05 April 2006
SETI League
PriUPS Project



I have a Prius hybrid car.  This blog is hosted on my PriUPS site, where I spend many electrons regaling all with the advantages of using the Prius as a generator.  But I am not immune to the seductive lure of using the car for transportation as well.  I have been known to actually disconnect the car from the house and drive it to work.  In fact, here in New Jersey, where we have roads and stores and everything, I spend more time at work than I do coping with power emergencies!  And, like many Prius owners, when I drive I'm obsessed with getting the best gas mileage.    How obsessed?  For example, see this blog entry, in which I babble about the ability to time traffic lights with the GPS.  In other words, more than somewhat.

But somewhere, buried deep within, I also continue to embody a minicum (half a modicum) of common sense about the subject.  There are various techniques for improving ones gas mileage that would result in hazards for others on the road (or, in California, risk of assassination,) and I tend to eschew those techniques.  So I was intrigued by the offer of a company called EDrive Systems for what amounts to an additional battery pack that would allow me to double my mileage during my daily commute.  Basically, this is a system that replaces the Prius NiMH battery pack with a lithium ion system.  Between replacing the 1.3kWh Prius battery with a 9kWh one, and using 80% of the charge instead of 25% as the Prius normally does, their system delivers 24x more energy.  This is enough to drive the car mostly by electricity for the first 50 miles after charging.  Beyond 50 miles, the car will drive as it normally does.  (Their FAQ explains this in much more detail.) 

How intrigued was I?  Very.  Do I want to get one?  Read on...  (Let me assert here that I have no affiliation with the company, and know nothing about their system beyond what is on their web site. I don't, for example, even have any pictorial evidence that their device exists!  I'm taking their every word at face value, and have no reason to disbelieve any of their assertions, which seem reasonable albeit unconfirmed.)

The major economic justification for buying a hybrid is the gas saving.  You spend extra money for the car when you buy it, you save money every week when you fill up the tank.  There are any number of second order considerations, pluses like tax credits, minuses like battery replacement.  Allow me to ignore them here - they are a subject for an essay or even a book, but - please! - not now.  Let's just plug in a few numbers, and shamelessly round them to boot:

Hybrid technology adds significant cost to a car.  Let's say $5,000.  If you double your gas mileage, you will spend half as much on gas.  If you spend, say, $16 a week for gas for the Prius, it means that you are also saving $16 a week compared to your now-traded-in semi-guzzler.  $16 per week translates to  $4200 over five years.  Does this mean you shouldn't buy a Prius?  No!  It means you can either carry out the economic analysis to more and more second order terms and more guesses about the future and the price of gas, or you can say "I like the hybrid concept and I'm going to get one."  (Of course, you can instead come to the opposite conclusion.  My point is that the economics are both imponderable and inconclusive.)  As I pointed out elsewhere, I bought the Prius primarily because of its electrical system.

Getting back to the EDrive system, let's again look at the first-order economics.  Making the most favorable assumptions based on their data, i.e., that you drive about 50 miles per battery charge, and you keep your speed to 65mph or less, you will double your mileage yet again, to 100 MPG!  Assuming (as they do) that it costs about $1 to charge the battery to accomplish this, you are now talking about $8 a week for gas, plus maybe $6 a week for electricity.  Thus the economics for this addition to hybrid technology comes to $10,000 for the modification, and a saving of $2 per week, or $500 over five years.  Whoops.

I guess this answers the "Do I want to get one" question.  With the Prius itself, I could argue "maybe it will cost a little, maybe it will save a little" and get one because I felt like it.  With the aftermarket battery pack, the economics are daunting!  And that's in the most favorable case.  With the Prius itself, you save money on gas for all but extremely short trips.  With the battery pack, your driving must be "matched" to its characteristics.  If you drive less than a battery charge worth of miles, you're spending a lot of money for batteries you're not using.  If you drive more than that, you not only get no additional benefit, but your mileage is decreased by the extra 180lbs the pack weighs*.  In order to benefit economically from the EDrive system, electricity costs will have to remain as they are, gas will have to increase more than dramatically, and you will have to drive about 50 miles per charge without speeding. 

So is EDrive being stupid?  Why would anyone bring out a product that can't possibly be justified economically and seemingly doesn't make much sense even if they give it away?  Maybe they're not.  First, gas could increase dramatically, and electricity could remain as it is.  But instead of focusing on the price, focus on what they're offering.  Essentially EDrive is offering an improvement to the Prius hybrid drive.  Are improvements possible?  Hell yes!  I could save a few MPG if I could tinker with the computer program.  If the Prius battery had modestly more capacity and were used more aggressively, both achievable with some additional R&D, the car could eke out better mileage.  Adding battery capacity, especially for commuting, is a great idea!  And EDrive is doing it.  So why the dreadful economics?

  • The obvious - it's too expensive
  • The less obvious - it should be matched to your driving habits, but may well not be
  • The argumentative - gas is still too cheap

The last is likely to resolve itself, but that's by no means certain.  For example, if hybrids were much more popular, not only would we not need imported oil, everyone would be able to afford the gas for an SUV.  (Economics has feedback loops, too.)

The first will resolve itself, too.  Stored energy is critical to our continued existence.  We'll get there, slowly but surely.

Meanwhile, if EDrive could consider an intermediate solution, e.g. half the weight, half the range (recharge the batteries at work and at home!), and somehow 1/4 the price, they might be close to selling some on an economic basis in addition to the handful that will go to the four-pen nerds* and coal magnates.

**I understand that the weight of the battery pack will be decreased by 100 pounds in the next iteration.  That pretty much eliminates the argument for decreased mileage beyond the battery charge range.

*I'm a three-pen nerd.  I dream of my next promotion!

Richard Factor