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The lithium battery is sealed in when manufactured and will draw only a few nano-amps (one nano-amp is one thousand millionth of an amp) in sleep mode. This has little or no affect on the batteries electrical life. Only when the transceiver mounted on the car sends a radio signal does it wake up the sensor and reads its low power transmission. Transmission time is around 0.85 miliiseconds and uses around 600 nano-amps. It has to be appreciated this type of electronics works on the nano-scale and is distinctly different than that of conventional electronics. It is estimated that a car will only be actually driven for 5% of its life therefore the demand on the battery is extremely low. The life of the sensor is governed by the typical shelf-life of a lithium battery which is currently 10 years which is probably less than the average car.
There are 2 development on the horizon. One where there are four (not the usual one) transceivers - one at each wheel thus reducing the transmission distance and energy requirements - and also the possibility for not requiring each wheel sensor to use a different wavelength.
Additionally they are developing mini internal moving flaps which act on piezzo current generators which in turn will recharge the batteries. The main difficulty in this development is the centrifugal forces (G-force) within the rotating wheel can actually inhibit movement.
It appears Renault engineers where hoping this development would improve safety as it is calculated that aound 6% of motorways acidents can be attributed to incorrect tyre pressure. One other aspect of TPMS systems that tends to get overlooked is that it can drastically reduce tyre wear and in fact over a 10 year period if you saved 2-3 sets of tyres it would probably pay for itself.
For those who want to know more and are looking a little bed time reading have a look through the link below.
Currently in madnoel10's garage:
Honda Civic 1.4l