Dual-cell Battery Concept
The battery of an electric vehicle (EV) must fulfil specific power and energy requirements. Since the available energy on board is limited, also the overall system efficiency is of key importance. These basic requirements depend on the purpose and the size of the vehicle. For hybrid applications, high power batteries with limited energy demand are required. For large full electric vehicles, high energy batteries are required that can pretty easily meet the power demand. However, there are applications for which both power and energy demands are key requirements:
- Small full electric passenger cars
- Medium size electric vehicles with range extender
- Electric light duty delivery trucks with range extender
- Plug-in hybrid electric medium duty vehicle
These categories of vehicles will be the first EVs in the market and will dominate the next few decades rather than other electric vehicles.
The batteries for these vehicles share a common feature since the batteries are relatively small (15-20 kWh) with respect to their energy content, but they must provide significant peak power (up to 150kW) required for acceleration, recuperation, and fast charging. Therefore, advanced energy storage systems are needed that provide both high power and reasonable energy storage density. However, so far such batteries do not exist! Since the battery in the mentioned applications has typically only an energy content of about 15kWh it cannot accept the power of up to 150kW and more during acceleration and recuperation. Today’s batteries are either optimised for high power (for e.g. full hybrid passenger cars like the Toyota Prius) or high energy (for e.g. full EVs like the Tesla). This dilemma of high energy vs. high power requirement is shown in the Ragone plot below. Looking at the Ragone plot it can be seen that a trade-off between high energy (HE) and high power (HP) exists. Without changing the technology, one cannot create a battery with a maximum of energy and power at the same time.
Plot showing the advantage of the dual-cell battery
The orange dots represent the trade-off between existing technology for high power and high energy technology whereas the yellow dot represents the target for the SuperLIB project, which can be achieved by combination of existing cell technology. (Source: AVL)
To overcome the existing limitations, HP cells can be combined with HE cells. If the HP cell utilizes the same chemistry as the HE cell then both cells can be connected in parallel and a charge balance between both cells occurs automatically. However, to optimise the performance of the battery, an additional charge-equalization circuit is required in between the HE and HP cell to use both cell types at their best operation points (max/min State-of-Charge (SoC) values, maximum peak current, maximum State-of-Charge swings) and to protect the high energy cell against high currents in order to reach lifetime targets of at least 10 years.
Thus, a dual-cell battery is obtained, which extends the operating modes of current Li-Ion batteries with respect to power and energy. In addition, the HP and the HE cells can be operated at different SoC levels. The HP cells will be operated at a partial SoC range only (e.g. 40-60% SoC), whereas the full SoC window of the HE cell can be utilized from 5 to 95% without limiting the functionality of the dual-cell battery. In addition to the charge-equalization circuit, improved SoC calculations functions will be developed, which are combined with a smart self-learning algorithm for controlling the energy distribution between the two cells.
The SuperLIB project focuses on smart control system solutions for traction batteries. The general concept consists of a combination of measures for
- a dual-cell battery concept comprising high-power and high-energy cells,
- including an electronic architecture for an efficient energy and current distribution,
- managed by a smart control strategy, and
- monitored by cell integrated temperature sensors