Monitor and manage customers' battery current.
The ECM and/or BCM remain the brains of most charging systems, but there are standalone systems as well, notably GM's Stand Alone Regulated Voltage Control (SARVC). In either case, a small current monitor built into the battery terminal or placed around the cable provides information regarding current flow in or out of the battery. By adding up all the current flowing in and out of the battery, the ECM or a standalone module can determine any net gain or loss to SOC.
Sensing Current, Voltage and Temperature
Current sensors built directly into the battery terminal from Bosch, Hella, Delphi, and others provide current, voltage, and temperature sensing. GM uses a Hall effect sensor placed around the battery cable for current monitoring and monitors temperature and voltage elsewhere in the system. Non-Hall effect designs typically utilize micro-ohm shunts capable of measuring large amperage flows during cranking all the way down to parasitic draws of 30mA (milliamps) or less. Hybrids such as the Prius extensively use current monitoring for both main battery and motor monitoring functions. Because of their widespread adoption, techs will need to become familiar with current sensors. Most will display amperage data as a Parameter Identifier (PID) on a scan tool providing a value which can be compared to a reading taken from a clamp-on ammeter.A shunt-style current sensor is merely a very low value resistor, in the range of 50 to 100 micro-ohms (millionths of an ohm). This provides enough resistance to create a measurable voltage drop but not enough to interfere with cranking. A 200 amp starting current passing through a 50 micro-ohm resistance will only produce a voltage drop of 0.01volt.
Current sensing is only one piece of battery management. Battery temperature sensing is already in widespread use to vary the charging rate on vehicles. Traditional charging systems adjust the alternator setpoint (12.8-14.5V) depending on the temperature of the regulator. Certain Chrysler vehicles use temperature sensors plugged into the battery tray. When a battery is cold, it can be charged harder and faster than when it's hot. Thermistors incorporated onto certain older GM vehicles' battery cables perform a similar function.When temperature sensors fail, get damaged or unplugged, "strange" charging behaviors can result. For example, a "failed high" sensor on a Chrysler can cause the alternator to refuse to put out sufficient charging voltage. It can also prevent certain emissions monitors from running, an interesting "gotcha!" A Battery Volts Out of Range DTC P1630 may accompany a failed GM thermistor. Typically a battery temperature sensor is a simple thermistor and can be checked against specs with a Digital Multimeter (DMM). Normally DTCs will set for a shorted or an unplugged connector — but not always, as was recently reported on iATN (www.iatn.net) in a techhelp posting on a 2007 Caddy with an underseat battery.
Sending Sensing Information
A current sensor may transmit information to the ECM directly or via a low-cost LIN bus, Flexray or CAN bus connection. A databus-attached sensor that fails to respond to a query may set a DTC. In the case of GM, the Hall effect sensor utilizes a direct connection to the ECM or battery control module. Generator performance is monitored through the generator field duty cycle signal circuit. This signal is a 5 volt PWM (pulse width modulated) signal of 128 Hz. PWM ranges between 0 and 5 percent and 95 and 100 percent are used for diagnostic purposes.
Sensors built directly into the battery terminal or tray are located in the worst possible place for corrosion and damage from battery acid and fumes. When a sensor fails to provide correct current flow information or loses its connection with the ECM, erratic charging behavior is often the symptom. But be sure you actually have a problem.
Seemingly "unusual" charging behavior such as no alternator output at idle can trip up techs. It may actually be the work of a system working as intended. Take for instance GM Regulated Voltage Control, whether in its regular or "standalone" version. This charging system has six different modes listed in the attached chart. One even attempts to reduce sulfation of the battery. Someone accustomed to seeing the voltage gauge on the instrument cluster pegged at 13.8V is likely to wonder when he or she first encounters a GM vehicle where the voltage varies anywhere from 12.4 to 14.8 during normal operation. Hundreds of Internet postings attest to owners bewildered by this phenomena. A GM TSB attempts to alleviate customer concerns.
Issues and Problems
Current monitoring isn't exclusive to GM. Lexus, Toyota and BMW all are adopting current-based battery management. These systems have had a few bugs that needing ironing out. On BMW, the alarm system performing a self-check was tripping the current sensor, which expected only "normal" parasitic flows resulting in a High Current Drain message being presented to the driver, along with fault code 2DED (Power Management, closed circuit current monitoring). A reflash is available to correct this problem. GM quickly added monitoring of tow/haul switch settings to adjust RVC behavior to accommodate vehicles that tow trailers/campers and need to recharge auxiliary batteries.
