Why does 12V Battery SoC % Increase After Being Parked for Several of Hours?

[Hans Moleman]

When I park 12V battery SoC is around 83%. After I come back several of hours later the SoC is 90%. This happened several of times. Is there something wrong with the battery?

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[Snakebitten]

Did the ambient temperatures climb considerably between the two moments?

 

[Hans Moleman]

Did the ambient temperatures climb considerably between the two moments?

Not really, from 50*F to 60*F

 

[PaulGrun]

When I park 12V battery SoC is around 83%. After I come back several of hours later the SoC is 90%. This happened several of times. Is there something wrong with the battery?

My understanding is imperfect, but here's what I think is happening:
- it is not possible to measure SOC directly, outside of a laboratory environment.
- however, there is a close correlation between the battery's so-called resting voltage and its state of charge.
- unfortunately, the battery's "resting voltage" is all over the map until after the battery has been at rest for several hours.
- because of this, the BMS only re-calibrates the 'actual' SOC based on an accurate reading of the resting voltage after the battery has been at rest for several hours. I believe that the 90% SOC reading you saw after being parked for several hours represents the BMC's "recalibrated" view of the SOC.
- in the interim between recalibrations (i.e. while the truck is in use) the BMC simply estimates what it thinks the current SOC is based on the last
calibrated value and an estimate of charge added to, or subtracted from the battery in the interim. I believe that the 86% you mentioned just after turning off the truck is the BMC's then current estimate of the SOC.

If I'm right, (a big 'if'), there are two observations: 1) I'm amazed at how close the estimate is to the actual (86 vs 90%), and 2) the system is working extremely well in your truck.

Don't worry, be happy!

 

[Hans Moleman]

Ok so the opposite happened today. While driving the SoC was 90% then all of a sudden it dropped to 78%. At 78% it was only charging at 2 amps. Could it be a bad cell that sometimes work and sometimes not?

 

[JohnTrigger]

The BMS tries to also help Ford save fuel to meet CAFE standards. So, it will alter how much your alternator is being used, so I think the drop to 78% above is it just doing its thing.

 

[PaulGrun]

Ok so the opposite happened today. While driving the SoC was 90% then all of a sudden it dropped to 78%. At 78% it was only charging at 2 amps. Could it be a bad cell that sometimes work and sometimes not?

I doubt it.
Remember how we said that there is a relationship between SOC and the battery’s resting voltage and that the BMS uses that property to gauge the battery’s current state of charge?
Well, that relationship between resting voltage and SOC is represented by a graph relating the two. It turns out that for AGM batteries, the curve is relatively flat, meaning that the difference between 90% SOC and 78% SOC is in the millivolt range.
Meaning that if you lost an entire cell (~2v) you would expect to see a very dramatic drop in SOC.

 

[roadPilot]

It's like your BAC (blood alcohol concentration/content) immediately after stop drinking (especiallly if you pound one or two quickly before stopping). Even though you stopped drinking, your BAC will continue to climb as your body metabolizes the alcohol you already drank. Eventually, BAC starts to drop after there's no more to metabolize.

Actually, I don't know if that's why the battery does that. I just made it up because it sounded good! ?

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