25 kilograms: That’s the weight of the hybrid battery in the 2010 Mercedes-Benz S-Class S 400 HYBRID. The battery consists of 35 cylindrical, rechargeable lithium-ion batteries and delivers a peak power of up to 15 kW (20 HP). The electric and thermal protection of this little bundle of energy is one of the battery ECU’s most important tasks. The ECU’s algorithms were developed in a joint venture between Johnson Controls and SAFT. dSPACE TargetLink is used to generate the ECU software. 

Mercedes-Benz S 400 HYBRID
The S 400 HYBRID is a mild hybrid in which the electric drive is used for engine start-up, the start-stop function, boosting and energy recovery. To save space, the compact, disk-shaped electric drive is installed in the enclosure of the torque converter, between the engine and the 7G-TRONIC seven-speed automatic transmission. This external rotor motor is a 3-phase rotary current permanent magnet electric motor with a maximum power of 15 kW (20 HP) and a start-up torque of 160 Nm at an operating voltage of 120 V.

The S 400 HYBRID has a mild hybrid drive. The high-voltage lithium-ion battery is installed in the engine compartment.

So much space is saved by installing the hybrid battery in the engine compartment in place of the conventional starter battery that the vehicle’s spacious interior and trunk do not need to be altered (Figure 1). The lithium-ion battery is not only an energy store for the electric motor. Via the voltage converter, the battery is also connected to the 12-volt electrical system that supplies the headlights and consumer comfort features. As with traditional autos, engine start-up is the biggest demand on the vehicle’s battery. If the battery charge is low due to self-discharge, low outside temperatures, and so forth, it first manifests itself during engine start-up. If the charge ever becomes too low, the hybrid system supports jump-starting.

A 12-volt lead-acid battery is installed in the trunk. It supplies power to standard consumer features and also to the monitoring system for the high-voltage components. Thanks to support from the lithium-ion battery, it can be considerably smaller and lighter.

The high-voltage lithium-ion battery weighs 25 kg. It has a high-strength enclosure with integrated cooling.

The mild hybrid has a specially designed combustion engine. It utilizes the advantages of the Atkinson principle, which increases the engine’s thermal efficiency and at the same time reduces fuel consumption and toxic emissions. The disadvantages of the Atkinson principle, such as a relatively low torque in the lower speed range, are compensated by the electric motor.

Comprehensive energy management ensures that all the components in the hybrid powertrain (battery, electric motor, voltage converter) respond optimally to the vehicle’s requirements. The electric motor supports the combustion engine during acceleration and acts as a generator with an energy recovery function during braking. Especially during the start-up phase, the hybrid battery provides electric power for the vehicle electrical system via the voltage converter. Moreover, suitable shifts in the working point ensure that the combustion engine always runs in its optimum efficiency range, even in such different situations as cross-country drives and urban traffic.

The high-voltage battery is made of 35 lithium-ion cells.

Cooled Hybrid Battery
The heart of the modular, very compact, very efficient hybrid drive is the new high-voltage lithium-ion battery (Figure 2), which was specially developed for use in vehicles. Its essential advantages over conventional nickel metal hydride batteries are greater energy density and higher electric efficiency, plus more compact dimensions and lower weight. The hybrid battery supplies a terminal voltage of 128 V at a maximum current of 200 A in the charge and discharge directions. It consists of 35 lithium-ion cells with a nominal voltage of 3.6 V each and a capacity of 7 Ah (Figure 3). For cooling, the battery is connected to the vehicle’s climate control. Battery cooling always has priority over the driver’s air-conditioning settings and can demand full cooling power even if the climate control is switched off.

Battery Management System
The battery management system (BMS) is the control center for the electric, thermal (physical) and chemical processes in the battery. The BMS is an independent ECU installed in the battery and has the following functions:

• Safety functions (e.g. voltage cut-out)
• Charge indicator (via the instrument cluster, see Figure 4)
• Computation of current, voltage, and power limits
• Temperature management
• Monitoring of battery aging
• Balancing (equalizing charge differences)

The vehicle reduces its speed to less than 60 km/h. The electric motor acts as a generator and converts the vehicle’s mechanical kinetic energy into electric energy to charge the hybrid battery (energy recovery). The instrument cluster shows this process as a green energy flow towards the battery. The battery is 50 % recharged, and charging continues.

Besides performing these control functions, the ECU also acts as a black box, that is, it stores all the battery data permanently so that it can be retrieved via diagnostic functions. To guarantee safe operation at the high voltages and currents, there are numerous safety functions that ensure that the battery’s high-voltage contacts are not live unless the battery is in operation. The battery is therefore completely safe to install, transport and store.

The vehicle accelerates to over 50 km/h. The electric motor consumes current from the battery and supports propulsion by the combustion engine. The instrument cluster shows the process as a red energy flow towards the wheels. Because of previous energy recovery, the battery charge is now at 51 %.

The vehicle brakes to a standstill (speed 0 km/h; the combustion engine and electric motor are turned off). After this new braking and energy recovery process, the battery’s charge has increased to 52 %.