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CalCars Working With Electro Energy on New NiMH Battery Pack for Plug-in Prius

CalCars is working with Electro Energy Inc. (EEI) on a custom prototype battery pack based on the company’s bi-polar NiMH batteries for use in CalCars’s Prius+ plug-in hybrid testbed.

CalCars hopes to use these batteries to demonstrate that NiMH battery systems are capable of powering effective production-class plug-in hybrids (PHEV).

Instead of using the conventional cylindrical coiled electrode or flat plate prismatic designs, the EEI design uses individual sealed flat rectangular wafer cells that are stacked on top of each other to create a series-connected battery.

Schematic of the wafer bipolar battery design.

Each wafer cell contains one positive electrode, a separator material, one negative electrode, and outer faces that serve as positive and negative contacts of the cell and contain the cell’s potassium hydroxide electrolyte.

To construct a multi-cell battery assembly, identical cells are stacked with end contacts and end plates.

This bi-polar design is more compact, exhibits higher power capability, and presumably will be lower in cost that the conventional cylindrical and prismatic designs.

According to EEI, limitations of the edge seal of bipolar battery designs have limited their usefulness in the past. By fabricating each individual wafer cell as a self-contained unit and sealing the perimeter of each cell, the EEI design overcomes this historic problem of attempting to seal the edge of the multiple cells in a stack.

EEI has been working on a design for hybrids based on USABC’s target specifications (25 kW discharge power and 30 kW recharge power). The preliminary battery configuration selected to meet these requirements is a battery containing 280 of EEI’s 6 Ah cells.

The CalCars spec, however, will use 360 cells. The prototype consists of 6 modules of 60 cells/module, 216V nominal, 34 Ah. The eventual pack will use 90 cells/module, and cut the weight down from 420 pounds to approximately 292 pounds.

CalCars will use two parallel strings of modules, allowing the disconnection of the first-fully-charged of two parallel strings of modules during charging to ensure that both strings get fully charged.

CalCars expects the EEI bipolar NiMH pack to support an EV-only range of 20–25 miles, and support 90 mpg US for 40–50 miles in an average mixed-driving scenario. Charge time should be 6 hours.

EEI is extending its wafer cell technology to Lithium-Ion chemistry as well.

EEI has been in business since 1992, and has produced and delivered prototype bipolar NiMH batteries for the US Army, NASA, the defunct Partnership for a New Generation Vehicle, NAVAIR and U.S. Air Force, National Institute of Health (NIH), and the Department of Energy (distributed energy and power quality).

The company recently received a $1,050,000 contract with the DOE for the further development of its Bipolar Nickel Metal Hydride energy storage battery for utility energy storage applications, including power regulation, and bulk storage.

It also received a $1.1 million U.S. Air Force contract supplement to continue development of manufacturing processes for a bipolar Nickel-Metal Hydride (NiMH) aircraft battery.




Any word on how this will ultimately impact cost? IIRC, CalCars biggest problem was that their kit was so damned expensive. Lower the price and they'll have a shot.


Not definitively, although it should push the price down. CalCars isn’t commercializing a Prius+ kit—that’s EDrive, with the Valence Li-ion battery—although CalCars did work with them.

Lance Funston

It's a shame that Toyota won't really get behind all this work. I'm having trouble understand their reticence around this. Except for the "some people don't understand the plug-in issue already" problem... Which is really not something that has appeared to have affected sales much so far.

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