Key Fob Battery Lasted 3 Years!

Thanks to this thread I went by Radio Shack and picked up two CR1632 batteries for $12.78. I replaced the one on my key fob and measured the voltage of the existing battery versus the new battery. Battery rating is 3 V.

Existing battery = 2.84 V (95% SOC)
New battery = 3.24 V (108% SOC)

How long would the existing battery have lasted before starting to fail? At +$6.00 I cannot think of a good reason to wait and find out?

I will change my wife's when she gets home tonight. It will be interesting to see the voltage on this one since it is seldom used.

 

What's the point of changing it before it's dead? If it's dead, you can still open the car with the key, and you can still start it by holding the fob up to the power button. The car will even give you directions on how to do that if it thinks you're trying to start the car with a dead fob.

So you can't get stuck anywhere just because your fob is dead, you just lose the convenience of the SKS. I plan on waiting until mine's actually dead, but whatever - to each his own...

 

Don't confuse the voltage with the SoC. The voltage will drop as the SoC drops, but it's not even a linear relationship. The battery will be dead probably even before it hits 2.0 V. It's close to an exponential curve; the first 10% of the SoC happens in a very small voltage range (say 0.05 V, from 3.24V down to 3.19V), while the last 10% might be much larger (like half a volt or so). More than half the charge is gone by the time it gets down to the rated voltage, probably.

As an example, Toyota publishes this data on the SoC versus Voltage for the 12V battery:
12.6-12.8V Fully charged
12.2-12.4V Half charged
11.5V-11.9V Discharged

As you can see, if you take the middle of each range, the battery drops only 0.4V in the first half of the SoC, but 0.6V on the last half.

 

Considering it is a 3V nominal cell I think it's safe to say it was pretty much "running on fumes" at 2.84V. I've had trouble with equipment using these lithium cells when I measured them at 3V! Replacing it fixed the problem. I'm pretty sure the FOB won't run at anywhere near 2V. Probably not even at 2.5V.

I believe the appropriate statement is "It's dead Jim!"

Would your wife know how to get in and start the car if -her- FOB battery died? Better instruct her!

Remove key from FOB and use it to get in the car.
Hold the FOB right next to the steering column with the brake pedal depressed and touch the "on/off" button.

 

Usually range on a wireless device with a questionable battery suffers just as much. So that small area of wireless lock/unlock near the doors shrinks appropriately, or is hit and miss. I'd much rather be able to count on it, so will probably switch out the batteries for new between 2 and 2.5 years as preventive maintenance.

 

We purchased our Prius in mid May 2009 and are approaching 80,000 miles
So far no problem with the key fob, works just like when the car was new.

I believe that the batteries are like $5 each and our Toyota dealership
install them for free..

 

Yes, while changing my wife's fob battery I took the opportunity to remind her again on how to use the mechanical key to get in the car and to hold the fob close to the power button to start. She appreciated that as she vaguely remembered and did not know that she would have known what to do.

 

I finally got around to checking the key fob battery on my wife's fob. It was 2.9V versus my 2.84. Hers is not used very often while mine is used every day.

I was really more concerned about hers than mine. I would rather hers not die on her leaving her to remember how to hold the fob up to the power button to start.

 

Mine is almost 3 years old. Thanks much for the info & link to buy.

 

I just replaced a key fob battery at 3 years. I got no warning before it failed. The service folks told me that carrying around a cellphone shortens key fob battery life.

 

That's a terrible price. If there's a Fry's near you, I see them for $2.99 at FRYS.com | RAYOVAC. A quick check on 1632 battery - Walmart.com shows a pack of 2 Rayovacs for $6.88 near me.

I believe there's a warning to that effect in the manual. Essentially, anything that gives off RF that might cause the circuitry inside to be more active/do more "work" than when not near a strong RF source might drain its battery quicker.

 

How far apart should you keep the cell phone and the fob?

 

p. 63 in the manual says...

 

That recommendation's a bit unrealistic in today's world. Still, mine lasted nearly three years.

 

Hmmmmm........ hadn't read all these posts when my Prius gave me the "low battery" warning. Called the dealer to see how serious it was. Like, do I need to replace it immediately. And can I do it myself. He strongly warned against doing it myself: The batteries only cost $7 and we'll do it for you. Do you realize the fob costs $?? and are not guaranteed if you might do something to damage it. So I've been waiting to take it to the dealer. Foolish me. Never listen to a dealer's repair advice....

 

I sold my 1996 BMW 318ti hatchback a few weeks ago, and the original fob batteries were still working after sixteen years. Of course, unlike the Prius, there's no drain on the fob battery unless you push a button, but that's still pretty impressive.