It’s a normal school night. You are driving across town on a dark, snowy evening to pick up your child from after-school practice. You have the radio blaring and the navigation system on – you never turn them off. It’s a cold night, so the heater fans and seat warmers are running at their highest level.
You pull up to a red light, and the car’s automatic stop-start system shuts the engine off to conserve fuel. When you hit the pedal to move off, however, nothing happens. You try to start the car manually, but the starter just whines and the headlights go dim.
What’s happened? All the fancy features and new technology in the car have drained its battery, and now you are stranded in traffic with a child waiting for you across town.
This scenario is all too familiar, and is becoming more common as new vehicles come equipped with an increasing number of innovative systems that rely on the one component of the electrical system that has seen almost no innovation since the 1950s: the lead-acid battery.
Lead-Acid Battery Technology
The lead-acid battery has been an integral part of a vehicle’s electrical system since 1912; Cadillac was the first to harness its power, and turned the automotive industry upside down by introducing the self-starter. And the lead-acid battery is still the most viable energy-storage unit for vehicles, because of its performance, ruggedness and low cost.
Many different technologies have been mooted as replacements for lead-acid; all have been either too expensive, too fragile or too large. It is true that the lead in car batteries is highly toxic. But the environmental concerns are mitigated because lead is the most recycled metal in the world. It is estimated that some 97% of all battery lead in the US is recycled.
Unfortunately, improvements to the lead-acid battery have not been anywhere near able to keep up with the technological advances seen elsewhere in cars and trucks. According to the 2010 Battery Council International Technical Subcommittee, which conducts studies to find the failure modes in recently removed batteries, plate/grid-related failures had actually increased by 9% compared to five years previously. It is generally assumed that this increase in failures is caused by the extra electrical stress on the battery caused by the increase in the number of electrical systems in new vehicles.
Why Lead-Acid Batteries Die
There are three main causes of lead-acid battery failure. They are corrosion, sulfation, and acid stratification. They result from over-charge conditions, under-charge and abuse, respectively.
Corrosion is when the lead plates break down over time; eventually, portions of them might deteriorate completely. Corrosion of the lead plates in a battery is inevitable because they are immersed in acid. This process accelerates when conditions such as over-charging, high depth of discharge and over-temperature occur. The key to extending the life of a battery is not to stop corrosion but to manage those causes of it that are controllable.
Sulfation occurs when a battery is not allowed to be fully charged. This is a condition that has been made substantially more common by the increased number of electronics systems in new cars. After starting a vehicle, the lead-acid battery needs time to properly recharge. If the car goes straight from starting to idling in traffic, the reduced engine speed will not allow the alternator to adequately recharge the battery. If the motor is not allowed to speed up, the battery dies.
Stratification may have the same causes as sulfation. It occurs when the battery is held at a low state of charge, only moderately cycled and never fully charged. While the cause may be the same, what happens is very different. In a stratified battery, the electrolyte separates from the liquid mixture contained within the battery, and accumulates in the lower regions of the battery. The light acid in the upper regions causes the plates in these areas to be more likely to succumb to corrosion, while the highly concentrated lower region causes sulfation on this section of the lead plates.
In addition to the long-term effects of corrosion and sulfation, stratification causes short-term effects in the form of reduced cranking performance when starting the car. Stratification also causes the battery to have a false rise in the voltage reading, which makes the battery look more charged than it actually is to most measurement systems.
Keeping a Car Battery Healthy
The natural question resulting from this explanation of lead-acid technology is, how can these modes of failure be prevented from happening?
The sad answer is that they cannot. These failures will inevitably occur, and even under the best of operating conditions the battery will eventually die from corrosion.
Early death, however, can be prevented. What is more, the driver can be notified of impending natural death if the vehicle is provided with a sophisticated Battery Management System (BMS).
A BMS will be able to accurately monitor all battery parameters, including current, voltage and temperature. If the device is unable to monitor all of these parameters it will not be able to identify whether a battery is in good or bad health. For instance, the increased voltage caused by stratification will be misleading to a meter that only considers voltage measurement when determining a battery’s State of Charge (SOC). The BMS should take these three measurements together, and provide them to a higher level controller for an estimation of SOC.
The SOC is basically an estimation of how much energy is left in the battery. Like a vehicle’s fuel gauge, it shows how ‘full’ a battery is. The SOC can be calculated by coulomb counting. Coulomb counting is a method of measuring the current into or out of a battery, and integrating it over time. If the battery’s capacity is known, it is simple to calculate how full it is.
In the case of automotive batteries, however, it is not so straightforward. This is in part because corrosion and sulfation occur throughout the life of the battery. This means that a battery will be losing capacity over its entire life span. Therefore a fully charged battery will not contain as much stored energy after a few years of service as that same fully charged battery when it was new.
State of Health (SOH) readings help the measurement system to compensate for this decline in capacity over time. An SOH value gives an estimation of the percentage of the original maximum capacity. A new battery would have an SOH of 100%, whereas an older battery might have an SOH of 85%.
If an older battery is fully charged it will show a full SOC. Since it is older, however, the maximum that the battery can be charged to is 85% of the original capacity, meaning that even though the system says the battery is full and the charging system stops as it should, the car will still know that the SOC level will fall more quickly since it is being measured on a smaller scale. SOH estimations made with a good BMS thereby remove the false capacity readings that other measurement systems might produce.
Using Information from a BMS
So how could the scenario presented at the start of this article have been prevented by the use of BMS data? A BMS would monitor the SOC and SOH of the battery. Therefore the vehicle could have turned on warning lights or alarms weeks or months before the battery died on that night.
Of course, drivers routinely ignore warning lights. So the system could have sensed imminent danger and shut off non-essential systems (such as seat warmers and radio) to help maintain the battery’s SOC. In addition, the vehicle’s control system would have prevented the car from shutting off the engine at the red light, given the risk that it would not start up again.
Vishay Dale Intelligent Battery Sensor
An example of an ideal sensor for use in an automotive BMS is the Vishay Dale Intelligent Battery Sensor (IBS). The Vishay Dale IBS measures the voltage across the battery’s terminals, the charge or discharge current flowing through the battery, and the temperature of the battery measured through thermal conduction between the battery post and the IBS unit itself, using a WSBS8518L100 shunt resistor (see Figure 1).
|WBPK600L0A Product Summary|
|Voltage Range||4V to 18V|
|Current Range (Continuous)||±600A|
|Current Range (Pulsed)||±2000A|
|Temperature Range||-40°C to +115°C|
Figure 1: key features of the WBPK600L0A battery sensor
All three measurements are taken almost simultaneously to ensure accurate measurements even when operating conditions are changing rapidly. The Vishay IBS uses a LIN communication protocol to send the results of these measurements to the vehicle’s ECU or other control systems.
The Vishay Dale IBS is built to handle the full range of automotive operating conditions. Its -40°C to +115°C temperature rating allows the IBS to survive conditions that would damage even the newest lead-acid batteries. The voltage measurement range lets the unit continue to retrieve data under both battery over-charge and under-charge conditions. Through proprietary software the device is able to monitor the full current at both ends of the voltage and temperature extremes, with minimal loss of accuracy.
Future automobiles will continue to have more and more electronic systems that will make it even harder for the lead-acid battery to maintain the correct output over many years of reliable operation. A high performance BMS provides both the driver and the vehicle’s control systems with the information they need to prolong the life of the battery and to manage the risk of failure effectively.