Smart key battery drain due to cell phone interference?

Well, that's a good question. The answer is dependent upon the conditions of the test. I do not know what frequency the FOB uses to xmit RF. I do know however, that it is relatively low power. That being said, if, the FOB is placed in close proximity, and the frequency of the FOB is actually being picked up by the phone, and if that frequency is overloading the front end of the phone, the phone could think it has lost comm's with the nearest cell site. When that happens the cell will start transmitting, looking for an acknowledgement from the site. If it does not "hear" that acknowledgement it will keep polling until it does. The power consumed by the phone in Transmit mode is far greater than just standing by waiting for a call. That's why the phones are rated in Standby Battery life, vs. Talk Time! I find in the building where I work, and I am in the basement, my phone is constantly polling looking for a site and the battery just drains its little heart out. The phone actually gets warm, due to the output transistors pumping out all that RF energy!

That might explain the phone problem, but the FOB on the other hand, is not a bi-directional system, like a cell system is. It just puts out a constant little RF field, so there is no excessive current drain to contend with!!! Now, with that in mind, if your phone is constantly transmitting, it may be overloading the front end of the SKS receiver and that would explain why your having SKS problems!

RF is a trickey beast to play with. I have been working with RF Systems for the over 40 years now, and I am never amazed at what kinds of problems I encounter, and what some of the wackey fixes have been to get a RF System to operate correctly and meet FCC spec!!!

Hope this sheds some light on your problem! Good Luck!

Addendum .... This question got me wondering so I called Toyota.

The SKS specs can be found in TSB ELO15-06 Smart Key Specifications

The SKS FOB operates on 134.2 Mhz, with a power output of 92dB microvolts/meter at a measurement range of 3 meters. It also uses Amplitude Modulation, or AM to transmit its data to the car.
With that in mind, I would say with 99% certainly, that the SKS would not overload the cellphone causing it to constantly XMIT and kill its battery. Like I mentioned before, RF can be a trickey little bitch to work with so there may be a SLIM (1%)possibility that the SKS is contributing to the cell battery going dead prematurely. Vice Versa, I really do not think a cell phone operating in the 1.8 and above GHz range, and putting out approx 600 milliwatts that it is interfering with SKS. But there is always that 1% chance that it is!!!

Hope this does not confuse the issue further.

 

Your most welcome friend!!!

 

To add to KK6PD's information, I offer the following:
92dBuV equates to about 62.5 microwatts, or about the power dissipated by a 36,000 ohm resistor across a 1.5V battery. Or, if I didn't screw-up my calculations, about 3.9 nano-watts per square meter. The sun has a radiation power level of about 1000 watts per square meter at most points of the earth. So, the keyfob is radiating 256 billion times less radiation than the sun!

Now, regarding frequency, any transmitted signal spits out harmonics. Most transmitters have filters before the antenna to prevent these harmoncis from getting out and interfering with somthing. The fob transmits at 134.2MHz. Its 5th harmonic is 805.2MHz, 6th harminic is 939.4MHz, and 12th harmonic is 1744.6MHz and 13th harmonic is 1878.8MHz. Now, the power level in these harmoncs drops off drammaticly with each subsequent one. In fact, they drop off so fast that most designers don't worry much about harmonic radiation much past the 5th harmonic.

So, as you can see, I wouldn't stay up at night worrying about your fob causing your phone to go dead!

 

The short answer is yes. A strong cell phone signal can cause the SKS fob to respond. Responding uses more power than just sitting there, shortening the life of the battery. It can also interfere with the operation of the SKS.

The owners manual that came with your Prius recommends keeping cell phones and fobs separated. Putting them in opposite pockets works well.

Tom

 

The fob receives on 134.2 KHz, and transmits on 315 MHz. I assume that transmitting is a major drain on the battery, as an idle fob (receive always active) lasts at least 5 years. So it takes a 134.2 KHz signal close enough to a legitimate request in order to trigger the higher power 315 MHz response. Note that 134.2 KHz is the same as 0.1342 MHz.

Two possibilities. The cell phone could be producing electrical noise at the 134.2 KHz frequency. The other is that the cell phone is producing a normal signal at 850/1900 MHz, and the signal is so strong that it overpowers the fob's ability to select only the 134.2 KHz frequency. Or both.

 

A cell 'phone's battery drain sounds more like the 'phone is looking for a Blue Tooth connection. You might want to turn this feature "off" while not in use, to see if it makes a difference.

I doubt that the cell 'phone makes much difference to the key fob, as the fob does not "poll" anything for a response.

Marshal

 

It is fairly common for RF signals to swamp sensitive circuits and induce noise. Look at what GSM cell phones do to audio circuits, where they inject that annoying "DaDaDat, DaDaDat, DaDaDat" sound. Obviously the cell phone signal is even further removed from audio frequencies, but it still induces a signal. I think you put too much faith in the selectivity and shielding of the fob electronics.

Tom

 

Same with me, I have spent loooong hours on the open range and anechoic chamber to get products to pass FCC certification.

Standard text book practices do not always work and requires "Black Magic" fixes we called in our profession.

 

Sometimes you have to sacrifice small animals. Black chickens by the full moon work best for RF problems. We used to use Vestal Virgins, but they're too hard to find these days.

Tom