So its been a year since I purchased my Two Eco. Having read that the fob battery should be replaced in a timely manner, i.e. before it dies and leaves you stranded, I went ahead and changed them out today. Pretty simple so I won't go into the procedure, but the interesting bits are as follows: Primary fob was down to 3.08 volts after 1 year of daily use. Secondary fob was down to 3.07 volts after 1 year of never being used. Obviously this fob was never put into power saving mode (turning off SKS). Brand new lithium 2032 battery read 3.28 volts out of the package for both. So down, probably 0.2 volts from brand new. The interesting thing for the next year would be to note how much turning off SKS on the secondary fob would save the battery.
IIRC, I think SKS turns itself off after a while if not used and the fob is more than 6 feet from the car (or in a faraday pouch).
When I got Samantha, I hid away one of her FOBS, sans battery. Just last week, almost 2 yrs old, I took battery out of the one I had been using, hid it away instead and re-started the "spare" as my normal use FOB. The used battery was only marginally lower than the unused one - I didn't record the figures - but both were still well in the "GOOD" zone on my tester. I've done this with my last 3 cars too - when I sell them at 3-4 yrs, they've still had the original batteries.
a dead fob will not leave you stranded, just hold it up to the power button as you push it. i have a 10 year old cr2032 in a spare garage door opener that reads 3.29 volts. i was shocked. i don't replace the batteries until they die, so as not to hate batteries.
And the Gen 4 gives you plenty of warning on the MFD when the battery is low (apparently, as mine haven't run down yet).
I think you'd get a warning on the dash, well before the fob battery dies. I took a quick look in the 4th gen manual, didn't see anything stating that, but that's how it is on our 3rd gen. I actually just got that warning the other day, it's been probably close to 3 years. When I get the warning for one fob, I replace the batteries on both.
