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Characteristics of Hitachi’s 3rd-gen Li-ion power cell used in GM’s eAssist; 4th gen power cells and PHEV cells coming

Input and output power characteristics of 2nd and 3rd generations. Source: Hitachi Review. Click to enlarge.

GM’s eAssist light electrification system—currently offered as the base powertrain in the full-sized 2012 Buick LaCrosse and offered on the 2012 Buick Regal—uses a 115V, 0.5 kWh lithium-ion battery system built with Hitachi third-generation power-optimized cells. (Earlier post.)

In a paper in the current issue of Hitachi Review, Hitachi engineers describe the characteristics of these cells—targeted at hybrid electric vehicle applications—as well as their current efforts on the coming fourth-generation of power-optimized cells for hybrids (earlier post) and efforts on a prismatic cell for plug-in hybrid electric vehicles.

The third-generation cylindrical cells feature a manganese-based cathode and amorphous carbon anode, and offer a high output power density of 3,000 W/kg together with small size and light weight. In developing the third-generation, Hitachi engineers optimized the proportion of lithium and other metals in the cathode and controlled the microstructure to reduce the charge transfer resistance of the active material surface.

The battery’s structure was also designed to minimize the length of the cathode and anode leads. Improvements to the welding methods helped to reduce the electrical resistance.

Calendar life comparison. Click to enlarge.

Improvements in the anode structure and new electrolyte additives resulted in improved calendar life over the prior generation of cells; the improvement in lifecycle of batteries kept at high temperature allowed the battery size to be reduced.

Hitachi also developed packs to GM’s requirements. Each pack consists of two 16-cell modules comprising two blocks of eight cells with the battery monitoring module located on top of the battery modules. The pack is air-cooled; cell layout, cooling flow, and other parameters were optimized to reduce cell temperature rise, minimize the variation in temperature between cells, and reduce pressure loss in the flow path. To ensure the safety of the vehicle cabin if an abnormal situation arises, the pack is designed so that the cooling draft and gas exhaust grooves are entirely separate.

A new custom ASIC (application-specific integrated circuit) with an enhanced self-diagnosis function was developed for use in the battery monitoring system. Each ASIC can monitor four or six cells.

Hitachi currently has production capacity for its third-generation cylindrical batteries of 300,000 cells per month.

Hitachi automotive Li-ion HEV power cells
 2nd gen3rd gen4th gen
Status In production Entering production in 2011 Under development
Format Cylindrical Cylindrical Prismatic
Cathode material Manganese (Mn)-based Improved Mn-based Newly developed Mn-based
Anode material Amorphous carbon Amorphous carbon Amorphous carbon
Capacity (Ah) 5.5 4.4 4.8
Weight (kg) 0.30 0.26 0.24
Power density (W/kg) 2,600 3,000 4,500

Fourth-generation and PHEV cells. Hitachi’s fourth-generation HEV power cell offers a power density of 4,500 W/kg. The cell uses new manganese-based materials designed with optimum grain size using microstructure control and other methods. Measures such as making the electrodes thinner were also used to reduce charge transfer resistance.

The fourth-generation cell demonstrates 1.5 times the output power density of the third-generation (4,500 vs. 3,000 W/kg) and good heat radiation performance. When operated continuously at 11 C under natural convection flow conditions, the temperature rise was approximately 23 ºC and the temperature variation within the cell approximately 2 ºC.

Hitachi is also developing a prismatic battery that combines high energy (for EV operation) and high output (for HEV operation) that can be used as a power source for PHEVs. Hitachi was able to meet these sometimes conflicting objectives by using a low-resistance structure for the prismatic battery together with optimization of the electrode thickness and the composition of the active materials. The new cell also features the use of a ceramic separator and development of new electrolyte additives.



What's the energy density and the cycle life? So many breakthrough batteries seem to leave this out.


Great news, on battery development. The highest fuel savings come from having more vehicles with smaller batteries.

An eCVT is simpler than an automatic transmission, and the hybrid premium is no more than people choose in extras for a vehicle


Probably at least 4k+ cycles so as to meet the GM warranty specs.


Energy density is likely only in the 60 wh/kg range if that its all about power density even in the 4th gen cell.


Recent 30 Wh/Kg super caps could almost compete with this battery and would last much longer.



Who is selling 30Wh/Kg super cap, the best that Maxwells sells barely reach 10Wh/kg or I missed something ?


GM said that the original BAS could run just on ultra caps, so I would suppose the BAS+ could as well.


Lol.. my electric bicycle has more KwH than this.

Can't be a very large or expensive battery.


Assuming 4 volts per cell, the energy density of the 4th generation cell is 80 Wh/kg.


Treehugger...Electric Double-Layer Capacitors (EDLC ultra-caps) reached 30 Wh/Kg in 2010 and up to 85 Wh/Kg in Lab.

Maxwell Japan recently made twin ultra-caps EDLC modules for HEVs with performance approaching current NiMH batteries.

EDLC's energy density are increasing at a faster rate than Lithium Batteries and may soon reach 100 Wh/Kg. If this can be done, it could become a worthwhile alternative energy storage unit.

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