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Modular rather than one-fit-all battery exchange

I was delighted to watch the Tesla S battery swap demo and to see the gradual unfolding of a refuelling infrastructure for an all-electric car fleet that was only a vision for me 30 years ago. I used a powerful simulation technique in the 1980s to find a least-cost infrastructure that would enable electric cars to match the surveyed travel pattern of passenger cars in the UK. Battery technology would have advanced sufficiently by 2020 for the average electric car to travel at highway speeds for at least two hours and to have good enough acceleration for my simulation to become a reality.

As I increased driving ranges from 100 to 200 miles so the demand for battery swaps diminished from around 2500 per day per 100,000 cars to 500 per day per 100,000 cars. Home and opportunity charging points provided 95% of the annual total electricity needed to complete most of the short trips (less than 40 miles) for the 200 mile range cars. If it were acceptable to drivers to lengthen their two to three hour journey times by 25 minutes, a ‘super charging’ network would provide a much cheaper solution by far to a network of battery exchange stations. In the end my doubts about the detrimental effect of fast charging on battery life resulted an infrastructure proposal that included ’lease-only’ battery swaps to match the electric car travel pattern to the surveyed travel pattern based on thousands of car diaries.
If super-fast charging is technically and economically feasible, is there a need for battery exchange stations? The answer is a resounding YES, but not for stations stocking and swapping one-fit-all battery packs but stocking and swapping interchangeable battery modules! The simulation also revealed the potential for reducing electric car costs by having a network of such stations.

If 100,000 cars were served by 100 battery module swap stations, the electric cars could do with only 100 mile range instead of 200 or 300 mile range. On trips longer that 100 miles the cars would call at these stations, a few times in a year, and have their range extended to 200 or 300 miles in a matter of minutes. If each station is stocked a set of battery modules, equivalent to fifty 200 mile range full sized battery packs that can be charged in 45 minutes, the cost saving to the car fleet would be the cost of 100,000 100 mile range battery packs less 10,000 100 mile range packs less battery exchange station cost of $500 thousand per station, roughly $9800 per car owner (I assumed $500/kWh battery cost and 24 kWh per 100 mile range).

I was recently granted a patent on the mechanics of swapping battery modules and would be ready to submit my proposal to the Tesla Engineering team for a quick scrutiny and comments.

Laszlo Gyenes (laszlo.gyenes@talktalk.net)


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