Wh/mi increasing with age?

[ThosEM]

Why do you believe that any of those rated range values are reliable? I am particularly suspicious of rated range close to zero - I suspect that the 'number miles below zero' is one of the most variable elements, on the supposition that the whole reason for having miles below zero is to cover up inaccuracies in the range calculation. And the fully-charged miles is also an estimate based on what the system believes is the battery's health - presumably you are doing your own calculations because you don't trust the rated range (full) to tell you the truth, yet you then factor it into your numbers.

(edit: re-reading this post, sounds a bit aggressive - please read it in the spirit intended of seeking after the best measurement, not attacking yours as such).

I don't believe the rated range numbers, except as a measure of battery charge state, which is why I'm trying to construct a battery capacity estimate that is based only on things we know.

I *think* we know the energy consumption and total energy are pretty good numbers, and so is the distance traveled. The rated range when full and the rated range used could just as well be % of full readings, i.e. 100% and whatever the display shows at the end of the trip, subtracting to get the % used. So my suggestion doesn't assume that rated range (or ideal range) has any relationship to performance, just that they are proportional to battery state of charge.

Does that make sense?

 

[AmpedRealtor]

I drove 50 miles home from the service center today and used 255 Wh/mi. Can you believe that? I was driving an average of 70 MPH on the freeway, with occasional bursts to 75/80, my AC set to 72 (range mode OFF), and outside air temps at 103ºF. I was shocked!

 

[jerry33]

I drove 50 miles home from the service center today and used 255 Wh/mi. Can you believe that? I was driving an average of 70 MPH on the freeway, with occasional bursts to 75/80, my AC set to 72 (range mode OFF), and outside air temps at 103ºF. I was shocked!

Hotter air temperature helps Wh/mi because it's less dense. The A/C doesn't use all that much electricity once the set temperature is achieved. My own experience is that range mode doesn't help all that much in the summer (or in the winter if it's a long trip--really helps on short trips)

 

[arg]

I *think* we know the energy consumption and total energy are pretty good numbers, and so is the distance traveled.

Agreed here - all indications are these are real measurements, and no reason for them to be fudged.

The rated range when full and the rated range used could just as well be % of full readings, i.e. 100% and whatever the display shows at the end of the trip, subtracting to get the % used. So my suggestion doesn't assume that rated range (or ideal range) has any relationship to performance, just that they are proportional to battery state of charge.

Unfortunately, I think this is a totally incorrect assumption:

• Percentage state of charge is fundamentally very hard to measure with any degree of accuracy. So we can't assume that an accurate % of full is even an input to the algorithm used to generate the number on the display.
• We know that the numbers aren't simply (%-of-full-as-best-it-knows) * (rated-full-range), there are at least two adjustments: the fully charged number - which presumably does in fact correspond to 100% charge - is not fixed at the EPA value for rated range, but rather a number based on how healthy the system believes the battery is; and zero miles is somewhere above 0% state of charge, by a rather variable amount.
• The algorithm has changed significantly between software versions, so if you are trying to do this to measure long-term trends, you won't be able to compare sets of readings taken with different software.

The display algorithm might be as simple as:

• After a range charge, pick a number based on how healthy the battery appears to be.
• While driving, decrement the displayed number based on the power drawn, at a fixed Wh/mile.
• When charging short of a range charge, increment the number based on the energy put in to the pack, again at a fixed Wh/mile, with some adjustment to make it line up with estimated SoC based on voltage.

It's probably more complex than that, though the display while driving certainly seems to decrement at a fairly constant Wh/mile. It would be interesting to know if that varies between cars or with driving styles on the same car (given the same software version and configuration - can't compare your car to mine, since UK cars have a different scaling of the rated/typical miles compared to USA rated/ideal). If it does vary, it is probably more a case of the software compensating for things rather than being a direct measurement of anything.

This is of course different from the proposition made by the OP that the actual Wh/mile is changing. That ought to be independent of the battery (if the power measurements are calculated on actual current/voltage coming out of the battery), and also ought to be independent of changes to the battery management software - assuming these are direct measurements, there's no reason for them to change between software versions. However, it is wildly variable on all sorts of other factors (weather, driving style, load in the car) so quite hard to isolate trends.

 

[gregincal]

I'm also in the decreasing wh/mile camp. I can remember getting less than 300 wh/mile during my commute being a rare thing during my first 6 months of ownership, and it's a regular occurrence now. My lifetime average continues to steadily decline over time (after 5k miles it was probably 320 wh/mile, after 10K it was 316 wh/mile, now after 18 months of ownership and 15K miles it's at 313 wh/mile).

 

[stevezzzz]

Same here: after my first winter with the car, in May of 2013, I was at 338 Wh/mi; at this time last year I was at 318 Wh/mi. Now, with 32,000 miles in just shy of two years of driving, I'm at 313 Wh/mi.

 

[physicsfita]

I wonder if those you you seeing improved Wh/mi are going through the same thing Prius drivers often do: the energy displays do a good job of teaching you how to drive more efficiently, and your economy improves as a result.

 

[mknox]

I wonder if those you you seeing improved Wh/mi are going through the same thing Prius drivers often do: the energy displays do a good job of teaching you how to drive more efficiently, and your economy improves as a result.

Well, not for me. I had learned to slow down in the past primarily to save fuel. With the comparative cost of electricity so low, I find myself driving both more and faster now.

 

[cschock]

I am also in the steadily decreasing lifetime Wh/mi group. After 3 months we were at 320 or thereabouts, 314 after a year and it is now at 311 at almost 18 months. My last 1000 miles was at 304. My guess is lifetime is heading down to mid 30x over the next 12 months. Part of this is probably break in on the tires (19" original tires) as well as my driving a bit more sedately after the initial thrills :wink: