Was this on first use after changing the battery ?  If so, the car goes through an ignition cycle before recognising the new battery as it remembers last state until that state changes. 

On 2/22/2021 at 5:06 PM, Eric Bloodaxe said:

Just picked up the car from MOT etc, and noticed the "Key Battery Low - replace soon" message came up. Service also mentioned this on their report. Changed the battery and message gone - and key still works - phew!😀

When I leave the car for service I always use my spare key rather than remove all the personal keys from the one I normally use. So this is only the third time I've used this one, the message has not come up yet on my "usual" key.

So just curious if key batteries go flat quicker when not used much?

I'm sure it's to do with the key sending a constant signal. My 2018 st line keys have both had new batteries! 

On 4/12/2021 at 2:14 PM, Edge of Reason said:

Was this on first use after changing the battery ?  If so, the car goes through an ignition cycle before recognising the new battery as it remembers last state until that state changes. 

It was, but after locking/unlocking a couple of times to see if the remote worked ok. It didn't happen when I changed the battery in the first fob, though, nor when I re-changed the battery in the second fob. Both those batteries worked straight away with no problems.

With the suspect battery the "key battery low" message just kept on coming up even after the car had been locked/unlocked and ignition on/off several times.

In future, though, I'll try and remember to test a new battery before I completely reassemble the fob, to save having to take the fob apart again if the new battery turns out to be a dud!

 

 

I got the " Key fob battery low" the other day. Changed the battery and no problems afterwards thankfully.

On 2/22/2021 at 7:42 PM, Jim H said:

I do the same keep them in a metal box. I changed the battery of my second key last week, just before the MOT. Easy to do, I didn't want to be charged £8 from the Stealer again. I am surprised how long they last. I don't think I ever changed them on previous cars. I wonder if it's to do with keyless start, not a proper key in a lock/switch.

Just out of curiosity, does a low key fob fail an MOT?

 

I have a late 2018 Ford Fiesta.  I am constantly getting this message but changed batteries in both keys at a reputable dealer two weeks ago.  It's just so annoying.  Spare key (which is hardly used)  is kept in a drawer and a lot of the time the other one is in my pocket or a drawer.

8 minutes ago, Margaret Brooks said:

I am constantly getting this message but changed batteries in both keys at a reputable dealer two weeks ago.

So how many times have you changed the batteries ?

Have you been fitting Panasonic or Energizer CR2032 batteries or just whatever was around at the time ?

Did you clean the new batteries with IPA or acetone before fitting ?