Battery charge drifting

[jsamp]

There have been several discussions previously about the % charge 'drifting' while the car is parked.  I've noticed that if it's in the 'goldilocks' zone temperature-wise (72°F give or take) the battery will stay at the charge % where you left it.  If the temp is significantly higher or lower either when you parked, or when you get back in the car, you will notice the charge % having changed while parked.  This is assumed to be because Li-ion batteries are temperature dependent and thus their relative charge changes.  So you are not losing or gaining any electrons from the battery per se, but the capability of the battery to deliver those electrons is changing.

 

That brings me to the other morning when I was going to leave for work.  I charged as usual, and unplugged at 88% around 7:30 in the morning.  I was delayed leaving for work that day so about an hour later when I got in the car to leave, it said 100%!!  It was still cool (~64°F) and the car was still in the shade so I wouldn't think temp would have changed that much, unless the battery was hot and cooled down into the ideal zone.  This 12% change surprised me and makes me wonder how much over 100% it goes when we charge to 100% and then it overshoots?  It has been stated on here that there is a .5kWh buffer built into the battery, which I think is wise, but I wonder if that is truly enough?  It also makes me wonder how much this affects the kWh test we use to measure battery capacity.

 

12% overshoot =~.7kWh

 

I rarely charge to 100% just because of my understanding of it's effect on Li-Ion battery longevity.  Too bad Ford doesn't make it easy to stop charging at anything other than 100%.

Edited August 10, 2019 at 10:51 PM by jsamp

[murphy]

Someone used an OBD II scanner to determine that an indicated 100% is actually around 95%.

If you don't charge to 100% the 12 volt battery will never be charged using wall power.

It gets charged after HVB charging is complete.

[jsamp]

Someone used an OBD II scanner to determine that an indicated 100% is actually around 95%.

If you don't charge to 100% the 12 volt battery will never be charged using wall power.

It gets charged after HVB charging is complete.

 

The 95% is what I am referring to (I heard 93% but either way).  If mine climbed by 12%, and there is only 5-7% buffer, these cells are getting pretty stressed if you charge to 100% then the temperature changes and the % charge goes up.

 

True on the 12V, however it does get charged while driving, at least when the ICE is running.  I have gone months without charging to 100% and so have others without a 12V problem.

[jsamp]

Similar but slightly different thing happened this morning.  I checked when I woke up and it was at 77% and charging, which is about what I expected.  25 minutes later I checked and it was at 85% which is about right (I typically see ~3 minutes per % on 120V charging).  I went out 6-7 minutes later to unplug, figuring I'd be at ~88%, and the thing was done with the light ring out.  I checked and it was 100%!?!?!??

 

Somehow it took 25 minutes to gain 8% but 6 minutes to gain 15%?  Something funny going on here.

 

I'm going to hit 70k miles this week, so I'll run the kWh test over the weekend when it cools down (96°F today) to see if there is any significant change.

[jsamp]

Just got back from doing the test.  5.4kWh and 28.7 miles.  Only slightly less kWh than when I tested at 60k miles,  I tried to keep to the same route and the temps were the same as well (74°F average on my trip).

 

So nothing that would indicate why the battery is topping out so quickly upon charging.  The good news is my battery at 70k miles is only ~5% degraded from new if you count only the EV portion of the battery.

[murphy]

Brand new the EV Only portion of the battery was 5.6 kWh.

 

EV Only     5.6

Hybrid        1.5

Protection  0.5

 

Total           7.6

 

120 volt charging is very inefficient and is not linear.

[rbort]

Haven't been here in a while, but let me just say a couple of things:

 

a) the 12v battery is charged all the time while charging the HVB, its not only at the end.  Who said it was at the end and why did they think that?

b) Driving the car on very hot days on the battery and charging the battery on very hot days is bad for the HVB.  You should refrain from using it as much during the hot summer weather.  Don't charge it as much and use the engine more.

c) 120v charging is very good for the battery and better than 240v.  Forget about efficiency that everyone raves about, its about not building as much heat on the battery and in turn not degrading it as much.  In fact you can use 120v charging at the end of a drive late at night to lower the HVB temperature.   You can read about it here back on the Cmax forum:

 

http://fordcmaxenergiforum.com/topic/8197-240v-30amp-hvb-temperature-rise-or-lack-at-69f-outdoor-temperature/

 

You should use 240v charging sparingly especially during the summer months.  In the winter time it doesn't matter as much, however when not in a rush 120v charging is always better than 240v in my opinion (forget about efficiency)

 

-=>Raja.

[jj2me]

Some recent charge drifts I recorded.  All were night to next day drifts.

 

66 - 76%

76 - 82%

91 - 94%

 

Those were from the car's displays and MyFordMobile (they hardly differed when I checked).

