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Mike Brown ©1998 Mike Brown

"Can I charge my electric car with solar panels?"

Solar power is to electric vehicles like fries are to burgers. People refer to EVs as solar electric cars. The only true solar electric cars are the ultralights that run in the Sunrayce in this country and in the World Solar Challenge in Australia. People ask, "Why can't you put solar panels on the roof of a car to keep the batteries charged?" Unfortunately, there is not enough roof area on the average EV to mount enough solar cells to do any good. Reality changes the focus to, "Could you use a stationary solar array to charge an EV?" Answering that question is the topic of this column.

The practicality of charging your EV with solar panels is dependent upon many factors. The most important questions to ask are: "How is the EV used?" and "When is it going to be charged?"

In the ideal scenario, the EV would be used early in the day. It would consume half (or less than half) of the charge in the batteries. With this type of use, an appropriate solar array could charge the batteries directly, and the EV could be driven daily. If more of the battery capacity were used later in the day, the charging could be started that day and finished the next. With this plan, there is a chance that the batteries might not be charged enough to allow use of the EV the second day. Also, leaving the batteries partially discharged overnight could be harmful in the long run.

If the car is like most EVs and spends all day away from home base while being driven close to its maximum range, then charging overnight is necessary. This is where things get expensive and complicated. For a battery-to-battery charging system, you will need a stationary battery pack, charge controllers for both the stationary pack and the EV, and a solar tracker panel rack to get the most sun to the panels. All of this is in addition to the solar array itself. If you are going to use a standard ac-powered EV battery charger, you will need to add a heavy-duty inverter. Since the ability to charge the EV affects your mobility, a suitably sized gen set should be included in the system for cloudy days.

With the system described above, the stationary battery pack will be charged during the day while the EV is being used away from home base. When the EV comes home at night, it is charged either directly from the stationary pack or from its built-in ac charger through the inverter. This is the equivalent of a system for a large home in the 13 kWh per day range and costs around $28,000.

If you are in the position to use a grid-intertied system, you could eliminate the stationary battery pack. This would be effective only if you got credit for the electricity you produced during the day to off-set the cost of the electricity used at night to charge the EV. This is the system that the Sacramento Municipal Utility District uses to solar charge their EV fleet.

As you can see, solar charging an EV would take a commitment of time, money, and personal energy equal to or greater than the decision to drive an EV. That is not to say that it can't or isn't being done. While researching this column, I talked to several individuals who are doing it. They all have their own unique circumstances that allow them to solar charge, and reasons for doing it. The bottom line is: examine your mission for the EV, your lifestyle, and your budget, then use the resources of this magazine to design a system that suits these factors.

Please contact me with feedback, questions, or EV technical problems.

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Mike Brown's TechTalk, Electro Automotive, PO Box 1113-HP, Felton, CA 95018-1113 • 408-429-1989 Fax: 408-429-1907 • Email: [email protected] ||)y

Below: A 3,000 Watt PV array charges a Voltsrabbit at MREF '92.

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