Got "smart key battery low" message so spent silly money on a new coin battery only to find it still reported "low" message. Same with both keys. Changed the battery again and still got "low" message.. Decided to do the old faithful - disconnect main car battery terminal (negative only) left for 5 mins then reconnected. You will have to apply hand brake and reset one shot return on power windows but the message returned but hey presto once the car started it cleared. Remember don't shut the boot after disconnecting the battery - it won't open again as it needs battery power. There is always some gremlin or other in modern cars I find. No doubt the dealer would charge £100 + vat for this 5 min job.

 

When you replace smart key batterys, it takes two or three start/stop cycles for the message to clear. You should not have to do anything with the vehicle battery.

 

Not my experience Redsky - wouldn't clear without a car battery disconnect for 5 mins.On previous changes (had the car a few years) new smart key battery fitting immediately stopped the pesky message, this time only a car battery disconnect did the trick. well worht a try at least!

 

Not my experience either. I usually change them for customers in the service lane and have never had to disconnect the main battery.

 

If your Smartkey battery has been replaced at least once, it's possible that the prongs have been spread a little bit and now aren't making good contact with the battery. Instead of bending those prongs and risk breaking them (which means that it'll cost you big bucks for a new Smartkey plus programming), try a CR2035 replacement battery. It's identical to the original CR2032, except for the thickness. They're both 3V and 20mm in diameter, while the CR2032 is 3.2mm thick and the CR2035 is 3.5mm thick. That additional 0.3mm of thicknesses might be all you need to keep it working reliably.

https://www.ausbatteries.com/cr2032-cr2035-difference

 

After getting the smart key low battery message I changed the fob battery. But as posted above the message was still there even after multiple starts and stops.

Since this often fixes other little problems. I used the fob giving the low battery error to lock the car and pressed lock again to get the horn beep. Walked the key away from the car so it's out of range. Then went back unlocked the car an using the same fob started the car without the smart key low battery message.
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I would never change the battery in something like a key fob without bending the contacts a little, and cleaning them with sandpaper. A little copper grease or dielectric (silicone) grease prevents further corrosion.

After I did this on the XJ, it took a few drive cycles before the error went away.

 

Just out of curiosity
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is there a reason why
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you always use
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three dots at the
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end of your posts?
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Yes
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I recentely got the „smart key battery low“ warning, too. I changed the battery and the message was gone at the next engine start. No further action required.