AUBURN HILLS, MI - Chrysler Group announced it is temporarily withdrawing its test fleet of plug-in hybrid electric vehicles (PHEV) in order to upgrade the vehicles’ battery packs. According to the automaker, three pickup trucks, out of a test fleet of 109 vehicles, equipped with plug-in hybrid powertrains were damaged when their prototype 12.9-kWh lithium-ion propulsion batteries overheated.

Chrysler stated that no one was injured due to the overheating batteries, and that the incidents occurred when the vehicles were not occupied. Another parallel project in which fleets are testing 23 PHEV minivans will also go through the battery upgrade process, though Chrysler noted that no vehicles in that fleet experienced the same issue that the three pickup trucks in the other test fleet did.

Overall, Chrysler reported that vehicles in the project achieved improved fuel economy performance, with the pickup trucks recording peak average fuel economy of 37.4 mpg. The minivans achieved peak average mpg of 55.

One of the main program goals was for the vehicles to be able to transfer their power to the grid, which the automaker said could generate revenue for fleet operators. The trucks are also capable of linking to one another to form independent “mini-grids.”

Chrysler stated it plans to use different battery chemistry in the next phase in both projects. The batteries used in the program have a high energy density, which allowed the automaker to battery weight and size on the vehicles in the pilot program. The company also produced the batteries without using the NMP solvent used in most battery-manufacturing processes.

So far the PHEVs have been evaluated by 16 partner organizations, including a number of municipalities and utilities in 20 states. Chrysler reported that the fleet accumulated more than 1.3 million miles in a range of environmental conditions.

For the next phase of the project, Chrysler said it plans to focus on grid interaction and improving safety. The company said the program began last year and will end in 2014. The pickup truck and minivan PHEV projects are jointly funded by Chrysler Group and the U.S. Department of Energy.