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bobkart
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Anomalous Efficiency Reading

Mon Jan 25, 2021 3:43 pm

I'm sure some have seen higher readings, but I usually accumulate them for a month, so I've never seen anything much over 5.0.

Image

In this case I had a problem that caused my stats to reset, at a charging location that's mostly uphill from home. So this reading was captured on the way back home (at a red light).

This makes me wonder what the reading would be for an all-downhill trip, one that ended up adding to the SOC. Would it report a negative miles-per-kilowatt-hour number? (I.e. ten miles 'using' negative one kilowatt-hour, that ratio is -10.)
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WetEV
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Re: Anomalous Efficiency Reading

Tue Jan 26, 2021 8:19 am

bobkart wrote:
Mon Jan 25, 2021 3:43 pm
This makes me wonder what the reading would be for an all-downhill trip, one that ended up adding to the SOC. Would it report a negative miles-per-kilowatt-hour number? (I.e. ten miles 'using' negative one kilowatt-hour, that ratio is -10.)
I used to drive the Leaf downhill (from altitude of 130 m to 10 m) from home, and would reset the m/kWh when leaving. At the bottom of the hill, about 2 km from home, if I had been careful driving and wasn't using any climate, I got 99 m/kWh multiple times.
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bobkart
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Re: Anomalous Efficiency Reading

Tue Jan 26, 2021 9:55 pm

WetEV wrote:
Tue Jan 26, 2021 8:19 am
I got 99 m/kWh multiple times.
That makes sense, with just two digits before the decimal point, and no sign. Plus some people might get confused on seeing a negative number there.

At first blush, a negative efficiency number seems like a bad thing. But technically, miles-per-kilowatt-hour can be negative, with lower (in absolute-value) readings being better (more efficient) than higher ones. I.e. -4mi / kWh is better than -5mi / kWh. To help myself understand why that is, I'll phrase it as 'it takes four miles of driving to put one kilowatt-hour back into the battery', which is better than needing five miles to do it. Probably moving the minus sign into the denominator would help, because that what's gone negative, as opposed to the miles: 4mi / -kWh.
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coleafrado2
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Re: Anomalous Efficiency Reading

Sun Jan 31, 2021 12:20 pm

bobkart wrote:
Tue Jan 26, 2021 9:55 pm
WetEV wrote:
Tue Jan 26, 2021 8:19 am
I got 99 m/kWh multiple times.
That makes sense, with just two digits before the decimal point, and no sign. Plus some people might get confused on seeing a negative number there.

At first blush, a negative efficiency number seems like a bad thing. But technically, miles-per-kilowatt-hour can be negative, with lower (in absolute-value) readings being better (more efficient) than higher ones. I.e. -4mi / kWh is better than -5mi / kWh. To help myself understand why that is, I'll phrase it as 'it takes four miles of driving to put one kilowatt-hour back into the battery', which is better than needing five miles to do it. Probably moving the minus sign into the denominator would help, because that what's gone negative, as opposed to the miles: 4mi / -kWh.
Cool. There's a small singularity at 0 mi/kWh, right? Approaching from +0.1 mi/kWh, we're driving spectacularly inefficiently, while approaching from -0.1 mi/kWh we're putting 10 kWh into the battery per mile. I think I understand Wh/mi a bit better now, as it doesn't suffer this inverted weirdness. More energy per unit distance is always larger, less power per unit distance is always lower, even into the negatives.
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bobkart
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Re: Anomalous Efficiency Reading

Sun Jan 31, 2021 12:39 pm

coleafrado2 wrote:
Sun Jan 31, 2021 12:20 pm
There's a small singularity at 0 mi/kWh, right?
It's when total energy consumption is zero that you get infinite miles-per-kilowatt-hours (assuming nonzero distance travelled).

The watt-hours-per-mile phrasing suffers from similar singularity when speed is zero. But at least speed can't be negative . . . even when going in reverse, we can just take the absolute value (i.e. distance travelled can never be negative). So if we see a negative Wh/mi reading, we can more easily associate the negation to the Wh (numerator), instead of imagining it as moving to the denominator (for mi/kWh).
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Re: Anomalous Efficiency Reading

Sun Jan 31, 2021 12:43 pm

Nothing commutes when there's friction involved! :D
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bobkart
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Re: Anomalous Efficiency Reading

Sun Jan 31, 2021 12:43 pm

coleafrado2 wrote:
Sun Jan 31, 2021 12:20 pm
There's a small singularity at 0 mi/kWh, right?
I think I see what you're saying . . . 'inefficiency' is infinite at 0mi/kWh (energy is being consumed but no distance being travelled).

Reciprocal of that being that efficiency is zero.
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DaveinOlyWA
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Re: Anomalous Efficiency Reading

Sun Jan 31, 2021 1:29 pm

bobkart wrote:
Mon Jan 25, 2021 3:43 pm
I'm sure some have seen higher readings, but I usually accumulate them for a month, so I've never seen anything much over 5.0.

Image

In this case I had a problem that caused my stats to reset, at a charging location that's mostly uphill from home. So this reading was captured on the way back home (at a red light).

This makes me wonder what the reading would be for an all-downhill trip, one that ended up adding to the SOC. Would it report a negative miles-per-kilowatt-hour number? (I.e. ten miles 'using' negative one kilowatt-hour, that ratio is -10.)
I reset mine daily so nothing extreme although one day heading west from my old place (150 feet elevation drop in just under 2 miles) I hit around 15 miles/kwh.

The other day, I was at 12.3 miles/kwh with 3 miles on the trip computer but because I was headed to West Oly and decided to take the downtown route which meets the southern apex of Puget Sound and was lucky enough to not use friction braking at all. Glorious yes, transient, absolutely. Sadly, I was only 5.3 miles/kwh by the time I crested the hill on the other side.

As far as negative? Not possible since the BMS is constantly using power along with the 12 volt system.
2011 SL; 44,598 mi, 87% SOH. 2013 S; 44,840 mi, 91% SOH. 2016 S30; 29,413 mi, 99% SOH. 2018 S; 25,185 mi, SOH 92.23%. 2019 S Plus; 19,653 mi, 93.47% SOH
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bobkart
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Re: Anomalous Efficiency Reading

Sun Jan 31, 2021 4:12 pm

DaveinOlyWA wrote:
Sun Jan 31, 2021 1:29 pm
As far as negative? Not possible since the BMS is constantly using power along with the 12 volt system.
I would think that high-enough levels of regen would make the net energy flow out of the battery be negative. I know I had a much higher SOC at the bottom of Hurricane Ridge than I had at the top.

I think the readout just caps it at 99.9, not wanting to wrap around through infinity to negative values.
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