Issue: Oct 2009


Not realized by many is the degree to which lithium batteries need care to avoid expensive damage



SOME CAR LITHIUM ION BATTERY CHARACTERISTICS EMERGE

by Bob Brooks

Hoffman Estates, IL: At a recent rally event here of the Midwest Automotive Media
Association, AUTOMOTIVE INDUSTRIES, talked to Seneca Giese, sales
representative for Tesla Motors Inc., San Carlos, California. Giese displayed a Tesla
production electric car and outlined a number of features of the $109,000 very
small, carbon fiber, 2 seat roadster;155.4 inches long, 92.6 inch wheelbase, 44.4 inches
high…total weight with battery: 2700 lb. The electric motor is rated 240 HP 3 phase with
rev limit of 14,000 rpm

The vehicle is powered by a 992 lb, 6800 cell, 400V, 53 kWh capacity, lithium ion battery mounted with the motor and electronic controls behind the 2 seats driving the rear wheels. Economic and operational factors explained by Giese follow:

When the vehicle is sold new, for an additional $12,000 ¸the owner can purchase guaranteed future replacement of the battery after 7 years use if needed. . If the original battery is still functional after 7 years, a portion of the $12,000 is returned to the car owner on a schedule basis that was not explained.

Driving range depends on car owner use (speed & acceleration) which Giese explains is not unlike conventionally powered vehicles. Battery life is similarly impacted. The Tesla car’s computer maintains information about speeds and acceleration which is both stored and goes to a instrument panel readout of battery energy remaining expressed on a count down basis as miles the vehicle can travel before charging is needed.

The vehicle can be programmed for best range of 244 miles which limits power to about
half for moderate speed and acceleration performance .Programmed instead for
performance, range is cut to about 195 miles.

Battery charge is limited to between 90% and 10% of full capacity. Outside these
limits, battery life is compromised. Full charge up time on 240V 30 amp input is 3.5 hours or 24 hours on 15 amp 120V input. An important characteristic of the system is the need to maintain battery temperature at 70F at all times. For this reason vehicle owners are advised to keep the car plugged into an electric source whenever not in use and whether charging is or is not needed. Cooling or heating of the battery is achieved with the vehicle’s on-board systems that draw current in proportion largely to ambient temperature impact on stand-by battery drain. If left unplugged with battery at low charge level and significant battery temperature maintenance required, severe damage to the battery can occur quite soon, perhaps in as little as one day.

Passenger cabin heating and cooling can be a byproduct to a varying degree of the need for battery temperature control depending on ambient and operational conditions.

Information was provided on an advanced Tesla “Model S” electric car planned for production in 2011 said to have seating for 7 people, range up to 300 miles, 45 minute quick charge time and charging from 120V, 240V or 480V sources. Commercial 480V is needed for quick charge and is anticipated at future charging stations perhaps where food and comfort facilities are available. The Model S forecasted price is $49,900 said to include Federal tax credit of $7500. Add $12,000 for battery replacement insurance if that carries over from the current program.

Taken together, it is evident that if the Tesla car is representative, owners of lithium ion powered vehicles will have to adjust to special requirements for overall operation. Not the least of electric car owner requirements is access to plug-in electric source whenever the car is not in operation or at least not for any length of time.

If fuel economy of the little roadster is in the ball park of 4 miles per kWh, this translates at 14 cents/kWh for recent Chicago total residential electric cost to about $1.00 per 30 miles whereas gasoline cost for a similar conventional small car is close to $3.00 per 30 miles. Whether this advantage offsets total lithium ion battery powered vehicle cost and use limitations, remains for consumers to decide. Electric power has carbon emissions to be considered and the questions about road tax collection are unknown.


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