So I rebuilt the battery pack, and now that my new multimeter arrived today (£4 from eBay, can't go wrong) I reassembled everything to see what happened.
The old battery pack had a voltage across it of 1-1.5V. Not good. The new one had a voltage across it of 2.9V. Still not good but better, evidently these batteries don't come fully charged from the factory which is a shame but to be expected with NiMH cells.
I plugged the batteries into the siren circuit and screwed everything tightly back together. Checked the voltage across the main battery first before I did anything and it measured 10.7V. This isn't ideal, however given that it is an almost-new battery and has been sat around for several months doing nothing and I won't actually be cranking the car over until October it will do fine for my purposes and can be recharged at a later date.
When wiring the ammeter in series, the following observations were noted:
- Locking/unlocking the car creates about a 3.5A surge.
- This drops to 0.4A very quickly afterwards.
- About 1 minute later this drops to 0.08A.
From reading around, the ideal maximum for a parasitic draw on the battery is about 50mA, without the siren plugged in my car is registering about 80mA. Almost there, but not quite and yet not entirely out of the realms of acceptability.
I disconnected the battery again, and plugged the siren in. No siren as yet, all good. Upon reconnecting the battery however, the siren sounds on and off for 20s at a time much as it did before, the only difference this time is that when I disconnect the positive pole on the car battery, the siren doesn't stop, merely gets a little bit quieter (but still ear-splitting).
From this, I have drawn the following conclusions:
- The batteries in my siren were in fact dead.
- The siren needs more than 1.5V and less than 3V to operate constantly and be really annoying.
As I couldn't remember how to get the siren to re-sync to the immobiliser system I ripped it out and left it for now. On the plus side, the indicators now flash when the siren goes off, they didn't before with the old batteries
Hopefully when the car goes into the dealer to have the keys reprogrammed the alarm can be re-synced too. Oh, and all through this experiment the bonnet catch sensor was disconnected. Leaving it connected causes problems with the alarm when you try to do tests and whatnot...