TAG BATTERIES:IAG plans for battery deaths

IAG tags were the first major deployment of battery-powered active style transponders, previous deployed tags having been mostly passive backscatter tags, which make use of the tollster’s transceiver for most of the power to drive the RF signal. (Many backscatter tags have a battery for signal amplification.)

IAG and the sole-source supplier Mark IV Industries have been testing post mid-life batteries to estimate their remaining life. Preliminary results suggest considerable variability, according to Michael Ascher, head of the TBTA in a paper at IBTTA/Madrid. Like people, some batteries konk out before others, and average lifespan is no assurance for an individual.

TBTA and the IAG have begun to wrestle with the issue of: When do you replace a tag? Ascher says that a “close to zero tolerance level” for battery failure must be adopted. That is because tags that fail become an operational nuisance.

“For some agencies, the gates will stay down causing traffic backups, while for other agencies, tag failures will quickly overburden existing VES systems that will lead to costly lookups for a transaction that was precipitated by an E-ZPass customer with a malfunctioning tag and not a toll violator and the possible loss of toll revenue.”

Software only?

Mark IV and the customer group are investigating whether remaining battery life can be estimated through system software additions. One idea being investigated is whether a slowdown in the data rate between the transponder and the reader or an increase in the number of “handshakes” between the tag and the reader reliably identify a tag with a weak battery. A problem in using the handshake number as a proxy for battery strength is the motorists who don’t mount the tag on the windshield but keep it in the glove box or on the passenger seat and belatedly wave it at the toll equipment.

Ascher says for the moment the only sure way to anticipate battery expiry is to replace tags based on the date of manufacture. Assuming that tag failures are normally distributed, Triborough B&T will begin replacing tags that are between 2 and 3 standard deviations of the mean useful life of the batteries, as determined by the final results of the test. This will ensure reliable customer service for about 99 percent of all customers but it also mean that they will be retiring many tags prematurely.

The mass of IAG tags have batteries inside a sealed case. Some agencies have tried tags with a replacable battery, notably Illinois and Florida, but Ascher says the results are “mixed.” He calls for next generation tags to have a more dependable battery or powersource, and also with a method for measuring remaining battery life, to reduce the cost of premature retirement.

He suggests either system software, or outside contacts on the transponder to hook up a meter to the tag. Mark IV says it is working on the problem but there is no easy solution. (Contacts Triborough Auth/MTA B&T 646 252 7417 Mark IV 905 624 3025)