miles per kWh doesn't compute

According to the instrumentation on my 2011 Nissan Leaf I am averaging 4 miles per kWh. But it didn't seem right.
So I did a test.
I charged to 100%, drove 42 miles, then charged again to 100% with a
Kill-o-Watt meter in-line. The charging energy consumption was 16.2 kWh giving 2.6 m/kWh. (A friend who own a Tesla says he's getting 2.5 m/kWh).

(It's possible the instrumentation is based on 100% charging
efficiency but that might account for 15% based on lead acid technology
(14 volt charging, 12 volt discharge)).

Details: 25 degrees F, 54 min trip time, slightly hilly NH roads,
daylight, no heat, 11 miles range left. Battery test 82 miles, 1 bars (same as a year ago).

I plan to do again in warm weather.

That is the rate from the wall that includes losses in efficiency and charging, the number on the dash does not measure this component so it is always higher than the wall.

^^ Two things:

1) As EVDRIVER says, charging has efficiency losses; it is believed that charging at Level 1 (120 V, 12 Amps) is about 75% efficient. So 2.6 miles/kwh measured from the wall at L1 would be roughly 3.5 miles/kWh on the dash.

2) Are you resetting your dash miles/kWh meter for each trip or is it some sort of long term average? If the latter, your numbers will have little meaning because the mileage efficiency can vary widely depending on weather, speed, heater use, and other driving conditions.

Charging at Level 2 (240 Volts) is considerably more efficient than Level 1 (86% to perhaps 92%, depending on the charging amperage and some other factors). Also, a Tesla is considerably less efficient than a LEAF, in part due to the size of the car and in part due to energy required for running the Temperature Management System (TMS) that adjusts the battery temperature (which the LEAF doesn't have). 2.5 miles/kWh from the wall sounds pretty good for a Tesla, so my guess is that number is from Level 2 charging. Or, perhaps, the friend with the Tesla was referring to a dash number?

its simple really. Its KW from the battery, not the wall.

If you want to "track back" to the wall, you will need to measure how much a full charge takes. You will draw different amounts at different points during the charge cycle so a partial charge is not truly representative of power from the grid.

I reset the miles/kWh meter before this trial trip.

The Kilo-o-watt meter measures the cumulative total of energy drawn so a varying charge rate has no impact.

I am surprised that the efficiency at 120 is so low.

Another way to approach this is I traveled 42 miles and had 11 left. If I assume a 22 kWh battery at this point, then I got 2.4 m/kWh.

My friend with the Tesla was basing his number on the instrumentation readings. I believe he installed a 22 kW (!) charger at home. He says someone drove up Mt. Washington (7.5 miles to the top). They lost 42 miles range but got back 27. This seems plausible but he pointed out that the net range lost was 15 and the miles traveled was 15 so the regenerative braking was 100% efficient.
According to the Car Talk column regenerative breaking returns a little more that 1/2. He plans to try it himself sometime.

dhad0 said:

...The Kilo-o-watt meter measures the cumulative total of energy drawn so a varying charge rate has no impact...

Click to expand...

A clarification: while it is true that the meter will measure how much energy is used independently of charge rate, a varying charge rate does have an impact on charging efficiency. With most EVs charging speed slows as it approaches 100%, to protect the battery. But the some of the charging overhead losses will remain constant (cooling pumps, electronic systems, and the like). So, as the rate of charging slows, the charging efficiency drops. That means that charging between, say, 20% to 80% will be somewhat more efficient than charging from 40% to 100%. The latter charge cycle takes longer and uses more electricity even if the number kWh that ends up in the battery is the same. FWIW.

Charging my 2011 at 240 volts with my AeroVironment EVSE and measuring the energy from the wall with a revenue grade kWh meter typically yielded 3.0 miles per kWh while the dash display (in front of the steering wheel) typically indicated 4.0 mi/kWh. These numbers were consistent for the life of my vehicle. The slightly lower efficiency measured by the OP is due to the 120-volt charging and the cooling system overhead.

