How Dual Use of Batteries Could Happen

James Greenberger addressed the mismatch between energy storage for utilities and power for electric cars could be resolved.

Published: 28-Mar-2011

Several weeks ago I wrote about the need to develop a standard dual use battery for both automotive and grid-connected storage applications. The advantage of a dual use battery is that it can leverage two different markets to achieve the economies of scale necessary to bring down battery costs in plug-in electric vehicles (PEV’s). The high cost of PEV batteries is the greatest single barrier to meaningful electrification of our national vehicle fleet.

Certain automobile OEM’s already use a kind of dual use battery to hold down the cost of their vehicles. Tesla Motors, for example, uses the same 18650 lithium-ion battery cells that are used in consumer electronics. Those off-the-shelf cells provide the Tesla Roadster will a relatively low cost per kilowatt for power (though no one would accuse the Roadster itself of being low priced). For large format advanced batteries, of the type more likely to be manufactured domestically, grid-connected storage is the most probable secondary market that can be leveraged.

As many noted in response to my article, the concept of a standard dual use utility/automotive battery does not account for the fact that cars and utilities need different types of energy storage. Electric energy storage consists of two basic components: power and energy. By analogy to a water can, energy is the size of the can and power is the size of the spigot. Automotive batteries require a lot of power, especially hybrid vehicle batteries, which have a power-energy ratio of >20:1. The batteries in pure electric vehicles have power-energy ratios of between 2:1 and 4:1.

The requirements of grid-connected energy storage are different. Since a community energy storage unit does not have to accelerate from 0 to 60 mph in six seconds, energy is much more important than power. Accordingly the specifications for community energy storage developed by AEP envision a power-energy ratio of somewhere between 1:1 and 1:3. Skeptics often point to this mismatch between the optimal power-energy configurations of automotive and grid-connected applications as the reason why a dual use battery will never be developed and why aftermarket automotive batteries will never find widespread secondary use in grid-connected applications.

While the skeptics have the technical problem right, they may have the future wrong. Today, as far as I am aware, all major automotive OEM’s producing PEV’s use a single type of high power battery cell in their battery packs. But this does not necessarily have to be the case. On July 21 of last year, NAATBatt produced a webinar on hybrid battery-capacitor systems (see http://naatbatt.org/publications/webinars/), exploring the possibility of using a combination of high energy batteries and high power ultracapacitors to power electric vehicles. These hybrid systems are possible, and even have some advantages over battery packs using a single type of battery cell. The reason they are not used today in PEV’s turns largely on the fact that the advantages of a hybrid power pack do not, in the minds of OEM’s, outweigh the risks of additional system complexity.

But the OEM’s cost/benefit analysis is based on current market conditions. In the current market, energy and power cost pretty much the same, and they both cost a lot since the market for both high energy batteries and high power batteries is incipient and very small. Different market conditions might yield a different answer. What if, for example, high energy advanced batteries were produced in gigawatt quantities for use by electric utilities in electricity distribution systems around the country? High energy advanced batteries might become commoditized by such high production volumes and their costs could fall dramatically. It is relatively easy to envision under such circumstances automotive OEM’s abandoning the single cell battery pack design for some sort of hybrid system. Such systems, combined with low priced high energy batteries, would permit OEM’s to reduce the cost of PEV’s—and permit more consumers to buy them.

We are still in the earliest stages of the markets for PEV’s and advanced battery technology and there is no telling how those market will evolve. But deploying advanced batteries in grid-connected applications shows great promise, not just for making the grid smarter by adding storage to it, but also as a way of reducing the cost of PEV’s and reducing reliance on imported petroleum. Government and industry must jump on the opportunity to create the dual use battery by encouraging the rapid development and deployment of distributed energy storage technology. And don’t write-off ultracaps just yet either.

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