Why fast-charging and lithium-ion battery solutions?

Electric buses are one of the most exciting technological developments within public transport in many years. So why has Volvo Buses decided to opt for fast-charging and lithium-ion battery solutions?

Although Volvo Olvo Buses’ Electric Hybrids and full-electric city buses are new to the market, the choice of technology was made long ago, said Patrik Pettersson, chief project manager at Volvo Buses.

“The Volvo Group was early in adopting hybrid vehicles. In the case of buses, it was the natural and cost-effective step in the development from hybrid to plug-in hybrid, to enable them to be fast-charged with electricity at the bus terminals. For charging, this is the best thing about buses — they drive on a predetermined route and always come back within a certain time,” said Pettersson.

With an existing infrastructure including charging stations on the routes, it is important to have vehicles that run entirely on electric power, while in other cases hybrid buses are better. Everything is geared towards meeting a significant number of urban transport needs with a flexible, modular and environmentally sound solution.

The concept for the driveline was determined after several different concepts were evaluated. Apart from an in-house concept, several others were considered, including one with two electric wheel motors and a Volvo Buses-owned Nova Bus.

“The decisive factor was a total of 13 different criteria, such as battery life, LCC (life cycle cost), product cost and energy efficiency. It then became clear that our in-house solution was by far the best,” said Fredrika Berndtsson, chief project manager at the Volvo Group’s global truck technology and product development organisation, who is responsible for the delivery and development of the electric powertrain.

In addition, it was important to think long term and find a solution that benefits the entire group. “We are doing something that we do not see competitors do, namely aiming for an electromobility platform that meets the future needs of the group which will function for trucks and construction equipment, for example,” she said.

The ESS, energy storage system, contains a large helping of in-house-developed functionality, said Niklas Legnedahl, global system responsible for the ESS battery at the Volvo Group’s global truck technology and product development organisation.

“We buy the battery pack, but the control and functionality around it is our own. We are far ahead in this respect,” he said.

ESS has many advantages. It is so flexible that it is possible to switch to other types of batteries as technology develops. It also works for both electric hybrids with a single battery and full-electric buses with four batteries. The battery chemistry is lithium-ion, with 192 cells connected in series. It is capable of a discharge power around 170 kW and a total energy of 19 kWh. The usable energy of 8.5 kWh can be charged in six minutes after around half an hour’s driving. The batteries need to be replaced after four years in an electric hybrid and six years in a full-electric bus. This may sound like a short period, but it is equivalent to a capacity of up to 60,000 kilometres a year. Fredrika Berndtsson, Niklas Legnedahl and Patrik Pettersson see great benefits in the chosen technology.

One particularly relevant question about batteries relates to weight versus capacity. More cells provide greater capacity but also higher weight. Niklas Legnedahl is convinced that the lithium-ion battery solution is the best right now, but he emphasises that developments are ongoing.

“I’m sure that an even smarter storage medium will emerge. However, if you’re not on the intermediate stage of technology development, it is difficult to take the next step forward,” he said.