Andykin

Hi there - my key fobs seem not to be working. I've changed with new key fob batteries but still no luck so I'm sorting the battery. I've looked up and most people highlight that 100ah is the correct one. My current one is 75ah 650a which I assume is not the best one. I can buy a 100ah 900 amp one from Halfords (yuasa). Do you know if this is the correct one? Would a poor battery account for the key fobs no longer working? (I can't get into the car as is currently locked and I don't want to manually open as I think this will trigger the alarm etc). I'm assuming that a new battery will sort this out :-)

whoover

The mechanical key won't trigger the alarm. Especially if your battery is so dead that the keyless entry won't work.

Chances are that it's something else. A dying battery usually has enough power left to do many things but not crank the starter. Totally dead would be unusual. I'd open the door and see if anything electric works. Then you can check the battery voltage with a meter. If some electric consumers work but keyless entry doesn't there's something wrong with it specifically.

bobterry99

This battery should be entirely adequate. It does store less energy than the original 100 amp-hour battery, but if the car is driven regularly it shouldn't matter.

Andykin

Thanks for your replies. I got into the back boot and am charging the battery to see what happen when I put it back. If I get access with a key, do you know if the car will likely start even if the keyless fob has previously closed the car? Is there an immobiliser which stops this?

Sorry for all the questions- I’m new to this! R129 is 1997 280 if that helps.

whoover

No, closing and opening with the key or keylessly are equivalent.

Andykin

Thanks for all you advice. I put the old battery back in. Got in with the key. The roof is now working fine after s reset and one key fob has come back into life! So it seems like the car needed a re-set and now all is well!!

Out if interest, is there any reason why the original vat battery was 100ah if a less powerful one can do the job?

whoover

Just over-engineering, taking very cold climates into consideration. Engines take more cranking when very cold and it's nice to have the overhead. Also, the 100Ah battery will have a longer life. That's because it will not get as discharged in normal use. Sulfation is what eventually destroys the battery and it happens when the battery is discharged. So Mercedes was probably designing for a 7-year life versus a 5-year life.