Chrysler is in the process of temporarily withdrawing its pilot test fleet of plug-in hybrid electric vehicles (PHEV) to upgrade their battery system.

Chrysler declared three pickup trucks, out of a test fleet of 132 vehicles, equipped with plug-in hybrid powertrains were damaged when their prototype 12.9-kwh lithium-ion propulsion batteries overheated.

The pilot test fleet is comprised of 109 pickup trucks and 23 minivans. The company produced the batteries without using the NMP solvent used in most battery-manufacturing processes.

No one was injured when the batteries overheated. All indications seems to point to overheating while charging or when the vehicles were plugged in.

All 132 vehicles will go through batteries upgrades before Chrysler will resume the pilot test.

One of the main goal of these test vehicle was to be able to transfer the vehicle power to the grid, which Chrysler says could generate revenue for fleet operators. The trucks are also capable of linking to one another to form their own independent small grid.

Chrysler plans to use different battery chemistry in the next phase of both the minivan and the pick up truck projects. The batteries used in this abruptly ended first phase of the program had a high energy density, allowing engineers to vary and optimize battery weight and size on the vehicles in the pilot program.

So far, the vehicles have been tested in 20 states and, according to Chrysler, the fleet accumulated more than 1.3 million miles in a range of environmental conditions.

Interestingly, the pick up trucks reached peak average fuel economy of 37.4 mpg and the minivans 55 mpg.

Funded by Chrysler and the U.S. Department of Energy, the pilot test started last year and was scheduled to end sometimes during 2014.