Drive-Motor Power and Battery Capacity

In designing the electrical drive components of a hybrid or all-electric vehicle, the drive power and the weight of the on-board battery must be optimized. One of the great ad­vantages of an all-electric drive is the possibility of operat­ing the drive motor for a short time at a multiple of its av­erage maximum power. The average power can then be chosen to be considerably less than the peak-load power, making the electric drive train smaller, lighter and more economical.

We have simulated these requirements with software tools that we developed ourselves. If we insert the design electric drive into a power-consumption simulation, we ob­tain histograms for the energy used (Figure 3). For a quite realistic driving cycle developed at the ETH in Zurich (the Zurich commuter cycle), they show that the efficiency must be optimized for speeds under 50 km/h and for typical cross-country speeds of 100 km/h. The drive train must be optimized in particular for the partial-load regime. This al­so defines the region of average power, where the electric drive should have its best efficiency. For a city vehicle, this could be for example a drive power of 23 kW at partial load and 45 kW for peak load.

Once the components of the drive train have been de­fined, then the required battery capacity can be determined as a function of the maximum power needed and the cruis­ing range desired. Bosch and SB LiMotive, for example, project a battery capacity of 35 kWh for a compact car with a 200 km cruising range and equipment and driving per­formance similar to those of a conventional automobile. This battery, however, including its technical peripheral de­vices, would weigh 350 kg. It is planned by SB LiMotive to reduce this to 250 kg; in addition, today’s price of 500 N/ kWh storage capacity is to be decreased to 350 N by 2015.

Then such a battery would still cost 12,000 N – it would thus still be a very major cost factor for the all-electric car.

The strategy of two other automobile manufacturers is also interesting: They announced serial-production plug-in hybrid models for 2010 and 2012. For the plug-in variant of the successful Toyota Prius, tests for the optimal design of the battery were carried out from 2009 to 2012. The dri­ving behavior in daily usage of a 3rd generation Prius equipped with two tried-and-tested Ni-metal hydride (NiMH) battery packs of the previous series is being stud­ied in cooperation with the French energy supplier Elec – tricitd de France. The NiMH battery weighs around 110 kg, while the future series production model using lithium-ion technology should weigh 55 kg for the same capacity, about 5 kWh. With electric-only driving, the cruising range lies be­tween 15 and 20 km, and the test vehicles attain maximal speeds of 80 km/h. When their internal-combustion engines are running, they can drive faster.

General Motors is planning to extend the battery ca­pacity of its Chevrolet Volt model from 16 to 16.5 kWh in 2013. Its current battery weighs in at 175 kg, correspond­ing to an energy density of 91 Wh/kg. High-energy cells to­day achieve up to 160 Wh/kg, but the weight of the whole unit, including all the control and safety electronics, reduces this to currently about 90 Wh/kg.