Too Busy For Words - the PaulWay Blog

Mon 17th May, 2010

Catching the EV bug

In the past year or so there's been a fair number of people who have come along to one Canberra EV group meeting and never come again. Based on what they were interested in and what the meetings have consisted of, I suspect they saw that we were (at the time) more dealing with the theoretical and policy ideas rather than practical vehicle making - Canberra EV has a vague idea of running a group project but no actual plan, no shed to build stuff, few tools, and most of the people are either not building one or have already built their own. Seeing what they perceive to be a lack of practical action on our part (myself, possibly, included), they go away again.

(Or maybe they realise that it's a bit more difficult than just having someone invite them over to convert their old vehicle in a weekend for the price of a couple of beers and a few lead-acid batteries. But who knows?)

Recently, a young guy has started coming along who's dead keen and a tinkerer besides. He built his own electric go-kart out of an old frame, a bunch of second-hand free starter motors, a bunch of second-hand free lead-acid batteries, and an on-off switch. It fuses starter motors fairly quickly but he's worked out its something that he can do. And I, in my own way, am keen to see him not leave because we're not doing enough to help new people - and I'm also interested in building an electric go-kart, too.

He lives in Goulburn and commutes to Canberra each day, and he wants an electric vehicle to do that. I've suggested to him that he gets an old ute - something with a bit of carrying capacity for the batteries. To give him some leeway in his journeys - no good having to do a run to the post-office at lunch and finding out you can't make it home - I've suggested to calculate for 300Km and a top speed of 140Km/hr. But how do we actually convert those abstract numbers into an idea of what to actually buy?

Well, let's do a bit of research. From the Green Vehicle Guide we know that the Tesla Roadster uses 231Wh/km (watt-hours per kilometre) and the Mitsubishi i-Miev uses 132Wh/km. So let's settle on 200Wh/km as a rough guess of how much our car conversion is going to use. We need to go 300km, so that's 60Kw that we need to store in the batteries. Picking 192 amps as a reasonable maximum for our motor - the Kostov 11" motors are rated at 192A - we can then derive from W=VA that we need about 312.5Ah in the batteries.

Picking two 160AH Thunder-Sky LFP160AHA cells to supply 320AH, we need 120 cells to provide 192V at the battery pack's standard resting voltage. That would deliver 960A continuous and up to 6400A peak, making the pack able to deliver 184 electrical kilowatts continuously. The whole pack would weigh about 660kg and cost about $24,960 from EV Works - or possibly less if you bought direct from the manufacturer.

Then you've got to buy the motor, controller, wiring, and various electrical accessories to run the traction side as well as the accessory side. And the car, of course. So you're easily looking at the thick end of $30,000 to do the whole conversion. It's not a way to save money, by any stretch of the imagination. But I think I've got the EV bug pretty badly, because calculating this kind of stuff is interesting to me.

Aside: Steve Walsh has taken the time to correct my statement that going at 120km/hr would use 'a lot of' petrol. In analysing this graph from this page on fuel economy in automobiles, we determined that for most of the cars on that graph there was about a 10% drop in fuel economy at 120km/hr compared with going 110km/hr. Someone going 130km/hr would be losing about 22% or so. The average loss between 55mph (~88km/hr) and 75mph (~120km/hr) is around 25% (taken from the study that the graph gets its data from).

Our car does about 15km/l in a 300km journey from Canberra to Sydney - with fuel at about $1.40/l, it costs about $28.00 for that journey. An extra 10% on that is about $2.80 - 25% would be about $7 extra. So not, perhaps, the 'lot more' that I'd speculated, but still needless. Instead of 2h43m to do 300km, it's about 2h30m - so they've saved 13 minutes for about $2.80. If that gives them a smug feeling of pleasure at being faster than those other mundane people that just do the speed limit, then I suppose it's cheap entertainment - unless they pick up a speeding ticket, where it gets a lot more expensive.

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