Alarm Siren Battery Failure – What’s Your Fix?

[franklin]

Hey folks, I’ve run into an issue with the factory alarm siren on my 67k mile 04 E46 M3. As many of you probably know, the siren has a self-contained battery, and once that battery fails, the siren stops functioning entirely.

Here’s what I’ve observed:

• No chirp on arm/disarm
• Panic mode lights up the tail and brake lights, but there’s zero sound from the siren
• OEM replacement sirens are no longer available from BMW

I’ve read that some people have cut open the siren, removed the dead batteries, and managed to restore functionality. I’m curious—what have you guys done to address this?

• Did you go the DIY route and rebuild the siren?
• Source an aftermarket solution?
• Just live without it?

Would love to hear your experiences or recommendations. Thanks in advance!

[Slideways]

Get a used one from eBay. Many BMWs, Range Rovers and other BMW group products used the same siren. Low cost and low effort

[heinzboehmer]

Sucks that they're NLA now.

You can open it up, but it requires a lot of cutting and you'll likely find that the battery has leaked and caused corrosion to the other components inside. Full replacement is the best option, IMO.

Here's my old one that failed:

https://nam3forum.com/forums/forum/main-forum/member-build-journals/13854-heinzboehmer-s-2002-topaz-6mt-coupe?p=228536#post228536

[LSB4Me]

Get a used one from eBay. Many BMWs, Range Rovers and other BMW group products used the same siren. Low cost and low effort

This. I installed a factory alarm/siren (and tilt sensor and cabin motion sensor—I regret the latter as my dog sets off the alarm). It was a few years ago, but all of these sensors cost about $100.

I would just buy a replacement siren on eBay and be done. Looks like they can be had sub $100.

[franklin]

All the eBay sirens are used as well. So I'd just be replacing junk with junk. Guess I will just try to cut mine open, get those worthless batteries out of the equation and hope the circuit board is not corrupted.

[sapote]

Here's my old one that failed:

How large is the capacity of these batteries and are they rechargeable? It's hard to believe these small looking batt could power the loud siren for a few minutes and working for so many years without charging.

[heinzboehmer]

How large is the capacity of these batteries and are they rechargeable? It's hard to believe these small looking batt could power the loud siren for a few minutes and working for so many years without charging.

I don't remember. But yes, they must be rechargeable.

[Shonky]

They can only be rechargeable. I'm surprised they last as long as they do. Service life that is. Then they're pretty small so couldn't run very long with the siren sounding.

[franklin]

Let’s be clear: The hack is simple—remove the rechargeable batteries from the siren. That’s it. Permanent fix.

Think about it. How many thieves are going to pop your trunk, peel back the velvet-lined carpet, pry out the plastic tray, and then cut your 4-gauge battery wire just to silence your alarm? Not happening.

The real threat is internal: once those rechargeable batteries fail, your siren goes dead. No chirp, no panic alert, no sound—just silence. Yank the batteries out of the equation, and your siren should last forever. No more relying on aging cells to keep your alert system alive.

[old///MFanatic]

Hey folks, I’ve run into an issue with the factory alarm siren on my 67k mile 04 E46 M3. As many of you probably know, the siren has a self-contained battery, and once that battery fails, the siren stops functioning entirely.

Wait, I thought the internal battery was for “backup power only”? Are you saying if the internal battery dies the whole siren dies regardless of being connected?
Are you sure it’s not only if the batteries corrode the inside electronics is causes it to fail. It should first work off the cars power source no?