Key Fobs & battery life

[mattkh]

It had to happen. Batteries in key fob died.

Any chance of a photo with the 2 batteries out of the way.

 

[AMGeed]

There's a mobile Car electrician (frequently on here) .
based in Carshalton who will supply a new fob for about £140.

Worked for me about a year ago, and no problems since.

If only I could remember his name...

JonnySBC ?
https://www.ecu-repair.co.uk/services


This company lists the R129 as a model they will supply a new key for.
Worth a call at least?

http://www.replacementmercedeskeys.co.uk/contact-us/

 

[RichardSEL]

Any chance of a photo with the 2 batteries out of the way.

Hi Matt -- thanks for coming back to me. I've had another look at this 'ere key fob internals' PCB. I'm pretty sure it's a 6V system with the two CR2025 cells placed one on top of the other with a sprung metal side clasp with a claw to make the top cell's positive connection. What was puzzling me was that only one of the two cells voltage was going down signifigantly over the discharge period of 15 days. But they're in series, how can this be?!!

Looking at the battery holder side there's a sprung clasp that's bent at right a right angle to form a claw to make the top cell's positive connection. But looking at that claw from the side and following it down to the PCB I can see that it bent out a little were someone in the past has just forced the two cells into the cells' holder without making sure the top claw was actually grasping the top of the top cell. So the claw's metal going down to the PCB is bent out a little. And when the fob's top cover is put back the lower of the two cells positive side can touch that bend out.

I'll try and draw a circuit (my ASCII art's not very good and this board's editor posts differently than its editor):

What it should be:
Clasp Claw Cell 1 (top) Cell 2 (lower)
_____
| V
| ------------------|.-----------------|.------|
|___________________ 6V_____________|

What I've got:
Clasp Claw Cell 1 (top) Cell 2 (lower)
_____
| V
| ------------------|.-----------------|.------|
|__________________ 6V________^_____|
bend out

This would mean there's an occasional short across the top cell between the "bend out" and the "clasp claw". Now, realise I'm probably clutching at straws here but if you try and bend back these cheeep sprung metals that are now 20 years old, they'll probably snap. So I put some PVC tape across the bend out to try to make sure Cell 2's positive doesn't touch.

Hope this all makes sense. Haven't posted a pic as to get the PVC tape in position and stay was a pain. To take it off and put on again would be even more...

 

[mattkh]

Hi Matt -- thanks for coming back to me. Haven't posted a pic as to get the PVC tape in position and stay was a pain. To take it off and put on again would be even more...

Well done for the closer inspection. Regards

 

[RichardSEL]

Well done for the closer inspection. Regards

No guarantee that I've found the fault. But if it's like this due to careless inserting of replacement cells, and I've seen other posts about fobs that discharge their batteries prematurely, hope it helps someone.
Always appreciate encouragement

 

[RichardSEL]

Well, that lasted a month before the FOB wouldn't transmit. The battery volts measured with a DVM were 2.73 and 1.37V for the lower cell. That still confuses me since the two cells are in series ;-) So opened the thing up yet again and repositioned the PVC tape so that there's no chance of the lower cell's edge (+ve) catching the bulge out of the claws's metal connector. And added another PCB tape piece across the PCB to make sure there's no connect that way

>>Any chance of a photo with the 2 batteries out of the way.

Here's a pic for you to see what I mean:

 

[mioba]

Love the determination OP.
You are gonna crack this.

 

[Altamar]

As for batteries not lasting long, I found Keyless Go fobs seem to drain batteries quickly...

My SL (with keyless go) suddenly refused to open/start the car so I now leave a spare battery in the sunglass holder thingy (which is too small for sunglasses so I had to find some use for it!)

I got into the car using the emergency key.

The batteries lasted way longer than a month though so I guess something else is causing the OP's to drain. Good luck with finding the solution.