R129 SL500 battery woes

[Roast Beef Racing]

The battery on my R129 SL500 keeps going flat.

The car is kept in storage over the winter on a C-TEK. I noticed the C-TEK had a red light signalling there was an issue with the battery and it wasn't charging. The battery can't be more than 4 years old but I replaced it as it was cooked.

Today, 2 weeks after changing the battery for a new one, it is flat again! It has been on charge but this time with a different C-TEK so it's not that. The car hasn't moved anywhere since I replaced the battery but I did start it to reset the windows when I replaced the battery.

Any ideas where to look for the issue?

[Discombobulate]

Roast Beef Racing said:

The battery on my R129 SL500 keeps going flat.

The car is kept in storage over the winter on a C-TEK. I noticed the C-TEK had a red light signalling there was an issue with the battery and it wasn't charging. The battery can't be more than 4 years old but I replaced it as it was cooked.

Today, 2 weeks after changing the battery for a new one, it is flat again! It has been on charge but this time with a different C-TEK so it's not that. The car hasn't moved anywhere since I replaced the battery but I did start it to reset the windows when I replaced the battery.

Any ideas where to look for the issue?

An easy place to start is the boot light. Put your phone on video in the boot and shut it. Wait 15 secs then open and check the light goes out on the footage.You wouldn't be the first...

Edited to add: but a CTEK should cope with that draw, so must be a bigger parasitic drain, so ignore me. Sorry.

[Gerradi]

I had similar ,turned out to be a loose mirror cover on the sunvisor making light come on intermittently.

[0a]

My r129 did exactly this - it turned out to be an aftermarket immobiliser.

In the end my mechanic gave it to his car electrician mate who diagnosed it in minutes.

[Jon39]

Just in case the situation might be similar to Aston Martins having failing batteries, if your car has a tracker, then that could be the culprit.
When tracker batteries start to fail due to old age, it drains the main car battery.

[pcn1]

If you have a current meter put it in series with the battery and just start pulling fuses one at a time until the current draw stops.

On my old Audi cabriolet, turned out the electric Ariel was faulty and constantly pulling power.

[Jon39]

pcn1 said:

On my old Audi cabriolet, turned out the electric Ariel was faulty and constantly pulling power.

Auto spelling correctors can be humorous sometimes.
My thought was that Ariel did not make an electric motorbike.

[Deefor62]

0a said:

My r129 did exactly this - it turned out to be an aftermarket immobiliser.

In the end my mechanic gave it to his car electrician mate who diagnosed it in minutes.

I had the same problem on mine. That would be the first thing I would check if one is fitted.

[Roast Beef Racing]

So...it's not the boot light, the vanity mirrors, ariel or immobiliser (doesn't have one). I used a jump pack to start the car and it seems to be holding charge. Very odd. Will get an auto electrician to take a look at it when I have time.

Thanks for all the suggestions. I remain bemused.