One of the critical parameters for a start-stop system battery is dynamic charge acceptance (DCA). In a recently published paper with colleagues from RWTH Aachen University and Digatron Industrie-Elektronik GmbH, Eckhard Karden of Ford Research and Advanced Engineering Europe noted that:

Stop-start and regenerative braking are the hybridization features used for micro-hybrids, while avoiding the need for a high volt (above 60V) electric motor and traction battery. In it most widespread and lowest cost implementation, the topologies of a micro-hybrid vehicle’s powertrain and electric power supply system are the same as in conventional vehicles with internal combustion engine, i.e., a 14V generator with modified control algorithms and one lead-acid battery (or sometimes two) perform the brake energy recuperation.

Together with novel vehicle functions like electrically assisted brakes and steering systems, or cabin pre-heating during key-off, micro-hybridization confronts the starter battery with substantially increased energy throughput by micro-cycling with a typical amplitude (depth of discharge, DOD) around a few percent state of charge (SOC) or less. Fast recharge of the battery during these micro-cycles under a broad range of real world usage conditions, like temperature and driving profiles, is a pre-requisite for consistently high availability of stop/start and other essential functions for the customer.

It has been proposed to characterize the recharge ability of vehicle starter batteries as dynamic charge acceptance (DCA). Fuel savings due to regenerative braking critically depend on the DCA of the battery. With an appropriate modification of the alternator control strategy, lead-acid batteries can provide brake energy recuperation functionality.

—Budde-Meiwes et al.

While the energy throughput of AGM batteries is generally sufficient, DCA is an acknowledged challenge as the battery can’t handle more than a half to a third of the power the alternator can provide.

Although other approaches to lead-acid such as PbC or alternative electrochemical systems such NiZn can offer weight reduction, promise longer service life and better charge acceptance, there are as yet unproven at volume and over time, and system integration into 14 V power supply systems could pose a challenge, Ford says.

AGM batteries are actually a very good solution for start/stop. They are used extensively throughout European applications. It is a more durable 12V lead-acid battery. It has been proven in bench testing as well as being able to deliver on the target.

Many of the patents associated with [the Fusion start/stop system] make sure that the system recognizes the battery capability and works to those. The power of our system is that the controls that are used to determine if the engine shuts down or is started take into account the battery, both in terms of short term charge availability as well as in terms of lifetime. We want to make sure that both are maintained in a healthy zone.

We look at what kind of current draw is being used in the vehicle, the powertrain controller makes the decision based on those measurements to ensure that the battery remains healthy.

—Birgit Sorgenfrei

The Ford system. Until now, most non-hybrid vehicles with Auto Start-Stop have only been available with manual transmissions, but US drivers overwhelmingly opt for automatic transmissions. The new Fusion is the first Ford vehicle to offer Auto Start-Stop with a self-shifting gearbox. Since an automatic transmission needs to maintain internal hydraulic pressure even with the engine off, Ford added an electrically driven pump to the transmission along with the upgraded starter motor and the absorbed glass mat battery.

In order to minimize launch delays after a restart, the engineers wanted to keep the transmission in gear even with the engine off as opposed to shifting it into neutral. This required some unique control algorithms for the engine and transmission, Sorgenfrei said.

The control software includes a simulation model of the electrical system that constantly monitors the accessory loads. The model factors in the current draw from features like headlights, climate control, audio system and window defoggers to predict how much power will be available with the engine off and how fast the battery will drain. If the electrical load is demanding too much from the battery, Auto Start-Stop may be disabled to prevent a rough restart or being stuck with a flat battery.

Voltage blending is another technology Ford is seeking to patent. While the engine is running, the alternator produces about 14 to 15 volts, but the battery only produces 12 volts with a full charge. When the Fusion is slowing down with Auto Start-Stop enabled, the load model tracks the vehicle speed and deceleration and then calculates when to ramp down the voltage from the alternator to the battery-only level before the Fusion stops. This blending helps to ensure the driver doesn’t experience any light dimming or sudden fluctuations in ventilation fan speed.

The Auto Start-Stop team has also filed several patent applications related to the signal monitoring and controls for the climate control system. In addition to the cabin temperature and humidity, Auto Start-Stop monitors the temperature of the evaporator core that starts to rise before the occupants even feel a change inside the car. When this happens, the engine will restart sooner.

The 2013 Fusion with Auto Start-Stop also provides coaching to the driver.

When Auto Start-Stop is disabled because of a high accessory load, we wanted drivers to understand why so that they could opt to switch some things off if it’s appropriate. If the rear defrost is still on but the window is clear, they can switch it off to gain the efficiency benefit of shutting down the engine when the car stops.Auto Start-Stop engineering supervisor Kirk Pebley

Resources

• Heide Budde-Meiwes, Dominik Schulte, Julia Kowal, Dirk Uwe Sauer, Ralf Hecke, Eckhard Karden (2012) Dynamic charge acceptance of lead–acid batteries: Comparison of methods for conditioning and testing, Journal of Power Sources, Volume 207, Pages 30-36 doi: 10.1016/j.jpowsour.2011.12.045