12V Battery SOC Tips

[ruffieuxh]

Hello everybody,

I had this very same problem on my MME with updates not working due to low SoC of the 12V battery.

I am a certified car engineer. So, I looked at the situation a bit closer, as a 2 years old battery in an EV should never have a SoC problem. We use the car on a daily basis, hence it should not show a low SoC either.

Here ist what I checked and found:

1. I connected a bluethooth dongle at the diagnostics board an checked the 12V battery using the car scanner app.
   
   
2. Car scanner reported a voltage of 13.5V when thar car was on (hence the 12V battery get's charged) and 11.7V when the car is off. Those are expected values.
   
   
3. Car scanner can also reports the SoC of the 12V battery and this value was (surprisingly) only between 45 to 50%! Hence the SW update process checks this very value before beginning the update.
   11.7Volts and 50% SoC does not match at all! Hence I figured a SW glitch.
   
   
4. I disconnected the negative battery terminal (this is the one with black cable connected and right under the black cover between the frunk and the wipers) and measured the voltage manually: 12.5V, which is exactly what should be expected.
   
   
5. After about a minute I reconnected the battery and reconnected car scanner. As expected the SoC value now got up to 97% without any manual recharging or letting the car run for hours.

Correction: Althoug the system reported 97% SoC after reconnecting, the next morning it was back to 50% and the OTA update did not work again.
This seems to be a rather common problem with the Mach-e 12V battery charging system. Many thanks to @Mach-Lee . He posted FAQ's on how the 12V battery monitoring system works here: https://www.macheforum.com/site/threads/12v-battery-faq.32744/

I hope this helps others

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[tonytaylor53]

12.5V is around 70% SoC for a sealed lead acid battery at room temperature. If it has not been disconnected from any charger and resting for at least an hour, the voltage will be artificially high and inflate that inference.

The car uses coulomb counting, so there is some drift. I'm curious if your charge voltage and disconnected voltage would be higher now that whatever was going on might be fixed.

 

[ruffieuxh]

12.5V is around 70% SoC for a sealed lead acid battery at room temperature. If it has not been disconnected from any charger and resting for at least an hour, the voltage will be artificially high and inflate that inference.

The car uses coulomb counting, so there is some drift. I'm curious if your charge voltage and disconnected voltage would be higher now that whatever was going on might be fixed.

Interesting, I didn't know how that SoC value is being processed. That makes total sense. Again learned something. Thanks.
But this problem I never had during my 10 Tesla years - strange. Why do you need to compute the SoC of the 12V battery at all? In principle you can judge the quality and charge level of a 12V battery based on it's voltage, when the car is idle. Any insights on that?

But I do not understand why, the charge voltage and disconnected voltage should change after my procedure. Can you elaborate your thought process on that?

 

[tonytaylor53]

Interesting, I didn't know how that SoC value is being processed. That makes total sense. Again learned something. Thanks.
But this problem I never had during my 10 Tesla years - strange. Why do you need to compute the SoC of the 12V battery at all? In principle you can judge the quality and charge level of a 12V battery based on it's voltage, when the car is idle. Any insights on that?

But I do not understand why, the charge voltage and disconnected voltage should change after my procedure. Can you elaborate your thought process on that?

It's called the Battery Monitoring System or BMS. Plenty of info out there, here's a popular video that covers it.

 

[Mach-Lee]

Interesting, I didn't know how that SoC value is being processed. That makes total sense. Again learned something. Thanks.
But this problem I never had during my 10 Tesla years - strange. Why do you need to compute the SoC of the 12V battery at all? In principle you can judge the quality and charge level of a 12V battery based on it's voltage, when the car is idle. Any insights on that?

But I do not understand why, the charge voltage and disconnected voltage should change after my procedure. Can you elaborate your thought process on that?

I believe your battery was actually at 50% SoC to start with. That is close to the minimum SoC value before it will be automatically recharged by an automated DC to DC power transfer.

When you disconnect the 12V battery and reconnect it, the BMS loses its volatile memory which contains the SoC value. The default SoC value is 100%. After the battery has been reconnected, the BMS will need to re-learn the current SoC of the battery, which is determined by the open circuit voltage in the off state during the six hour period after the car is shut off (it requires a long time to account for voltage relaxation). That resting voltage value is converted to a SoC. While the battery is in use, the BMS performs coulomb counting (via the current sensor on the negative battery terminal) to maintain a live SoC reading. By combining coulomb counting with periodic open-circuit measurements during the off cycle, the BMS can determine the estimated capacity (in amp-hours) of the 12V battery, similar to the BMS on a laptop battery. When you perform a BMS reset, it resets the battery's capacity to 35 amp-hours and 100% SoC, and will auto adjust from there. I'm not completely sure, but the intelligent battery sensor is likely made by Forvia Hella as the OEM.

In your case, the estimated SoC will adjust down to about 50% (or whatever it's actually at) the next time you drive and park your car for six hours. There is not a glitch in the software, that's just how it works. You cannot check and get an accurate battery voltage unless you shut the car off and wait an hour without disturbing it (opening the hood or walking nearby with a key would disturb it).

