We took another test drive of the truck. This time it was a little longer, and in the day time. We drove it 8.5 miles in total. In that distance I used about 20Ah from the battery. Using a nominal voltage of 141V for the battery pack, that works out to be 332 Wh/mile. That is not too bad I suppose. I had seen estimates of anywhere from 250 to 400 Wh/mi, so I am not unhappy with that.
But since the thing I measure is Amp-hours in and out, that is what I will use to determine my capacity and range. Using this very preliminary data to extrapolate my range, and using 80% as the maximum depth-of-discharge (DoD), my estimated range is 54 miles. The remaining 20% capacity is a reserve that I will never plan to use, and is about 13 miles. By limiting the maximum DoD to 80%, the battery pack will last longer.
As I said, this is very preliminary, and the range will be very dependent on where and how I drive, but I am happy with this performance so far.
Here is another test drive video (youtube), about 4.5 minutes long.
Comments
TomH
Posted on Wed 28 September 2011
I’m curious if you have compared the wh/m cost of electricity on your project car to your smart car gasoline mileage cost?
joe
Posted on Wed 28 September 2011
Tom, I haven’t done that, though you got me curious so I did some really rough calculations. At 40 mpg and $3.50/gallon, the smart car is about 8.8 cents/mile in fuel costs. I looked at my last utility bill, and including all the various charges and fees it looks like I am paying on average in the summer 13.1 cents/kWh. If the truck is 0.33 kWh/mile that comes out to be 4.3 cents/mile in electricity. However that is what the truck is using from the battery pack and not the wall. So I would derate that by a 90% charging efficiency from the wall and that gives 4.8 cents/mile.
So, that rough calculation yields 4.8 c/mile in electricity vs 8.8 c/mile in gasoline. However, the cost per mile of gasoline is highly variable depending on gasoline. I didn’t notice how much it was last time I filled up, but gas buddy shows around $3.30-$3.40/gal for premium which is what I am supposed to use in the smart car so maybe the $/mi for gasoline is a little lower than I calculated above. I believe that my electricity rate will be lower once I stop using so much A/C because if I am reading the bill right, the usage has a tiered charge structure.
However, to be fair, I should really compare the truck gasoline mpg to the truck electric m/wh, not truck vs Smart car. The smart car is much lighter and more efficient than the truck. The truck would get 20 mpg on a good tank. I could use regular so the gasoline would be less than the Smart car, so I would say that the gasoline cost for the truck is not quite double the smart car. So lets say 15 c/mile.
The big thing left out of the calculation is the batteries. In order to get cost per mile the battery life needs to be considered. The battery pack costs $8000. If it lasts 5 years and I drive 5000 mi/year, that comes out to 32 c/mile. So the battery costs pretty much swamp the electricity cost. However I pretty much am guessing at the battery life and the distance per year so that number could be highly variable as well.
The truck conversion was not an exercise to save money. I have no illusion about that. Mostly it is a fun (and costly) hobby. And I also think it is really cool to be able to plug it in and not have to buy gasoline for it. I have really enjoyed working on this project and learned a lot.