Getting on the Grid

Photos: Ray Ng

Entering the Hybrids Plus shop, the Sawyer car was just another black Prius [1]. Even before the car arrived, the shop’s technicians began assembling the replacement battery pack, including roughly 600 Chinese-made lithium-ion cells (each 26 millimeters in diameter by 650 mm long) [2], sold by A123 Systems of Watertown, Mass., and a battery controller, which monitors the performance of each individual cell, manages power output, and keeps the pack within operating limits. Here [3], a Hybrids Plus engineer wires the controller into the pack’s internal harness.

Next, the interior of the car’s cargo area is stripped of trim [4] and the original Panasonic 1.3-kilowatt-hour nickel-metal-hydride (NiMH) pack removed [5]. With the old battery out [6], the 4.5-kWh replacement pack built by Hybrids Plus can be lowered into place [7] and mounted securely, then wired into the car’s high-voltage and 12-volt harnesses and its internal communications system. Because lithium-ion cells have roughly twice the energy density of NiMH cells, the larger replacement pack more than triples the energy capacity while adding less than 15 kilograms to the Prius’s original weight of about 1310 kg. The optional extension pack adds 40 kg more.

With the replacement pack in place and the seats refitted, the pack’s electrical performance is tested [8]. The rest of the cargo-area interior can then be reinstalled [9]. Before the first road test, there’s one final task: connecting a standard three-prong heavy-duty extension cord to the carefully hidden plug and charging up the battery [10]! The plug for external charging is actually one of the car’s neater features, located within a special light fitting that replaces one of the two standard Prius license-plate lights. Its cables run through the same plastic tube as the rest of the tailgate wiring. After conversion, the only indications of the car’s PHEV status on the Sawyers’ Prius are some added instrumentation and a couple of dash buttons, the plug itself, and a slightly less spacious cargo area. Unless you hear it go by, of course—or rather don’t hear it, because it’s running quietly, just on electricity.

Photos: Ray Ng