Gasoline to Battery Range Comparison

One tank of gasoline contains hundreds of millions of Joules of energy (MJ), about 45 MJ/kg according to Wikipedia. Let’s say your typical car is 3,000 lbs in weight, has a range of 350 miles on a tank of gas, and gets about 30 mpg highway.

By contrast a Nickel-Metal Hydride (NimH) battery is good for about 0.22 MJ/kg. For the sake of discussion, using today’s technology how much battery mass would it take to provide the equivalent?

At 30 mpg it would take 11.67 gallons of gas to go 350 miles. If each gallon is roughly 6.5 lbs, then we have roughly 76 lbs or 34.5 kg of gas. Based on 45 MJ/kg that’s 1,552 MJ of energy.

We know a 350 mile range would be too far for batteries. So how much would the market accept as an alternative? Let’s assume 2/3 of that which would be 210 miles. For the same vehicle then 210 miles would require 2/3 as much energy which would be equal to about 1,035 MJ. But keep in mind that a gas engine is about 25% efficient whereas an electric power train is closer to 75%.

That means if you’re using 1,035 MJ of gas at 25% efficiency, you would only need about 345 MJ of electricity (1,035 x 0.25 / 0.75).

Using battery tech with 0.22 MJ/kg we’d still need an astounding 1,568 kg (or 3,456 lbs) of batteries. No wonder the ranges being discussed for plug-in hybrids (PHEV) are more often in the 50 to 100 mile range, using lithium batteries.

However, all is not lost. If we can pare the weight of the vehicle down from 3,000 lbs to say 1,500 lbs we can probably save another 1/3 in the energy for the same range. If that was done battery mass would come down proportionally to 1,045 kg (2,305 lbs). Still not practical but that’s for a 200 mile range. If the range were cut to 50 miles then it looks like we could get away with less than 600 lbs of batteries (2,305 lbs x 50 miles/200 miles = 576 lbs) if the car is very light. This does not yet account for the weight of the batteries either.

It’s clear there are only 3 main ways to increase PHEV range without additional fuel:

Improved battery energy density
More efficient powertrains
Drastically lighter vehicles

While there’s a lot of work being done on improving battery technology, it should be noted that dramatically lighter vehicles will strongly contribute to the growth of PHEV vehicles. And in doing so we’ll probably see a lot of refreshing concepts in the very near future.

Battery Recycling

According to a white paper by Firefly Energy (carbon foam cell battery maker) 90% of all lead acid batteries in the US are recycled. The infrastructure is all there because those batteries are considered to be hazardous waste.

Which begs the question, with the anticipated proliferation of PHEVs and BEVs how much of an issue is battery recycling going to be as we move toward non-lead acid technologies?

I read this article on AutoBlogGreen today, followed by this one on the Tesla Motors blog. Glad to see someone is addressing this.

If the claims are correct it seems like they went about it in a very clever way: Non-hazardous waste, modular, and with post-automotive applications in less critical areas (e.g. peak shaving).

Recycling programs need to be able to accommodate changing battery types and chemistries since technology is going to (hopefully) evolve at a much faster rate going forward.

Further more, hopefully battery life will continue to improve so that we will be able to recycle them less often, and when they are recycled the environmental impacts are also lower. We need to stretch the intervals.

Mobile Battery Charging

Some thoughts on the issue of charging batteries quickly. It seems to be the second biggest stumbling block with electric vehicles, the first being range. Right now, as far as I know, there are two main ways of storing electrical energy.

One is with batteries, the other is with capacitors. Capacitors charge and discharge very quickly but aren’t suited for slow discharges. Also, they have lower energy density than batteries. It takes multiple capacitor charges to equal the energy in a battery of comparable size.

What if there was a way to increase the energy of a capacitor to match that of a battery, and then find a way to charge batteries on-board a vehicle while it is in motion with a capacitor that’s quickly charged while stationary?

Peak Shaving

There’s been a lot of attention given to Plug-In Hybrid Electric Vehicles and their advantages. One of these is the ability to charge overnight using electricity when the demand or load on the power grid is least, and the possibility of supplying excess power back to the grid from the vehicle (V2G) during the day, when power demand is high.

I listened to an AFVi (Alternative Fuel Vehicle Institute) webinar today about these vehicles. One interesting slide covered V2G, and how that could help the grid. It was labeled “peak shaving”, and showed the potential to smooth out some of the peaks and valleys in power grid demand throughout the day. This is analogous to a previous post where about smoothing out drive cycles.

Whether it’s a power plant or a car, it’s quite clear that maximum efficiency is achieved within a very narrow operating range. And because of that, whatever we can do to decouple supplies and demands, no matter the system, means that higher efficiencies as well as lower overall peak power capacity can be achieved.

The Cost of Electricity vs. Gasoline

Just looked at one of my recent electric bills here in Virginia and my average annual cost for a kilowatt hour of electricity is 6.3 cents. Let’s put this in perspective and see how it compares with gasoline.

According to Wikipedia, gasoline has 32 to 34.8 MJ/L. So let’s assume 33 MJ/L. A gallon is 3.78 L so that’s 124.74 MJ/gal. A mega-Joule is a million Joules. So that’s 124,740,000 Joules in a gallon. A Watt is a Joule/second. So a Watt-hour then is equivalent to 3,600 (seconds/hour) times a Joule = 3,600 Joules.

A gallon of gasoline is equivalent to 124,740,000/3,600 = 34,650 Watt hours = 34.65 kWh. So let’s see, my cost to obtain a gallon of gasoline in electricity is 34.65 x 6.3 cents = $2.18295 or about $2.19. That’s not bad.

But it looks even better when I consider tank-to-wheel efficiency. A gasoline engine is maybe 30% efficient at best? Whereas when you compound the losses from a battery to an electric motor, it’s probably more than 60% efficient (say 70% battery efficiency, 85% motor efficiency). So if gasoline is at $3/gallon you’d have to buy $7 of it to get the equivalent work of $2.19 in electricity (70%/30% = 2.33; $3/gallon x 2.33 = $6.999).

Granted I don’t know if the electric meter in a house accounts for transmission losses from the power plant to the house, but I’m going to guess it doesn’t. So the electricity that comes out of your outlet is essentially what’s measured and what you’re getting charged for.

Not considering the environmental side of coal-fired power plants (or any other source) electricity seems to be very competitive with gasoline in terms of fuel cost.