To ensure sustainable individual mobility worldwide, despite the prospect of future limited crude oil resources, alternative powertrains based on hybrid technology, fuel-cell technology and battery technology are becoming increasingly important. This need will be underlined by the regulations in Europe (ACEA CO
2 Self-Commitment) and in the U.S. (ZEV regulation). There are three steps towards the “Energy for the Future” at Daimler: efficient cars based on improved internal combustion engines with or without hybridization, improved conventional and alternative fuels (e.g. biomass-to-liquid), and emissions-free vehicles with fuel-cell/battery drive. Additionally, improvements can be expected with respect to aerodynamics and (lightweight) body construction (see bionic concept car of Daimler).
Mid of 2009 Daimler AG introduced their first Hybrid car – the S400 HYBRID with the first Li-Ion-battery in a passenger car. Since end of 2012 smart EV vehicles with an 18 kWh battery went into market. This battery is going to produce by German ACCUmotive GmbH, a ninty percent daughter of Daimler AG.
Hybrid batteries have high power densities (1800 W/ltr, 1200 W/kg) with a low specific energy content (70 Wh/ltr, 35 Wh/kg). Cell capacities are between 5 and 7 Ah and C-rates up to 40 A/Ah. On the other hand EV batteries have a 3 times higher specific energie content (170 Wh/ltr, 100 Wh/kg) and the power densities are 2 times lower (800 W/ltr, 500 W/kg). Cell capacities ranges from 40 to 60 Ah, C-rates are low (e.g. 3 A/Ah). PHEV batteries have medium specific power and energy batteries with cell capacities between 20 and 30 Ah. The characteristics and design rules of this battery type will be described in the following article:
- Modular approach
- Cell design and chemistry
- Cooling system
- Life and performance aspects
- Cost aspects
The following summary can be made:
- Energy: The usable energy per litre of a PHEV battery should be greater than 70 Wh/ltr. The volumetric energy is more important than the gravimetric energy. The choice of chemistry is strong related to the vehicle requirement: 30-35 km or 50 km!
- Safety: Li-Ion batteries based on FePhO4-chemistry shows the best results, especially looking to the regulation requirements for the Chinese market. In the case of NCM chemistry for a vehicle range of 50km additional safety elements in the cell are needed (ceramic separator, CID, etc.).
- Performance, Lifetime: End of life peak-power for C-class cars at lower state-of-charges is 65 kW, for S-class cars about 90 kW. Batteries with FePhO4-chemistry have higher cycle life than NCM based batteries. Energy throughputs of PHEV batteries are between 65 MWh and 100 MWh (double as high as for EV batteries).
- Cooling: Air cooling is not an option due to volumetric energy density (factor 1.5 times higher than for water/glycol systems). On the other hand a contact of water/glycol and HV-connectors has to be prevented.
- Cost: 300 EURO/kWh until 2020 is possible (in comparison: EV batteries at high volumes can reach 220 €/kWh).
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