[sc489]

I have a keyless entry Mazda 3. I've had no problems but have just replaced the main and spare key fob batteries (CR2025) as a precaution after two years of use.

The new battery voltage open circuit is 3.2V and both old batteries were down to 2.9V. Does anyone know the minimum battery voltage for reliable key fob operation? The spare key has never been used used yet both batteries have depleted at similar rate. I expected my daily use battery to have a lower voltage due to its higher use. I wonder why both batteries depleted at a similar rate?

 

[skrubol]

It's probably that most of the voltage drop is due to age rather than use. The voltage of a lithium battery stays flat around 2.9v from about 95% charge down to around 30%. After around 2.7v they fall off pretty rapidly.

 

[Arakon]

Considering that for keyless entry to work, the amplifier has to be constantly powered, I'm not really surprised that both batteries are draining.

 

[sc489]

It's probably that most of the voltage drop is due to age rather than use. The voltage of a lithium battery stays flat around 2.9v from about 95% charge down to around 30%. After around 2.7v they fall off pretty rapidly.

That's useful to know. I found the following useful links showing the battery depletion plots. There could have been much more life left in the 2 year old batteries showing 2.9V but it's cheap to just replace them. The plots show that once the voltage drops to 2.8V, you had better change the battery soon.

http://data.energizer.com/pdfs/cr2025.pdf
http:/http://data.energizer.com/pdfs/lithiumcoin_appman.pdf

 

[MrBen]

Operation procedure for remote transmitter battery low warning

Place the remote transmitter inside the vehicle and close all of the doors.
Switch the ignition ON (engine off) and wait approx. 30 s .
With the brake pedal not depressed, switch the ignition off.
Verify that the KEY indicator light (green) in the instrument panel is not flashing.