half a mV. That is why a meter that cannot shift into a range of millivolts, is limited to 10 amps or more. Anything less than 10 amps is gueswork. The 4.7 amp test-load would be .00047v, or .47 mV.

Now if your meter can display .1 mV resolution, then you can discern every amp. Hence the 3.4 mV charging example (34 steps of .1 mV). Keep in mind, 3.4 mv is .0034 volts (34 amps).

Understood. I followed your adventures and understood what had happened. I misspoke when I said 1A. Thanks for the clarification

 

[Big Dog Daddy]

JJ Please share your findings on the parasitic draw test I'm very interested to hear what you find.


Mark, initially after shutting down my engine tonight, and allowing the interior dome lights to fade out, I measured 0.5 mV across the BMS shunt. This should equate to about 5 amps. This only lasted a few minutes, not sure, because I checked it 5 minutes later and the voltage dropped down to 0.1 mV. Then I checked it 5 hours later and the meter read zero, but I couldn't read any lower on the cheap meter I was using. It must have been less than 1 amp (I would hope so). I found one of my Fluke hand-held meters that should be able to read dc voltage with a 10 uV resolution, but it's 9-volt battery wouldn't hold up. I'll see if I can swipe one out of my grand daughter's metal detector tomorrow.

A sure fire way to measure mA current out of the vehicle battery would be to just lift the cable and insert an ammeter between the negative lead and the shunt. I will make sure I jumper the shunt to battery while I lift it off the battery post and insert the meter in series, as I don't want to interrupt the current, which may cause some sort of initialization that could take hours. Who knows. An added benefit of inserting an ammeter in series to this would be that I have a way of verifying the shunt's voltage drop with an accurate ammeter in series with the same current. I'll be able to verify the 10 amps per mV suspicion I have, meaning the shunt is truly 0.0001 ohms, as many common shunts in the 500-amp range are. I'll just have to be careful to not open the truck doors, as that could possibly cause a draw of more than the 10-amp rating of my meter. I can apply my own temporary load right in the engine compartment, say a light bulb, anything under 10 amps.

I would have my doubts about the BMS sensor going bad, because this is merely an ever so slight resistance. No moving or semiconductor parts. You can prove out your original one first by monitoring "battery current" in Forscan immediately after starting your engine as you simultaneously measure the voltage drop (in mV) across your shunt. This should literally take 5 seconds. I would first run your high beams and heater blower WITHOUT engine running for around 5 minutes, then shut them off and start your engine. This will insure your battery "requires" and soaks up a hefty amperage (40-60 amps) for several minutes to replace the defecit, giving you ample time to measure the expected 4-6 mV across the shunt. Then you could compare those findings as you perform this action again on the new one they send you. If you change the sensor first, you'll never know (unless you bench test the removed one with a known current going through it).

I would be VERY interested in the results of that comparison, as that could be a way to trick the BodyCM & PCM into sending the battery more charge current. However, this would be at the expense of current accuracy. I would think it's easier for Ford to simply make a software change and update the trucks wirelessly.

Thanks for sharing your expertise with us!
I believe the 0.5mv you witnessed for the first few minutes after shutdown is normal. After the interior lights fade and exit lights shutdown the truck is locked and the key fob is out of range. The trucks modules continue there shutdown for 3 to 6 minutes. You can watch this happen by walking back to the truck without the key fob, watching in the window at the blue halo around the usb ports. When the halo turns off the truck is sleeping. This sleep mode is very important for FDRS updates, on many updates you can't continue until this has happened, especially the apim it's normal to wait 1/2 hour before continuing.

I will try to recreate your test when I change the sensor later this week. I have a Fluke meter I use for work mostly 3 phase stuff. I think with the min/max hold button will capture the mv voltage drop when starting the truck but I'm not sure, I try it. Using the points marked in the picture during start should yield the voltage drop correct?

 

[Jersey Jim]

Mark, that's a nice meter. I see it has a millivolt range. Be sure to select "DC" with the yellow function button. I would use the mV position, as that is what we're measuring. More sensitive that way.

 

[Big Dog Daddy]

Mark, that's a nice meter. I see it has a millivolt range. Be sure to select "DC" with the yellow function button. I would use the mV position, as that is what we're measuring. More sensitive that way.

Thanks for the reminder, I've only ever needed the function button for AC Hz . I won't have time to do this until later this week bu5 I'm anxious to compare the results.
Thanks Jim

 

[Jersey Jim]

Mark, I forgot to mention about the black negative meter lead placement... I would try it on both the center of the stud and where you see it placed in the pic. You are looking for the "lowest" reading of the two. The higher any undesired added resistance will result in a higher mV reading. Maybe as much as .1 or .2 mV. (1-2 amps). And wherever you place the probes, scratch the metal clean first, with either the meter probe, small screwdriver, sandpaper, etc. Mine already showed slight hints of a little oxidation coating. These are extremely faint signals being detected. Environmental effects will have an impact.

