Fob problem

[alohart]

For the second time in 6 years of i3 ownership, the "Remote control battery is empty" message is being displayed. I figured, no problem, just replace the fob battery.

I replaced the batteries in both of our fobs previously at the same time, so I figured that both batteries might need replacing now as well. I placed both fobs in a small metal box whose metal top overlaps the body of the box when it's closed, so I assumed that this would work well as a Faraday cage. To my surprise, I was able to unlock the doors even though both fobs were in this tightly-closed box! Maybe the Comfort Access signal wavelength is so short that it could pass through tiny gaps that might exist between the top and body. So I placed my wife's fob in the box maybe 30' away while approaching the car with my fob. I unlocked the door and verified that my driver profile was active. I then placed my fob in the box and approached the car with my wife's fob. I unlocked the door and the same low fob battery charge message was displayed. I verified that my wife's driver profile was active. So both batteries appear to need replacing.

The voltages of the 2 fob batteries are ~2.95 V. The nominal voltage of a new 2032 button cell is 3 V, so I am surprised that 2.95 V is considered low. The new batteries measure just over 3.00 V. After installing the new batteries, the warning message is still being displayed. I don't recall having to do anything to clear the message.

Before buying another set of button cells, is there anything else that might be causing this message to be displayed? If you have measured the voltage of a fob battery that causes the low fob battery charge message to be displayed, what voltage did you measure?

 

[Fisher99]

Are you getting the same message with both fobs? If so, and given the voltages you found, it sure seems like something other than an actual low voltage battery.

 

[Neil]

I recently got the remote low battery message on my i3 Rex. I replaced the battery and it went away. However, I don't remember how long it was before I tried the remote again with the new battery. It could have been quite long, at least 12 hours or so. Perhaps it takes some time to register? Also, that type of battery has a shallow discharge curve, and it might have gone to a lower voltage than you measured when a higher current consumption operation took place, like a transmit which would require more energy than a receive. Perhaps you would have seen a lower voltage if you monitored the voltage while you approached the car or pressed a button. Let us know what happens.

 

[SwissDI3]

A new and unused FOB battery (CR2032) should show approx. 3.3 Volts when you test it with a voltmeter (outside the FOB - with nothing connected other than the voltmeter).

At 2.85V the battery is considered completely empty - that would explain your message that the battery is low (at 2.95V) !

My best guess: You bought some old batteries that already lost charge sitting on the shelf!

 

[alohart]

A new and unused FOB battery (CR2032) should show approx. 3.3 Volts when you test it with a voltmeter (outside the FOB - with nothing connected other than the voltmeter).

At 2.85V the battery is considered completely empty - that would explain your message that the battery is low (at 2.95V) !

My best guess: You bought some old batteries that already lost charge sitting on the shelf!

Thanks for that information! I should have used the Internet to learn more about these cells before posting. Open circuit voltage varies with brand, and as Neil stated, the voltage under load could drop considerably when the cell's charge level is low. I'll buy another set of a different quality brand which will likely solve the problem.

 

[alohart]

Problem fixed! Installed new button cells whose open circuit voltage is just under 3.3 V. Warning message gone. As SwissDI3 suggested, an open circuit voltage of 3.0 V might be insufficient even though that's the nominal new open circuit voltage of these button cells.

 

[Fisher99]

Good to know.