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Replaced my key fob battery

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Fresh coins measure 3.3v or higher. I've purchased some at 3.0v, but that's ok since they last so long anyway. Check the package for mfr date might help.

Yesterday I *had* to move car but fob was not recognized (and I did not know specific spot to place battery-less fob). Plus I could not find my 6 pack of fresh coins. I did find amongst my battery refuse a 2.7v (resting voltage) coin that luckily got the car safely into the garage. :)

Where does one place fob, with battery removed, to operate car?
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center cup holder armrest
 
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Went to BAtteries plus, and they tested the battery, it was 3.0v
Installed and nothing, either the fob went bad, or it really needs to be reprogrammed.

For annual service last week gave Tesla my spare fob which is rarely used, didn't think to give both. They replaced the fob battery and gave me a new battery to install in the 2nd fob myself... I didn't, then a week later the car asked me to.

So I pop the old battery out and check the voltage only to find 2.97V. Meter says new battery is 3.25V. Have every reason to trust the Fluke 79 I have been using for decades. Anyway, 2.97V on a nominal 3.0V battery seems like the Tesla key fob is awfully sensitive. Not certain the battery I removed was replaced last year or year before.

Saved my old fob battery because I have a tire pressure gauge that will use them down to 2.5V. So there is another year or more in this one. Then again under load it might be 2.5V or less.

So if your battery is 3.0 and something isn't working, perhaps the battery needs to be replaced again. I have not had good luck with eBay coin cell batteries, seems to be a lot of knockoffs being sold as name brands. Best luck had with "sold by Amazon" Maxell or Energizer brands.
 
Measuring the battery voltage when the coin cell is not under load is meaningless. As the battery ages its internal resistance increases. The only way that gets included in the measurement is to have a load on the battery. The best load is the fob itself while it is transmitting a signal. Unfortunately that is very difficult to do. It can be done with another battery holder and clip leads to connect it to the fob. It's a lot easier to just install a new battery.