If you’re like me, you probably have a drawer full of AA batteries at home somewhere, just in case you need them. They’re not as ubiquitous as they used to be – I try to buy mostly rechargeable electronics nowadays, but those AAs still go into so many things, from TV remotes to flashlights to bathroom gadgets. (And yes, I know you can get rechargeable AA batteries – yay!)
So what does that have to do with a Mercedes?
Most small items run on two or maybe four AAs, because they only need a small amount of electricity to do what they do. Electricity is measured in kilowatt-hours (kWh), a unit of energy that defines how many thousands of watts of power get used up by a particular electrical circuit in an hour (you can see the kWh in action on your home electricity bill). A typical AA battery contains about 3.9 watt-hours, or 0.0039 kilowatt-hours, which is plenty of energy to keep your small flashlight bulb lit for hours before you have to change the batteries.
But what about an electric car? A 3,000-pound vehicle is not going to go very far on the amount of energy it takes to run an electric toothbrush. Saturday Night Live recently tackled this question, in what they probably did not intend as a science education sketch:
So how many AA batteries would it really take to power a car? According to the sketch, the Mercedes AA+ requires 9,648 AA batteries — but could that actually work?
According to Edmunds.com, the battery capacity of the Nissan Leaf is 24 kWh, while the Tesla Roadster‘s is 54 kWh.1 If we assume that the imaginary Mercedes AA+ is more like a Tesla than a Leaf, we can calculate that the battery requirement would be 54 / 0.0039 = 13,846. For the Leaf it would be 6,153, so the SNL estimate turns out to be pretty realistic!
Okay, but could you even fit 10,000 AAs in the Mercedes’ engine compartment? The batteries are about 2 inches long with a quarter inch radius, so again at a very rough estimate you get a volume of 10,000 x 2πr2 = 3,925 cubic inches, or only 2.3 cubic feet. No problem, right?
Despite these seemingly reasonable calculations, the video does a great job of illustrating some of the impracticalities of executing this particular design. I shudder at the thought of having to make sure 10,000 batteries were all lined up and snugly connected end-to-end anytime I wanted to pull out of my driveway…
There are other reasons we don’t use AA batteries to power cars, of course. As we’ve written about before, electric car batteries use different materials (including nanomaterials!) that allow them to be re-charged over and over again without losing significant capacity. Long term reliability is not nearly as important a feature for rechargeable AA batteries, since you’re not going to get stranded on the freeway if your TV remote dies.
If you want to learn more about different scales of electric power sources, check out this infographic about Powering Up the Star Wars Universe (how many AA batteries does it take to burn a hole in a blast door with a light saber, anyway?). Meanwhile, thanks to Saturday Night Live for this excellent lesson in the complexities of electric car design!
- Batteries Lesson Plan: How Does a Battery Work?
- Discovery Education: A New Road activity on battery technology (grades 6-8)
- Reed, P. Electric Car Battery Basics: Capacity, Charging and Range. Edmunds.com [website], 2011, retrieved from http://www.edmunds.com/car-technology/electric-car-battery-basics-capacity-charging-and-range.html