Edited to add: The above numbers are without using climate control while charging. The 4.0 mi/kWh on the dash display is not realistic, but matches similar inflated numbers from CarWings.

Gerry

I have been measuring most of the charges since we got our Leaf:

http://ecomodder.com/forum/em-fuel-log.php?vehicleid=8730

We are averaging 3.472 miles / kWh. Which is almost exactly 1 mile less than the typical dash meter. The difference is losses in the charger, etc. and the battery heater that is required in colder weather.

NeilBlanchard said:

... The difference is losses in the charger, etc. and the battery heater that is required in colder weather.

Click to expand...

It is worth emphasizing that if one preheats (or precools) while plugged-in, the apparent charging efficiency drops significantly. Comparing my readings from the "wall" to the dash efficiency meter I tend to see charging efficiency numbers in the 86% to 91% range with my 3.3 kW charger when no preheating is used. For the months when I do preheat, on occasion, I don't bother to calculate the number because it is not meaningful.

My monthly numbers over the last three years:

I understand the OPs comments/concerns more than I understand the responses. I have both the Tesla and a Leaf, which makes it easier to make the comparison...
Per the Leaf dash info, I have been averaging 4.3-4.7 miles per kwh- so I start with a full charge, drive (for exanple) 70 miles with (dash reading of) 4.5 miles per kwh- I theoretically should know that it will take 15.5 kwh (12 bars) to fill back up, and or that I have 8.5 kwh left- Even when I give a .5 mile per kwh buffer for other loss, such is not the case. Not even close. I think THAT is what the OP is perplexed about, as am I.
In comparrison, I am finding the dash data an exceptionally good predictor in the Tesla. My average is 3.4 miles per kwh. Providing the same 0 .5 mile per kwh buffer, I can easily predict how many kwh it will take to fill and more importantly, how far I can go based on my driving efficiency.
Leaf owners with predictable use/ driving schedules learn what there car is capable of handling, but for someone like me with a highly fluctuating use... I depend on the guess-o-meter and efficiency reports on the screen.... And they dont seem to be serving me well thus far.

GerryAZ said:

Charging my 2011 at 240 volts with my AeroVironment EVSE and measuring the energy from the wall with a revenue grade kWh meter typically yielded 3.0 miles per kWh while the dash display (in front of the steering wheel) typically indicated 4.0 mi/kWh. These numbers were consistent for the life of my vehicle. The slightly lower efficiency measured by the OP is due to the 120-volt charging and the cooling system overhead.

Edited to add: The above numbers are without using climate control while charging. The 4.0 mi/kWh on the dash display is not realistic, but matches similar inflated numbers from CarWings.

Gerry

Click to expand...

This actually might be more helpful! If I presume the efficiency data to be inflated by a full 1 mile per kwh... That just might work

sklancha said:

This actually might be more helpful! If I presume the efficiency data to be inflated by a full 1 mile per kwh... That just might work

Click to expand...

That depends on whether you are looking at the actual efficiency of the car or the amount of power used from the wall. It is well known that there are losses during charging to run the charger/inverter and cooling system, so not all of the electricity from the wall makes it into the battery.

Level 1 charging is believed to be about 75% efficient; I have done so little L1 that I haven't measured it for myself.

Level 2 charging at 3.8 kW (240V x 16A) is about 86% to 90% efficient, depending on whether one is charging in the middle of the SOC or all the way to "100%". The latter lowers the efficiency somewhat because the rate of charge tapers at the end but the overhead loss remains much the same.

Level 2 charging at 6.6 kW (240V x 27.5A) should be better than 90% efficient, although I can't recall seeing anyone report hard numbers for it. It is also subject to lower efficiency when tapering toward "100%".

With all of these charging methods preheating/precooling will reduce efficiency because the energy used to run the CC system is a dead loss so far as miles of range is concerned and that number is highly variable, depending on conditions. Those who mostly charge in the middle range of the battery, say 20% to 70%, as I do, will see higher charging efficiency numbers than those who routinely charge to "full".