Citroen (2003), aux battery location

[SaleB]

My alarm system battery seems to be starting to act out (imagine that, after only 20 years). But, the problem is that I do not know where the battery is. It is a petrol RFN with a left-side steering wheel. Can you help?

Would it be possible for the unhealthy alarm battery to drain the main battery (assuming nothing else drains the main battery)? My alarm is not acting out, but since in the last two years I have not driven often, I get the dead main battery on the 7th day, and when I disconnect it the alarm sounds only for three rounds. Earlier, the alarm would go for at least 10 rounds when a dead battery would be disconnected and I could start it even after nine or ten days. The main battery is in working condition, but it gets drained down to below 8v which did not happen before.

[GiveMeABreak]

Several things to consider:

1) If your main car battery is over 5 years old, then likely due for a change.
2) The alarm (if fitted) will have a siren module with integrated battery. These will ultimately lose capacity to hold a charge and when that happens it will start causing the alarm trigger.

[SaleB]

1. No, it is a few months old, a well-chosen Exide 64Ah battery. It is the same story after a long drive or after charging it on an external charger. Since a friend of mine is the owner of one of the local shops that is also an Exide Warranty service center, we have tested it for factory malfunctions, and it is definitely still a good battery (does not self-discharge, tests well on the stress tests and electronic testers find it to be healthy).

2. Do you know where the siren is?
I hear it from somewhere below the rear bumper but haven't removed the wheel wells to see if it is somewhere there.

[GiveMeABreak]

If I remember, the siren module is in the rear left hand wing behind the trim.

[SaleB]

Thank you, I will search for it there first

[GiveMeABreak]

It's not meant to be easily accessible (i.e. behind the bumper) as that would defeat the purpose. On many later vehicles they siren module is behind and under the windscreen scuttle, so requires bonnet to be opened and trim to be removed to access it.