With the U.S. Federal Government proposing increased fuel economy standards and everyone concerned with the existing fuel economy numbers, automakers are looking for anything that can help eke out a few additional miles per gallon. Energy management systems are one means to obtain fuel savings. To address the sensing needs of vehicle energy management systems, Delphi developed an integrated current, voltage and temperature (IVT) sensor. The sensor allows the accurate measurement of a battery’s state of charge (SOC) and state of health (SOH) required for improved energy management.
“It’s kind of a next step in the evolution of a charging system,” says David Wright, director, Advanced Engineering, Delphi Packard Electrical/Electronic Architecture.
But improved fuel economy is far from the only reason for energy management.
“I have seen studies that show that battery life is an increasingly significant problem for the automotive industry,” says Wright. In fact, according to ADAC, the largest automobile club in Europe, in 2000, about 15 percent of all vehicle breakdowns were due to dead or defective batteries. Since that time, vehicle loads continued to increase dramatically and OEM implementation of battery management systems has increased.
Accurate SOC and SOH measurements can also allow optimized battery sizing (with associated lower mass), maintain engine cranking capability, provide improved incandescent bulb life by having a lower average voltage and ensure operation of safety-critical electrical functions by providing input to the energy management system.
Determining the battery’s state of charge requires measuring its current, voltage and temperature. “The analogy that I like to draw is it is like the fuel gauge on your instrument panel,” says Wright. “It tells you how much of the capacity of the battery is available. You can literally think of it as full, ¾, ½, ¼ and empty.” In contrast, the battery’s state of health determines the size of the tank. Those are two different calculations but they both require current, voltage and temperature measurements.
A battery that is in good state of health but is nearly discharged just needs a recharge. However, a fully charged battery that is in a poor state of health will not perform as required and cannot improve. “As batteries age, the size of the tank gets smaller,” says Wright.
There are different ways to determine these two key battery parameters, but all of them require combining temperature, current and voltage. “We do have an implementation where we can put the algorithm directly right in the sensors,” says Wright. The algorithms that determine SOC and SOH can be contained on the memory of the small microcontroller in Delphi’s IVT sensor.
An alternate configuration of the sensor supplies just the basic temperature, current and voltage parameters to a body or other computer that would run the algorithm. In either case, the sensor communicates data and diagnostic information over a Local Interconnect Network (LIN) or Controller Area Network (CAN) interface.
IVT Specs and Applications
The current measurement has three operating regions with corresponding accuracy. The low range from -1 to +1A, accuracy is ±10 mA. From -300 to +300A, accuracy is the larger of ±1 percent of reading or ±80 mA. For starting, where currents can range from -1200 to +300A, the accuracy is the larger of ±3 percent of reading or ±400 mA. The voltage sensing range from 6 to 18V has ±30 mV accuracy. From -40 to 85C, the temperature sensing accuracy is ±2C.
One of the challenges to obtain highly accurate current measurements is the range of the data. “You are talking about trying to measure 10s of mAs going through a 100 µO shunt,” says Wright. “We are doing this using voltage amplifiers.”
“The range of current measurement enables us to do a few things with capturing the battery’s internal resistance by being able to measure the full cranking current,” says John Caputo, supervisor of IVT sensor development at Delphi.
With SOC measurements as part of the energy management system, if the SOC is above a certain level, say 75 percent, the vehicle would function normally. If the SOC drops below 75 percent, one of the things that could be done is raising the idle speed to get more current out of the alternator when the vehicle is stopped. If the SOC drops below 60 percent, specific loads could be regulated such as allowing heated seats and rear window defogger to operate only at 50 percent and never at the same time. Load-shedding decisions that turn off non-critical loads to ensure operation of safety-critical loads should come from the battery based on SOC and SOH measurements.
On the charging side, in those systems that have control of the alternator’s output voltage, the voltage could be lowered from the alternator if the battery was fully charged. “Most of the systems today bias toward always making sure you are always charging the battery,” says Caputo. “If they are fully charged, that is waste heat but it can also shorten the battery life by boiling off some of the available electrolyte.”
“The other advantage is you pick up a fuel economy gain,” says Wright, because if you are charging a fully charged battery that is just wasting energy. The 0.5 mpg savings would be in medium to small vehicles that typically have a smaller engine that has to work harder to drive the alternator.
IVT Sensor Implementation
A natural location for the sensor is in the battery cable at the negative battery terminal. The technical requirement is that it must be at a point where it can measure the total battery current.
“We see a couple of different architectural alternatives and probably one of the most interesting ones is this idea of having a pre-fuse box right at the battery,” says Wright. The pre-fuse box would include very high-current fuses and basically the feeds to all the different electrical centers around the vehicle. The power distribution scheme could have an underhood electrical center, a passenger compartment electrical center and a rear electrical center that could be fed through a 125A or appropriately sized fuse. “We have customers who are implementing those today,” says Wright.