Car remote battery warning

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 Arcturus 18 Mar 2023

I have a 12 year old 5 Series BMW. It is giving me a warning to replace the key fob battery. This is the second time in the last two months and strangely both mine and my wife's fobs have the same warning within a few days. The original battery from when I bought the car new lasted 12 years, this problem has only just happened in the last couple of months. The fob battery is a CR2450 lithium and is rated at 3 volts. I checked the battery which is in the fob with my multimeter and it is giving 3.03 volts the one in my wife's fob is showing 3.04 volts. I checked a new battery straight out of the packet and it gives 3.32 volts (notwithstanding it says 3v on the packaging). The fob itself functions perfectly. It opens and locks the doors without hesitation and it starts the car fine. I've looked on Google and various BMW forums but mostly they are full of the usual amateur rubbish and none of them offers a convincing proper explanation. 

Does anyone on here have any insight into what's going on? Why should it originally last 12 years without a peep and now it is warning of a battery shortfall within weeks of a fitting a new battery which is still showing full voltage on a volt meter?

 SouthernSteve 18 Mar 2023
In reply to Arcturus:

Just an idea. Did you put bitter coated batteries in. Lots of devices don’t like these. 

 Mlewis 18 Mar 2023
In reply to Arcturus:

Far from an expert on this... I think the BMW keys charge themselves when they are in the ignition. 

I would guess the car looks at the voltage drop over a period to assess the battery condition?

Was it a genuine battery or a cheap aftermarket battery? 

 kipper12 18 Mar 2023
In reply to Arcturus:

My merc keys only last around 6-9 months.  I now use the timsons lifetime fee thing.  Annul front fee then batteries replaced no questions asked.  I’d go,for that. 

OP Arcturus 19 Mar 2023
In reply to SouthernSteve:

Thats an interesting idea, I haven’t come across it before but it sounds credible. The multimeter probes had no problem but it would be just typical of BMW to have some super sensitive circuits . It’s told me a few times in the past I need to replace the main car battery and when tested it’s been perfectly acceptable.

I don’t know whether or not the fob battery is bittered but I’ll give it a wipe with solvent and see if that makes a difference. 
Thanks for the suggestion.

 SouthernSteve 19 Mar 2023
In reply to Arcturus:

I only know this might be a problem as you can't use them in the small tracker that I have attached to my keys as I am always misplacing them and it wasn't working well with the 'fancy' safe batteries. They were useless - hardly any connection so I suspect this is not your problem. 

In reply to Arcturus:

I have a very old Zafira, it doesn't have any fancy remote whatnot. The other day it started showing an error code. Googled it... Fob battery low. I think the ECU had that as a possibility for a fancier car. I cleared the error and it's happy again.

Maybe your warning is just a phantom error like mine was?

Post edited at 20:30
In reply to SouthernSteve:

Learnt something new 😁, I'd never heard of bitter coated batteries. Initially thought your first post was some kind of snide "get over it" remark 😂

In reply to Arcturus:

Did you get the low voltage warning first thing in the morning?  Did you leave the fob somewhere cold overnight?

 PhilWS 21 Mar 2023
In reply to Arcturus:

My 2008 E61 had a rechargeable keyfob. Maybe you are being shown an error as it can't charge the replacement battery rather than due to any actual issue especially as the fob works as expected...

 SouthernSteve 21 Mar 2023
In reply to Michael Hood:

That's definitely my posting style!

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