Low voltage DC-DC voltage changes when moving vs slowing/stopped

[Runaway Tractor]

Here's a Ford software engineering mystery. The DC-DC converter is changing the voltage when you slow down / stop vs moving. When speed drops below 5mph, the DC-DC converter voltage increases from 12.8v to 13.3v. When you accelerate above 7mph, the DC-DC converter drops the voltage back down to 12.8v. Been watching this for days. 100% consistent. This is definitely a programatic setpoint change for seemingly no reason.

I cannot think of any logical reason for this. The LV load doesn't suddenly change with speed. The only thing this seems to "accomplish" is randomly charging and discharging the LV battery.

Here's a screen recording from Car Scanner. You'll see the DC-DC amps and LVB volts changing instantly and consistently with the speed.

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[Maquis]

It’s programmed to emulate the alternator on an ICE?

 

[bmwhitetx]

I’ve seen it change from 13.x to 14.x depending on 12V load. Your battery is at 100% SOC so it doesn’t need to be charged. That’s why you’re likely only seeing 12.8.

For example when I have AC on high speed, the voltage will increase. I used this back when trying to charge a low SOC battery while in the driveway when OTAs were failing due to low 12V SOC.

It may also be prioritizing motors at speed over charging the LVB.

 

[Runaway Tractor]

It’s programmed to emulate the alternator on an ICE?

Lol that's the first thing I thought of but then I realized it's the other way around.

 

[Ricks Lightning]

i noticed my low voltage battery was at 14.9 when i started the truck this morning. seems very high for a 12 v agm battery to charge at.

Rick

 

[Firn]

My guess is voltage increases to help support the higher fan load needed at a stop

 

[Effonefiddy Lightning]

Drive and enjoy.

 

[Adventureboy]

Here is my theory - 12.8v is the fully charged float for the battery and is where you want it when the battery is at 100% and no significant power is being drawn. 13.3v is the absorption charge rate that kicks in a few seconds after your truck needs more 12v power to run the accessories like brake lights, brake hold etc without pulling from the 12v battery.
I suspect you are right, it is programmatic because it knows it needs more than 12.8v to power the needed accessories when stopped to keep from drawing on the 12v. I wonder if it is tied more to power draw than speed. Maybe if you can, watch the LVB amps and see if there is an obvious trigger point for the switch from 12.8v-13.3v.

Incidentally, you'll see into the 14s when you first start and it is actively charging the 12v battery while running all of the accessories.

 

[Runaway Tractor]

I'm confident it is speed as opposed to LVB current. Been watching it all for a week. It's 100% consistent at those speed points.

I think your theory on being prepared for additional loads that often come at slow speeds and stopped is sound. Probably prevents voltage sagging even lower when those loads eventually turn on.

 

[mrau]

I'm confident it is speed as opposed to LVB current

Might be the radiator fan coming on at slower drive speed due to reduced air flow coming in.

Not sure if you are able to monitor the radiator fan speed. Just a thought.

 

[chl]

Here's a Ford software engineering mystery. The DC-DC converter is changing the voltage when you slow down / stop vs moving. When speed drops below 5mph, the DC-DC converter voltage increases from 12.8v to 13.3v. When you accelerate above 7mph, the DC-DC converter drops the voltage back down to 12.8v. Been watching this for days. 100% consistent. This is definitely a programatic setpoint change for seemingly no reason.

I cannot think of any logical reason for this. The LV load doesn't suddenly change with speed. The only thing this seems to "accomplish" is randomly charging and discharging the LV battery.

Here's a screen recording from Car Scanner. You'll see the DC-DC amps and LVB volts changing instantly and consistently with the speed.

The DC-DC converter is getting its input voltage from the HV battery.
When there is a larger drain on the HV battery (more current for the motors) the terminal voltage will drop slightly. The HV battery, like any battery, has some internal resistance to current flow, so some voltage is lost to this resistance when the current is flowing.
That is normal.
So if the terminal voltage of the HV battery drops a bit, the DC-DC converter has less voltage on its input and so the output charging the LV battery will fall slightly too.

That said, the charging of the LV battery in the Lightning's is problematic, many reports of low LV battery voltage causing OTA updates to fail, etc.
The LV battery is AGM and going below 50% SOC will shorten the life of an AGM battery.
The Lightning programming will allow the SOC to go down to 40% for 48hrs before recharging when the truck has been sitting for some time.
Many threads about the issue.

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