The battery is one of the most interesting components of the Ampera – and here in our blog as well. We want to explain it in detail. Horst Mettlach, Technical Integration Engineer Rechargeable Energy Storage Systems, helped us do that.
Why is it so difficult to develop a good battery for electric vehicles?
The battery has to offer sufficient capacity, performance, quality and durability. And of course it has to meet our safety standards, without compromises. The challenge of integrating it into the car is above all one of volume and weight of energy storers. Compared to a diesel tank, the mass of a lithium-ion battery is around 100 times more for the same energy content. But the much better efficiency of the electric drive helps. Nevertheless, based on the current status of technology, for today’s prevalent, expected range of 500 km, a battery system with a weight of around 830 kilograms and around 670 litres volume is needed – clearly too much for a passenger car. In the Ampera we use lithium-ion technology as it currently is the best energy storer. We see no alternative in the mid-term, but of course improvements are always possible. We elegantly solve the weight problem with the range extender: in this way a 198 kg, 16 kWh battery is enough. By the way our development centre for alternative propulsion in Mainz-Kastel played an important role in its development.
The Ampera’s T-shaped battery pack is comprised of 288 cells. The cells – just a bit larger than one DIN A5 page and only a few millimetres thick – are electrically connected (3 parallel, 96 in line). They are made of a manganese-based cathode (positive charge) and a graphite anode (negative charge) that are parted by a reinforced separator. The chemical process inside the cells is called lithium-ion shuttle – during charging the ions go from the cathode to the anode and during discharging back again. The cells are in so-called pouches (polymer layered aluminum casings) that we also call coffee bags, because they look a little like vacuum-packed coffee.
Batteries are temperature sensitive. How is this handled in the Ampera?
The battery must always be operated in a certain temperature zone. Cold lessens the chemical reactions, thus reducing its performance capability. Heat reduces the battery’s life cycle. We are proud that with our liquid cooled and heated battery we have found a way to ensure performance and durability. The system control constantly attends to the ideal operating temperature in real time. So the battery counters temperatures from -25°C to +50°C and the Ampera remains operational at all times. The fact that we always only tap a part of the battery’s capacity – just over half of its total capacity – benefits its longevity. This is one reason we can grant a warranty of eight years or 160.000 kilometres on the battery.