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Home 2011 Editions April, 2011

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It’s time to charge up tire dealers’ customer education campaigns for TPMS sensor replacement as the life expectancies of batteries molded into sensors begin to run out.
When asked how long TPMS sensor batteries will last, industry representatives provide a broad life expectancy range - five to 12 years and up to 100,000 miles.
Mandated TPMS systems were phased in from 2006 to 2008, so some of these vehicles’ sensors have been in service for five years. Still older direct sensors can be found in quite a few domestic and import models introduced earlier in the decade and equipped with TPMS ahead of government deadlines.

Direct TPMS sensors commonly use radio frequency technology to transmit measured tire pressure readings to a vehicle’s on-board electronic control unit and warn drivers of a 25% or more under-inflation level.

Mounted inside a tire assembly on valve stems or wheel rims, the sensors are usually powered by 3-volt lithium ion batteries, but some use 1.25-volt nickel metal hydride batteries. There are developments underway that promise battery-less sensors in the future, having the potential to dramatically change TPMS markets.

For now, though, the batteries – generally round and encased in a sensor’s molded plastic housing – have finite lives. Since the batteries are entomb­ed, a dead or dying one requires the replacement of its entire sensor assembly.

When asked how long the sensor batteries will last, industry representatives provide a broad life expectancy range – five to 12 years and up to 100,000 miles are mentioned when addressing estimated life spans. The follow-up qualifier is that driving conditions, such as frequent on-off cycling of a TPMS, tire pressure sampling frequency and temperature extremes, will significantly shorten battery life.

When Batteries Die
A failed battery means its sensor no longer is providing protection to the vehicle driver and passengers, just like an inoperable home smoke detector can’t warn occupants of a developing hazardous fire.

A smoke detector’s single, short beep at long intervals alerts a home­owner to change a battery, but how does a car or truck’s on-board computer know that a TPMS sensor’s battery has failed?

Brian Rigney, general manager for Dill Air Controls Products, conducted an experiment to determine how one vehicle reacts to a sensor’s lost transmission.

“A car’s computer recognizes that a signal interruption has occurred, Rigney says, “but it usually waits to react.”

Automotive makes and models have many different systems, he says. Generally, sensors are design­ed to transmit data to the computer at regular intervals, usually measured in minutes. If the computer misses a few sensor transmissions, it assumes interference has occur­red and refrains from alarming the driver. However, if the computer misses several more transmissions, it illuminates the TPMS icon to alert the driver of the malfunction.

Rigney tested a Pontiac model by completely removing a sensor from a tire assembly and driving the car. “It took 45 minutes before the TPMS icon warned me the computer was not receiving a sensor signal,” he says.

“My test vehicle’s system was set up for the 45-minute delay, but other cars are programmed for longer or shorter time periods.”

Real Life
It was time to ask for his battery life estimate.

“Each manufacturer has its own warranty, some are simply two years, some are years or miles,” Rigney explains. “An OE sensor battery is expected to last at least five years, which is usually outside the warranty time period.”

He says battery life expectancy is directly related to the number of RF transmissions, which are affected by driving conditions and the sensor design. “Does the sensor have a sleep mode? Is it designed to transmit every minute, five minutes or 20 minutes in normal conditions? Gen­erally, an OE battery will last five to 12 years and the average is seven years.”

Colder conditions generally allow batteries to last longer, according to Rigney, so batteries are expected to fail quicker in warmer rather than colder climates given the same driving conditions. However, driving habits vary greatly from tire customer to customer, and by the same token, the demand placed on sensors and batteries also fluctuates widely.

Sensors usually transmit less while a vehicle is stopped, more often while it’s in motion, and a lot more as it accelerates or decelerates. A constant speed, such as highway driving, allows sensors to transmit less often.

In general, short distances with numerous starts and stops will have a greater impact than overall miles driven. “In other words,” says Rigney, “10,000 city miles will result in lower battery life compared to 10,000 highway miles.”

Battery Replacement
A depleted battery can’t be ex­changed, so the entire sensor must be replaced.

Rigney says potting material inside the sensor housing secures the electronic components and protects them from the harsh environment inside a tire. “In order to remove a battery, the potting material would need to be melted. Heating the material could damage components and allow the battery’s lithium to seep out of its housing and into the environment.”

A NHTSA ruling, says Rigney, stated that TPMS and its 71 million tiny batteries represent a 2% increase in U.S. battery usage, but a much smaller overall increase in battery volume and chemical content to landfills. Balance that against the environmental benefits of TPMS-improved proper tire inflation rates that include improved fuel economy, reduced emissions and longer tire life.

When a customer visits a tire dealer, a technician can test each TPMS sensor’s battery life, but only if his or her scan tool can display battery life and the pressure sensor is equipped to output that data. Rigney cautions that it’s difficult to interpret the scanned data, because the readout could be a battery life percentage or a one-word description. “A displayed ‘10 %’ or ‘low’ could mean six mon­ths to a year of battery life remaining,” he says.

The lack of accurate information opens the door for a conversation to discover a customer’s expectations and explain the needs.

Rigney believes the TPMS and tire service industry should adopt standards. “Standardize sensors so they output battery life information and standardize scan tools so they display battery life,” he says.

He adds that the industry also should agree that a “low” or “15%” readout is the proper time to recommend to customers that they replace all four tire sensors and recheck the spare if it is TPMS-equipped. “It’s similar to replacing both headlights at the same time, even though only one is burned out.”

He adds that if a vehicle owner is buying new tires and one sensor battery is dead or low, the technician should explain it’s the best time to replace the sensors because:

• The tires already are off the wheels, so buying now means avoiding a future visit to the dealer and additional charges for a second balancing service.
• It could avoid the TPMS warning light appearing while on a trip or in traffic.
• It would eliminate the worry of dealing with a failed sensor.
• Replacement sensors, such as those made by Dill, can last seven to 12 years, which normally means never purchasing another set of sensors. So buy them now, instead of later.
• Some states require TPMS icons to be off in order to pass vehicle inspections.
• Replacing all sensors, before or when one fails, helps avoid a last-minute issue while preparing for inspection.
This article appeared in the April 2011 edition of Tire Review. You can read the entire issue on your phone or tablet by downloading the Tire Review app.

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