Consumers, who wire high-powered stereo amplifiers and other systems directly to the battery terminal and bypassing the current monitor, may set themselves up for problems. GM has released four generations of RVC to date. Battery current monitoring has only been added to the later generations. Techs are utterly dependent on factory or aftermarket information to both determine if such a system is operating properly, and how to properly test it. Charging systems on certain GMs, BMWs and other luxury vehicles now incorporate "load shedding" features to momentarily turn off power-hungry systems like seat heating if the alternator can't keep up with demand. That may happen on a cold morning with the heater, window and mirror defrosters, wipers, lights and other accessories turned on and the engine barely ticking over at idle. When these accessory systems become inoperative, diagnosis must now begin with an analysis of the charging system.
Better gas mileage?
How exactly can a battery monitoring system increase fuel mileage. Lower idle speeds are one. At a stoplight, a monitoring system can intelligently allow a battery at 80 percent SOC to dip to a 60 percent SOC charge before restarting the motor or boosting the idle speed — perhaps just long enough for the vehicle to get underway again. At higher speeds, continually applying 13.8 charging volts to a battery already fully charged merely produces a useless "surface charge," a waste of energy and heat that evaporates electrolyte and ultimately shortens the battery life. Operating lights and other accessories at 13.8 that are perfectly content at 12.6 is another a waste of electricity — although "normal" headlights and dash lights are noticeably brighter at 13.8 than at 12.6. Consumers may have to get used to dash lights which dim slightly at idle.
Benefits and Futures
Why the added cost and complexity of battery management? The ability to get away with slightly smaller, lighter and less expensive batteries may be behind manufacturer's adoption of these systems, along with the potential for .1 to .5 mpg gain. Perhaps not having to upgrade luxury vehicles to even larger batteries. Making sure that start/stop hybrids always have enough "juice" to restart at stoplights is very important in designers' minds.
There may be diagnostic benefits like the ability to identify a failing starter pulling more amps "than it used to." Alternator malfunctions could be detected with the ECM comparing commanded output versus actual under control of a scan tool – perhaps eliminating the need for a load tester like a VAT-40. Future charging systems may incorporate super-capacitors for temporary charge storage, and/or features to support regenerative braking. One way to do this is to increase integrated starter/alternator output while braking to help decelerate the engine. Current monitoring either enables, or makes all of these tasks easier to perform.
In addition monitoring can determine battery state-of-health, and flash a light to tell the driver when it's time to replace the battery — before leaving the owner of an expensive luxury car stranded at the airport and allowing important keep-alive-memory systems to erase. How can it do this? By dynamically measuring voltage versus current it can calculate the battery's internal resistance using Ohms law.
While battery monitors may initially seem like unnecessary complexity, added cost and a virtually guaranteed reliability problem because of their location near battery acid and fumes, in the long run, with regular battery terminal service (corrosion protection) they should cut down on consumer's costs for battery replacement, reduce service calls for dead batteries, and help OEMs meet ever increasing fuel efficiency requirements.
BATTERY VIEW
Half Full or Half Empty?
MOST TECHS are familiar with battery state-of-charge (SOC). Open-circuit voltage provides a direct indication of SOC. With each cell in a lead-acid battery capable of producing 2.1 volts, a fully charged battery will display 12.6 volts, slightly more if it has acquired what's referred to as a surface charge. A half-discharged battery will display an open-circuit voltage around 12.3 volts (surprise, not 6.3 volts). To accurately measure SOC little or no current can be flowing into or out of the battery.
What about battery state-of-health (SOH)? A lot of the new conductance battery testers directly display battery SOH. If a battery that originally offered 600 cold cranking amps can now only supply 400, even fully charged, you can figure one-third or more of its useful life is gone. Comparing a battery to a gas tank, SOC is how full the (gas) tank is, SOH is how large the gas tank is. As a battery ages, the "size of the tank" gets smaller, and it gets harder and harder to fill because of increasing internal resistance, placing an ever increasing load on the alternator.
Wade Nelson is an electrical engineer who has worked for GM, Motorola and a mobility van conversion firm, and has taught automotive electronics at San Juan Community College. He has also written service and wrenched.