 

For that last 91-94 drift, I also recorded the two PIDs SoC and BAT_TO_EMPTY_ESTIM:

89.2 - 91.2%

6.4 - 6.6 kWh

[jj2me]

The 95% is what I am referring to (I heard 93% but either way).   If mine climbed by 12%, and there is only 5-7% buffer, these cells are getting pretty stressed if you charge to 100% then the temperature changes and the % charge goes up.

 

This is an excellent point.

 

I got curious as to the safety ranges, so I sporadically recorded the dash display and two PIDs, SoC (State of Charge) and BAT_TO_EMPTY_ESTIM (estimate of kWh remaining to empty) for a few days.  Here's the link to the spreadsheet.  

 

Observations:

 

1. Dash display & MFM display & large battery icon all show 100% at 7.1 kWh and 0% at 1.5 kWh, at which point the car enters hybrid mode.

 

2. The SoC PID is not a percentage of charge of the whole 7.6 kWh HVB, but appears closer to 7.1 kWh (i.e., without considering the 0.5 kWh safety reserve).  I had to calculate to determine this (column E in the spreadsheet).  The average of the individual calculations is 7.18 kWh, I don't know why it's not 7.1.  Maybe it's representing some more sophisticated calculation, or maybe it's just the effects of the kWh PID being an estimate.

 

3. (Unexpected) The charge level in forced hybrid mode never went below 1.1 kWh (14.5% of 7.6 kWh), and settled in at 1.1 to 1.3 kWh.  This was tested in two 40-mile stretches in forced hybrid mode, where I did a lot of observations not recorded in the spreadsheet. Expected was that hybrid mode wants to operate at the midpoint of the hybrid reserve, which would be 0.75 kWh.

 

4. (Unexpected) The little hybrid icon looked to be

- full at 21.2%/1.5  (SoC/kWh)

- 7/8 full at 20.3%/1.5

- half full at 17.7/1.3 as well as at 16.7/1.2. 

- 1/3 full at lower SoC values for 1.1 kWh.  This was the lowest I saw.

 

So it looked like the battery level displayed in the icon reflected just the higher end of the 1.5 kWh hybrid reserve (maybe 0.9 to 1.5 kWh).

 

Expected was to see the little hybrid battery icon reflect charge levels between 0 to 1.5 kWh.

 

5. How much buffer was observed?

 

A. At the high end, about 6.6% (100% charged = 7.1 kWh). I.e., the 0.5 safety reserve.  But specifically haven't tested drift at 100% re jsamp's concern, i.e., can it drift at 100% to greater than 7.1 kWh?  (My battery and I don't want to test this.)

 

B. At the low end, about 14.5% (lowest observed = 1.1 kWh). 

Though it might go lower due to differences in cars or environments or when in EV+.  If I calculate the low end using observed lowest SoC of 15.2 rather than the kWh of 1.1, I get 16.3%. 

 

 

Takeaways: Plenty of buffer at the low end, safe to run at 0% nominal charge.  Not much buffer at the high end, threatened even moreso by drift.  

 

 

A minor thought about the BAT_TO_EMPTY_ESTIM PID, but something I can't test:  Since my car's battery tested at 5.5 kWh two years ago, how come that PID always records 5.6 (7.1 - 1.5)?  If a car with degraded HVB, like 4.0 kWh remaining, also reports that same 5.6 kWh range from the BAT_TO_EMPTY_. PID, then we'll have more info as to how badly that PID estimates kWh remaining.

[jsamp]

Very interesting data.  Great work.

 

We've always said 1.5 is the hybrid portion, but just like in the Fusion Hybrid you can never use the whole thing.  Seems there is a larger "no-go" zone at the bottom end than we assumed.

 

I, too would be interested to see what the SOC-PID values are after a 100% charge and then sitting for an hour like mine.

 

I've just taken to unplugging at 80-85% to keep it in a safer zone.  If there is any significant time between unplug and my departure (1/2-1 hour) it will climb into the upper 90's or even 100%.

 

Thanks! 

[jj2me]

No battery drift upwards overnight at 40-50°F temps.

 

Completely charged to 100% at night. Didn't check PIDs, but assume the usual 7.1 kWh estimated remaining energy PID reading. Next morning recorded:

- 54°F ambient temperature

- 50°F battery temp

- 100% SoC on dash display

- 98.6% SoC PID

- 6.7 kWh Bat 2 Empty PID <=== didn't drift up.

 

Maybe cold weather suppress upwards drift? We've seen instances of upwards drift in warm weather, but none were from fully 100% charged batteries. That's the concern, that it would push the safety reserve to 5% or less. That's still to be tested.