My observations and impressions about battery degradation

My car is presently at 38,000 miles so it's probably one of the higher mile units on this forum. It's been in service about 20 months.

I have believed that the usable capacity of a new battery is approximately 18.5 kWh (somewhere in the mid-18's). My reasons for believing this were based largely upon my acquisition of an OBD scanner about a year ago.

1. The OBD scanner showed ETE of generally 18.5 kWh after charge (varies slightly between 18 and 19)
2. The OBD scanner showed the range of SOC to go between 90 and 8% approximately (i.e., 82% usable)
3. The battery is rated at 23 kWh; 23 x 0.82 = 18.8

So everything looked generally consistent

I am now changing my view. More forum members are using OBD scanners, and they are frequently reporting ETE values in the mid-19's. Also, I found Idaho National Laboratory test reports

http://energy.gov/sites/prod/files/2015/02/f19/fact2013fordfocus.pdf

which show approximately 19.5 kWh for a new car.

So now I'm thinking 19.5 kWh might be the right number. It's also possible, I suppose, that 18.5 was the correct number for my specific FFE and Ford gave INL primo ones. The INL reports show tests of four separate FFEs, and the measured battery energy was between 24.0 and 24.8 kWh, well above the rated 23 kWh. Who knows?

http://avt.inel.gov/pdf/fsev/batteryFocus2578.pdf

While the better test is the one that INL has promised to perform but has not yet (retesting the same cars periodically) we can make some estimates. I believe, based upon the trip-meter (both driving and with a heater rundown test), that my current battery capacity is somewhere between 17 and 17.5 kWH. Let's call it 17.2.

So if 19.5 was the original number, it's dropped about 12% over 38,000 miles
If it was originally 18.5, then it's dropped about 7% over that distance.

I'd say these numbers help give some idea of the actual degradation with use. My car has been through two summers, one before I got it, one in my possession. It was built 24 months ago.

To put this number in perspective, a Leaf loses its first battery bar at 15% reduction. If we accept the generally but not universally accepted definition of "useful battery life" as the point where it has 70-80% of its original capacity, this suggests the FFE battery is good for somewhere between 80 and 120K miles, at least when used as I use it. There is a time factor also involved (separate from miles) but I am burning through the miles so fast this probably hasn't been much of a factor for me.

Hope this is helpful and I would value other data forum members may have.

Thanks for the helpful data. It would be nice if more members would use FORScan to measure their ETE with a full charge and report it either in this thread or in this one.

@michael

What is your typical WH/mile rating? And if you fully charge it overnight, what do you typically see for the GOM miles?

I think you are right about the 19.5. At least with my 2014, if I extrapolate on long trips where I nearly run it dry, I usually come up with about 19.5. The one time I did run it dry, there was an error in MFM (no--really!), but guestimating around that still got me to 19.5. Still, considering the number of cycles your mileage represents, things seem OK.

275 Wh/mile most of the time.

My GOM values at recharge are sort of meaningless because the last few miles of my commute are downhill, and the GOM looks at that, so I can get pretty large numbers, sometimes over 100

Your ETE was 18.5 about a year ago - What does your current full charge ETE read?

It varies of course. I've seen range 18.2 to 18.7 recently. I'd say about the same.

The difference is when I start to drive, over the first few miles the ETE falls faster than the trip meter consumption rises, then more-or-less tracks. Others have reported similar.

In other words, the amount actually available is maybe 1 or 1.5 less than the initial ETE that is shown right after charge.

Forgive me for double posting this, but it applies here as well...


This morning before starting out:

Outside air, 65F
Battery Temp 91
18.4 kWh ETE


Then I started my drive and recorded the usage (trip meter) and ETE along the way. The table below also shows the sum of ETE and usage. In theory, one would expect the sum of the two to remain relatively constant, ideally equaling the original ETE

Used ETE Sum

0.0 18.4 18.4
0.1 18.0 18.1
0.2 17.8 18.1
0.3 17.6 17.9
0.4 17.5 17.9
0.5 17.3 17.8
1.0 16.8 17.8
2.0 15.7 17.7
3.0 14.3 17.3
4.0 13.1 17.1
5.0 12.0 17.0
6.0 11.0 17.0
7.0 9.9 16.9
8.0 8.9 16.9
9.0 7.8 16.8


Desination

9.1 7.7 16.8

264 Wh/mi 91 Bat temp


As you can see, there is a significant discrepancy between trip meter usage and reduction in ETE.

This trip was generally 60-65 MPH, slight net elevation gain, a few hills

So you can see that about 1.5 kWh have somehow "disappeared" from the equation. I don't know whether this is typical or not.