I have a response to a question on Facebook that I need to post here. This is uber geeky electric car stuff.
The Better Place electric car navigation system, Oscar, which I’ve demonstrated in a YouTube video, gives you a choice of showing either battery percentage or the raw number of kWh. A kilo-watt hour is the basic amount that is most usually spoken of in electric vehicle circles when describing battery capacity. I’m not aware that any other EV gives an instantaneous readout of this value at all times (happy to be told otherwise).
For example, my car has a 22 kWh battery and the Tesla Model S car comes in either 40,60 or 85kWh varieties. In America they’ve developed a complicated version of Miles Per Gallon to express the electric efficiency of an electric car, called MPGe. I find this number confusing and dumb.
My car reports how many kWh it takes to drive 100 km (for me and my car this is usually around 18 kWh/100 km).
Anyway here is the screen that lets me choose % or kWh:
Next shot is the value just after unplugging in the morning. Theoretical maximum is 22kWh but in practice I see values between 21.2 and 21.8 kWh. If I was looking at percentage, it tends to list anything above 21.2 kWh as 100%. Below this value I think the conversion from kWh to % is linear down to some cut off above 0 kWh. I haven’t really had the time to experiment with the ultra low end of kWh but I did have the car down at 3% on Sunday of this week on the way to Jerusalem.
After driving 2 km and stopping briefly I get back in the car and it reads 20.6 kWh (bottom left of screen). The first two km have taken 0.8 kWh and I know from experience I can reach this destination with the % display still pegged on 100% sometimes. It’s mostly downhill though my child likes strong acceleration from the traffic lights so sometimes I indulge him.
The next number to the right is 0 kW and this shows an instantaneous snapshot of current use. I’m parked so it’s 0 of course. While driving if I put my foot down this peaks at around 85 kW and if I’m decending a hill using full regen the number goes green and peaks at -35 kW.
The number on the far right, 19.2 kWh is the estimate of my battery level at my next destination in 7.3km. Though this is a very slightly different route the one I’ll actually take. The box in the top left shows my estimated arrival time and distance to destination. The system can also handle multiple destinations and will show a constantly changing estimate for each way point.
It also (as I show in the video for route planning) knows about charging times or battery switching and calculates later way points energy level based on what happens along the way!
So what happened this morning? Well the estimate was a little pessimistic. I arrived with 19.8 kWh so beat the estimate by 0.6 kWh which is a 2.5% variance on a full battery. This journey is a bit short for a proper test and I believe that today’s estimate is somewhat skewed because of two trips to Jerusalem this week both of which involved some high speed driving.
The current system does try to estimate for topology but doesn’t do a great job. We are waiting for the next upgrade and I was told this will crowd source actual usage figures from all the other cars to learn what energy is used on each km of every road in Israel.
I’ve seen a sneak peak of the new system and it is prettier. It’s being tested in some of the Better Place cars at the visitor centre so here for the first time (an Israellycool exclusive I believe) is the new system:
Update: fixed the brain fart confusing kWh and kW over the display of instantaneous energy use thanks to Robin’s comment.