January 2007 Articles & Features
Precise control of the charging system yields improved fuel economy, reduced vehicle weight and emissions, plus several less obvious benefits. Expect this technology to become commonplace very soon.
Laptop computers and cell phones have something in common with automobiles: Each of these devices must operate exclusively from the battery for periods of time. To get the longest possible use out of the battery, the electronic devices have for years used energy management software. This same concept of a broadly based energy management system is now coming to the automobile. Enabling the software are new computing platforms and new types of sensor devices.
According to Rob DeAnna, strategic development planner for Delphi , â€œToday's typical vehicle is not able to measure battery charging and discharge current. The traditional methods for measuring battery current, voltage and temperature do not provide the level of accuracy and resolution necessary to provide meaningful inputs to the optimum energy management.â€
This inability to monitor the current entering and leaving the battery has several consequences. In a vehicle, the battery acts as a buffer between the alternator's output and the vehicle's electrical needs or loads. If the battery's state of charge (SOC) were accurately known, the output of the alternator could be limited to just that amount needed to maintain the charge. Overcharging requires wasteful output from the alternator that then has to be dissipated as heat in the battery. Delphi says that the potential is there for as much as half a mile per gallon improvement in fuel economy by effectively managing the alternator.
How much energy is available to power the loads of a vehicle depends on the two possible sources of that energy. In today's vehicle designs, that's the output of the alternator and the stored energy of the battery. In vehicles of the future, the stored energy could also be in super capacitors or in other parts of the system such as high-voltage storage (300 volts) or medium-voltage storage, such as 42 volts.
The output of the alternator is dependent upon engine rpm as well as the control strategy for the voltage regulator. The voltage regulator responds to the surface charge of the battery as sensed at the alternator terminals. This could lead to errors in charging, as the true state of charge is not correctly indicated by this measurement.
The available energy from the battery depends first on the storage capacity of the battery and then its SOC. Since the battery is a chemical storage device, its storage capacity is not only a function of how much lead and acid is in the box, but of the temperature of all those materials as well.
The ability of the battery to supply energy to a load also depends on the battery's state of health (SOH). SOH monitoring devices measure the impedance or opposition to current flows into and out of the battery. By measuring this ability to take current for storage or release it for use, it's possible to know how healthy the battery is and, therefore, what can be demanded from it.
The challenge of electrical system loads is that not only are they constantly changing, but that there's nothing that gives one load priority over another. The driver is in control of HVAC and convenience loads and expects they'll always be available. Engine management systems, braking systems and the like control themselves and likewise expect always to be able to get sufficient power to do so. To meet all of these possible needs, all at the same time, would require a very large-capacity electrical system, even at engine idle. Siemens VDO says that current peak loads can amount to as much as 3000 watts now, with 4000 watts being possible in the near future.
The truth is that the worst case load situation seldom happens, if ever. This is the opportunity for an intelligent energy management system. By controlling the loads and managing their use, it's possible to correctly size the alternator and battery for what is more nearly the average load instead of the peak load. Reducing the overkill made necessary by the lack of a centralized control strategy is expected to help with fuel economy, vehicle weight and, of course, emissions.
There are some unexpected advantages that come along for the ride. For example, the same feature that monitors the discharge of the battery can be used to watch for trouble. It's estimated that battery-related vehicle breakdowns could be reduced by 80% by the combination of SOC and SOH monitoring. A software strategy called Quiescent Current Monitoring (QCM) can extend the life of a battery by preventing key-off loads from running it flat. This same strategy could put an end to batteries going dead during vehicle shipment or long periods of nonuse, such as in a long-term airport parking lot.
The Hella Corp. points out that its energy management system allows new features to be implemented within existing charging system sizes. For 2008, BMW is planning to switch to electric water pumps that run only when coolant circulation is needed. The ability to guarantee system power at all times could make other technologies, such as the electric wedge brake and drive-by-wire, safe enough to actually get into production vehicles.
Implementing energy management in vehicles requires both software and hardware. Systems that do this are being offered by a number of suppliers, including Hella, Bosch, Siemens VDO and Delphi , among others. While there are differences among these systems, the basic concepts and the needed hardware are similar.
Hella calls the most visible part of the system the â€œintelligent battery sensor.â€ Delphi calls its approach â€œIVT,â€ because the sensor monitors current (I), voltage (V) and temperature (T).
There are a couple of things you should know about these sensors. The first is that typically they mount right at the battery, usually as a part of the terminal assembly, because the needed accuracy depends on the sensors being as close as possible to the battery itself.
The current is measured through a special device called a manganin shunt . This is a very precise low-ohmic resistor in the range of 50 to 200 micro ohms ( Delphi uses a 100-micro ohm resistor). The current flowing into or out of the battery flows through this very tiny but very accurate resistor. The current is determined by measuring the voltage drop across the sensor resistance.
What's remarkable about the manganin shunt is the broad current range that can be measured with amazing accuracy. The same device that's accurate to within Â±10 milliamps (mA) at a 1-amp flow rate is still accurate within 3% at starting currents of up to 1200 amps. The voltage accuracy of Â±30 millivolts (mV) over the range of 6 to 18 volts is equally remarkable. The battery temperature can be measured within Â±2Â°. It's this accuracy that allows battery charging to be so carefully adjusted for what the battery really needs.
In addition to the sensor, there's the computing platform itself. The systems being shown so far are stand-alone units. The sensor inputs are processed according to the software, then the loads are controlled accordingly by the energy management computer. Depending on the size of the load to be controlled, both solid-state and magnetic-type relays can be used to control and manage the loads.
The software and pieces of software have names that vary by supplier. Siemens VDO calls part of its software the â€œself-adapting battery monitor algorithm.â€ Its purpose is to â€œdiagnose the battery in terms of energy content and to understand the ability to provide vehicle functions with power,â€ according to Ward Randall, the company's business development manager.
Another piece of Siemens software is called â€œpower trader.â€ A Siemens press release states that this is â€œa global approach to vehicle energy management and can be used on any type of vehicle system, including hybrids. The underlying principle of the power trader is that the energy provided by the alternator, the battery and other sources (APU), and likewise the energy requested by the loads and the vehicle, can be traded like stock on a stock exchange.â€
Press releases aside, what this means is that through the hardware they know what power is available and what power is needed. The software then makes the call as to what the best allocation should be for any given operating condition. They have a system of tiers that prioritizes the need for power. Those systems basic to the function of the vehicle (say, the ignition system) get first choice. HVAC may get second choice, with the result that it could be turned off or cut back in favor of driver-demanded heavy acceleration, for example. The worst case situation might lead to comfort feature loads such as the seat heater being turned down or the HVAC blower motor speed reduced.
The other main thing to know about vehicle energy management is that, as these systems are implemented over the 2007 to 2012 time frame, they'll likely get more complex and be able to do more things. This is especially true where there are multiple voltages and multiple energy storage systems in place on the same vehicle, as is the case with both mild and full hybrids.
As usual, it will take a while for technology like this to become an integrated part of the vehicle and not simply an add-on feature. Still, the benefits are there, and this will become just another part of what you'll expect to see every time you raise the hood.