You may want to update your BCM software with FDRS if that's possible, that raises the minimum SoC before recharge from 45% to 55%. It used to be as low as 35% SoC in early software.

And if you're comparing to Tesla, Ford is a lot different with their LVB strategy. Tesla will keep the DC/DC converter turned on pretty much whenever the car is awake (as soon as you walk up to it, and on for a while after you walk away). Ford only turns DC/DC on when the car is ON or charging, or when the battery gets down to ~50%. Reason being to minimize any phantom loads on the HVB. The 12V lockout recall is the first major change to the 12V strategy, it now keeps the DC/DC on for 12 minutes after key off (more like Tesla). That helps keep the 12V battery a lot more charged. Previously the 12V battery would drain about 10% after you shut the car off, because there are a number of modules that take a while to shut down (it takes two hours for all the modules to fully shut down after key off).

If you want to know a lot more about the 12V battery, read this: 12V Battery FAQ
Or ask me.

 

[ruffieuxh]

Many thanks for you detailed explanation. I read your FAQ's in all details.

In fact it all went as you said, the next morning the SoC was at 50% again and OTA did not work!

Wow! By all means, why does this has to be so complicated?? A battery which never have the load of an ICE car, does not require such a complicated monitoring system. A better charging methode and a good monitoring of the voltage would be good enough.
So, now I leave the car running for a moment, since you cannot even charge the battery with normal battery charger.

If I still have a problem, I may need a check by the Ford dealer. But I definitely need to find the stud's. Without reading your FAQ's I would have connected the charger directly to the battery!

 

[Mach-Lee]

Many thanks for you detailed explanation. I read your FAQ's in all details.

In fact it all went as you said, the next morning the SoC was at 50% again and OTA did not work!

Wow! By all means, why does this has to be so complicated?? A battery which never have the load of an ICE car, does not require such a complicated monitoring system. A better charging methode and a good monitoring of the voltage would be good enough.
So, now I leave the car running for a moment, since you cannot even charge the battery with normal battery charger.

If I still have a problem, I may need a check by the Ford dealer. But I definitely need to find the stud's. Without reading your FAQ's I would have connected the charger directly to the battery!

Ah. If your OTA is failing, I have a topic for that too: https://www.macheforum.com/site/threads/how-to-get-failed-otas-to-install.29749/

The reason it’s so complicated is that most of the automakers use the intelligent battery monitor to reduce alternator loading and enhance fuel economy. Same on EVs, they want the 12V charging to be as efficient as possible which means you only charge the battery to about 90% instead of 100%. You need the battery current sensor to make that work.

 

[ruffieuxh]

Many thanks, I am currently trying the slow charge method.....

 

[ruffieuxh]

Ok, @Mach-Lee , the OTA now got installed, but after slow charging the HV battery (approx. 1.4kW) over 13 hrs, the 12V battery is still not over 67%.

What would your take be on that? Is my battery really dying? Or does the 12V battery charge cut off after several hours?

Many thanks for your highly valuable support.

 

[Mach-Lee]

Ok, @Mach-Lee , the OTA now got installed, but after slow charging the HV battery (approx. 1.4kW) over 13 hrs, the 12V battery is still not over 67%.

What would your take be on that? Is my battery really dying? Or does the 12V battery charge cut off after several hours?

Many thanks for your highly valuable support.

What method did you use to charge the battery?

 

[ruffieuxh]

I charged the HV battery for 13h (it was not only plugged in but charging) with my level 2 wallbox and understood from your FAQ's that during that time, the 12V battery will be charged via DC/DC converter.

 

[Mach-Lee]

I charged the HV battery for 13h (it was not only plugged in but charging) with my level 2 wallbox and understood from your FAQ's that during that time, the 12V battery will be charged via DC/DC converter.

Hmm. So did you check it at 67% right after charging finished, or was there possibly a delay? After 13 hours it should be above at least 80%. I charged all day and drove home, mine was at 98% (newest software though).

I guess I would try the blower charge method next, and watch if the SoC will keep increasing with Car Scanner. If your 12V battery is more than 3 years old, then I would recommend replacing it.

Also, have you tried scheduling the OTA to occur at 1AM? On newer software it should automatically pre-charge the 12V battery for scheduled OTAs (which it obviously can't do if you press "Update Now").

 

[ruffieuxh]

Hmm. So did you check it at 67% right after charging finished, or was there possibly a delay? After 13 hours it should be above at least 80%. I charged all day and drove home, mine was at 98% (newest software though).

I guess I would try the blower charge method next, and watch if the SoC will keep increasing with Car Scanner. If your 12V battery is more than 3 years old, then I would recommend replacing it.

Also, have you tried scheduling the OTA to occur at 1AM? On newer software it should automatically pre-charge the 12V battery for scheduled OTAs (which it obviously can't do if you press "Update Now").

Well, there were 2-3hrs between finishing charging and my check. In the meantime the OTA worked by slow charging over night (until 6am) and setting the update to 6:30am.

Now, I am just trying to understand why it does not charge the 12V battery to a higher SoC during overnight charging.

Ok, let me try the "blower" method......

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