I would be very surprised if something as simple as a current shunt would be defective, but then again, bad connections of the 2 small sensing wires may have been poorly attached. And you did say the part number is different now too. BTW, my part number is the same as yours...."ML3T-10C652-BA", and the same as shown on the first post of this thread. Mine made in July of 2021, yours in Feb. Clearly a different batch number. I am anxious to hear of the comparison. Also be sure to lightly sandpaper the "sandwiched" lead terminations once you separate them from the shunt. Could be as simple as oxidation forming between that junction. Good luck.

 

[TheWraith]

Ok so been reading all this thread as I have a 2021 5.0 with pro power option and my battery is starting to take a dive @ 40k which ford says after 3/36k it's not covered even having the best ESP possible oh well no biggie. Went to my local Costco and my jaw dropped for only a $5 upgrade compared to stock size. Essentially spending the $5 more for the extra 90CCA and capacity the BMS system won't really let me unleash the extra capacity this battery has to offer?

 

[Jersey Jim]

Kevin, thanks for posting the easy to read comparison. The length of the 80 Ah battery is 1-1/2" longer than the 70 Ah, so that shows that the plates are much thicker and can withstand more abuse. That is also evident in the 6-1/2 more pounds of lead. All conditions being equal, the bigger battery will take the abuse better of initial heavy charging for the first few minutes (over 100 amps), and heavy discharging. I would definitely spend the extra $5!

 

[Big Dog Daddy]

Ok so been reading all this thread as I have a 2021 5.0 with pro power option and my battery is starting to take a dive @ 40k which ford says after 3/36k it's not covered even having the best ESP possible oh well no biggie. Went to my local Costco and my jaw dropped for only a $5 upgrade compared to stock size. Essentially spending the $5 more for the extra 90CCA and capacity the BMS system won't really let me unleash the extra capacity this battery has to offer?

It's a good move to go with the H7 over the H6. My only problem with Interstate batteries is that there also made by Clarios the same company that makes OEM Motorcraft battery's. Check out the case and even the lifting handles on those it's identical to the Motorcraft.

 

[Jersey Jim]

I used to put Deka deep-cycle batteries in my boats. They're made in PA. These 8 AGMs don't even need venting, like flooded I've used in the past, as I set their charge profiles so they'll never build 2 psi to open their valve caps (VRLA). I am very pleased with these deep cycle's....

Small leads on each battery go to Battery-Balancer boxes

 

[TheWraith]

Kevin, thanks for posting the easy to read comparison. The length of the 80 Ah battery is 1-1/2" longer than the 70 Ah, so that shows that the plates are much thicker and can withstand more abuse. That is also evident in the 6-1/2 more pounds of lead. All conditions being equal, the bigger battery will take the abuse better of initial heavy charging for the first few minutes (over 100 amps), and heavy discharging. I would definitely spend the extra $5!

So is it going to mess with the BMS setup on my 760CC H6 stock battery vs this monster 850CC H7?

 

[Jersey Jim]

I don't think so Kevin. From what I'm observing with Ford's charging algorithm, and trying to make sense of their reasoning, the "constant voltage mode" that they're applying will allow the internal resistance of the battery itself decide how much power to absorb. Sure, your larger battery has 14% more Amp-hour capacity, but the chemistry is the same. If Fords set it's charging voltage limit to protect an AGM battery, than it will protect ANY agm battery. The larger 80 Ah battery will maintain a lower internal resistance for longer (when charging) than the smaller. This will allow more amps to flow into the battery, but the charging voltage will not go higher than what it considers the safe preset limit.

' Let's put it this way, your smaller battery will draw 40, 30, 20, then 10 amps, etc., all the way down to 1 amp float charge, with the same 14.xx volts applied. Your batteries internal resistance will climb, limiting the current.
So will your larger battery act the same way. Just that it will take longer at the 40, 30, 20, then 10 amp level, until IT'S internal resistance climbs high enough to taper off your charge current to 1-amp float charge.

I've used this analogy before....... constant voltage mode is like a toilet bowl tank. When empty, the water (current) flows fast and at a high rate when filling it. When that tank (battery) is almost full, the rising float limits the water (current flow) slower and slower near the full-tank point. Now imagine an aquarium sized toilet bowl tank (your larger battery). When low, the higher flow (high amps) will be applied longer because that float rises slower. Not that the supply is slower, but there is more gallons (Ah) to fill and hold.

The other day I ran my 70 Ah battery down to 42% SOC with engine off. It took a few minutes running before it's internal resistance climbed high enough to limit the charge current into the battery from 140 amps or so to less than 100 amps. If I did that with an 80 Ah battery, either that high current would hang in there a minute or two longer, or an even higher current would flow, finishing up the charge at about the same time due to the higher rate of charge. Interestingly, the battery was back at 80% SOC in only 20 or so minutes because it refilled at such a high rate of current. The 38% soc was removed in an hour or so at 26 amps, but replaced 3 times faster.

So I wouldn't worry about it. Our alternators can output 200 amps, if not more with some trims. That extra 10 Ah will go a long way with regards to letting the vehicle sit unused for weeks, or lengthy OTA updates. The parasitic load I measured (10-11 mA) equates to 8 Ah per month of sitting. That larger battery should negate that!

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