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Issue #209 December 2007
My Kingdom for a Battery
by Steve Ciarcia
There is no hiding the fact that I’ve always been a car junkie. This isn’t just vicarious enjoyment resulting from reading eight or nine car magazines a month. I have physically owned more than a few cars that would blow the doors off a normal automobile. The good news for the future health of the magazine is that crazy behavior tends to lessen with old age and I drive “safer” cars these days. Still, the “roar factor” associated with high-octane gas and a lead foot has a firm place in American culture.
So, why am I bending your ear about my fantasies? Well, just like reality has caused me to switch from Italian madness to German sensibility, perhaps there is a next generation in the way I think about automobile transportation too. As you can see from the three-part article on my new photovoltaic (PV) power system starting this month, I’ve made a commitment to use solar energy to power my home. (BTW, my electric bill for October was $0.) Being the technology junkie that I am, however, I’m already wondering about a plug-in electric car.
That’s where the enthusiast in me conflicts with technical reality. I like the technical concept of hybrid electric vehicles (HEVs), but I don’t like either their current market niche or hybrid technology. Aside from the Lexus 600H, which alludes to having some performance, all the other HEVs manufactured in reasonable volume are econoboxes that merely stretch gasoline mileage by adding a battery and an electric motor to improve the gas-motor efficiency. At least if I’m going to have to jam myself into a shoebox-sized car, give me the satisfaction of saying that I charged it from my solar panels, it gets 1,000 miles/gallon, and it goes 0 to 60 MPH in 4 seconds. ;-)
Of course, making an HEV operate from my PV system isn’t quite as simple as buying a Toyota Prius and just ripping out the nickel-metal hydride (Ni-MH) batteries and installing higher-capacity Lithium-ion batteries (Li-ion) like most people think.
Battery specifications only tell part of the story. Hybrid electric vehicles such as the Prius operate in what’s called charge-sustaining mode. The gas engine intermittently recharges the batteries while the car is running. Most of these cars use Ni-MH batteries because the power contained in the batteries only has to be short-duration and high-current (some cars go the first few miles on batteries before the engine starts). When the battery is depleted to a specific level, the gas engine turns on and recharges the battery (and for some brands also powers the car). The harder you drive the car, the more often the engine comes on, but the battery charge never drops below a certain specified charge level.
Battery electric vehicles (BEVs) operate in what’s called charge-depletion mode, where the car depends on a single charge for the full range and performance of the vehicle. When the battery drops below a certain point, the car stops. That’s where the concept of the plug-in hybrid electric vehicle (PHEV) has its attraction and why every Prius owner thinks he wants to change the battery pack. PHEVs operate in both charge-sustaining and charge-depletion modes. The concept is that the car contains a battery that can be charged by plugging it into any AC outlet. The car’s Li-ion battery has sufficient power density to enable the electric motor to power the car for most “local” activities (the gas engine may never come on) and the car just goes back on the AC charger again at night. If the battery gets low or highway driving is involved, the gas engine comes on, charges the battery, and you just keep on going. Sounds perfect.
Unfortunately, the physics governing the “state of charge” for each of the battery chemistries greatly affects whether it is suitable for PHEV applications. Ni-MH batteries have an operational window of only about 30% of capacity. In other words, long-term battery life will be severely affected if the battery is charged above 85% of full charge or depleted below about 55%. These batteries don’t last long if you run them dry each time you run the car. (That’s why the engine in the hybrid is designed to come on frequently to keep the battery at the proper level.)
Lithium battery technology increases the state of charge operating range and metaphorically “puts more gas in the tank,” but using Li-ion adds a whole new set of problems, such as durability, cost, and safety. For example, currently manufactured Lithium batteries are exceedingly sensitive to over- or under-charging conditions and high temperatures. The power capacity and charging cycle times for a viable PHEV battery requires virtually monitoring every cell in the string (and there can be thousands) just to keep the current distribution and cooling limits balanced or it self-destructs. Finally, if the occasional chemical impurities introduced during the manufacturing process that penetrate the separator materials and short-out the battery causing it to explode don’t concern you, perhaps a little gossip will. Ever noticed that the laptop you bought a couple of years ago only runs about half as long on batteries now? Gossip has it that current Li-ion batteries permanently lose about 10% to 15% of their total capacity per year by just existing. Raise the temperature (e.g., leaving the battery installed in a hot computer all the time) and it exists for even less time.
Hopefully, Lithium polymer or other new battery technologies can combine high density and safe storage, but I think that PHEVs probably still need a few more design iterations before I jump on the bandwagon. A plug-in hybrid that gets 300 to 400 miles between charges and incorporates a high-efficiency diesel engine is worth fantasizing about. Still, I may need some incentives to give up the wonderful sound of a big V8.
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