Cold Temperature Tests of Heated/Insulated Battery for AM General DJ-5E Jeeps

The U.S. Postal Service operates approximately 400 electric vehicles; predominantly AM General DJ-5E vehicles with a 1300-pound (590 kilogram), 3.22 cubic foot (9 x 10-² cubic meter), 54-volt Gould propulsion battery. A number of the vehicles were assigned to delivery routes in low ambient temperatures which affected battery performance. The field installation of foil-faced insulation on in-service batteries maintained a 15°F (-9.5°C) to 18°F (-7.75°C) higher electrolyte temperature than unwrapped batteries exposed to 15°F (-9.5°C) ambient temperatures. Overnight “cold-soak” in 15°F (9.5°C) ambient temperature did not significantly reduce battery capacity because of the heat retained after completion of the charging cycle. Weekend “cold-soak” in 15°F (-9.5°C) ambient temperature, however, reduced battery capacity such that the vehicles could not complete assigned routes.

The purpose of this effort was to determine practical solutions for maintaining the battery in an operable condition, i.e. electrolyte temperature above 50°F (10°C), over weekend “cold-soak” periods in ambient temperatures as low as 15°F (-9.5°C). Cold chamber tests were conducted on both the unwrapped battery and on the battery with a combination of various types of heat sources and insulation. Test results indicated that a heat source of about 300 watts with a foil-faced insulation cover and heat cycling by thermostatic control would maintain the battery in an operable condition. As a result of this effort, the vehicles will now be able to complete assigned routes in these low ambient temperatures after weekend “cold-soak” periods.