A123′s 20Ah battery – putting it together

A123 discussed their 20Ah prismatic battery cell and module (42 cells) recently, pushing them specifically for the E-REV and BEV market. How could you put a bunch of these cells in a car, and how will it perform?

The cell is rated at 20Ah, and A123′s chemistry results in a battery voltage of 3.3V, so thats 66Wh per cell. Its not the biggest cell I’ve seen announced though (the 50Ah Altairnano cell is still bigger, though its still teetering on the edge of vaporware).

The cell is supposed to have a 50% higher energy density than their M1 cell, which means the individual cell has a rating of about 160Wh/kg, with A123 noting that the module level energy density would be around 140Wh/kg (this is because of the battery harnesses, controller, etc). The module would hold 42 cells and result in a total energy rating of 2.77kWh. Stringing 6 cells together would result in a battery with 252 cells and 16.6kWh of storage capacity. But does it have the power? We don’t know since the specific power ratings weren’t disclosed, but we can find out what the minimum would be.

With 16.6kWh of energy at a cell energy density of 160Wh/kg, thats 104kg of cells (not including other components). For a small car with a 120kW motor, thats 476W/cell, or about 1,200W/kg. It seems feasible – their M1 cell (to which most of the energy density comparisons were made) has a power density of at least 1,600W/kg (2.8V at 40A, or 15C, higher power densities at lower discharge rates), at nominal performance (50kW), the current draw per cell would be 60A, or 3C. Again, based on the M1 cell, at 3C discharge the nominal voltage is 3.2V, so 3.2V * 62A * 252 cells is approximately 50kW. When acceleration is occuring and the vehicle would need more power (100kW) the cell voltage would drop to about 3.1V, so you’d have to draw 128A from each cell (or about 6C). These figures compare well with the M1 cell, which has a max continuous discharge figure of 30C and peak 10s discharge of 52C.

It seems the new cells are very well suited for E-REVs and EVs – as long as my assumptions are safe. The rumored configuration for these modules is an 8-module pack (22kWh) for a Chrysler 200C, which is larger than the Volt, so the additional modules would be needed for the larger electric motor (~150kW) and to propel the car the 40 mile electric only range.