Both units have the battery back up. The difference is one works with the remote pager the other doesn't. Like posted earlier you need the smart siren 2 to operate the remote.

 

I have the Siren II and pager. Dealer installed both. Siren works great, I don't mind the chirping when it arms. Pager has never worked (tried two). Can't get it to mate with bike. Dealer tried, too. Same result. Claim Siren II and antenna are installed properly. Dealer gave up and I guess I did, too. 08 RKC.

 

This doesn't exactly address your question, but if all you want is a siren, consider installing a Radio Shack piezo siren for $6. A tutorial is here.

I've had one for about six months and it works great. Be advised that it does not have a battery backup like the HD siren, nor does it mate with a pager.

 

Thanks for all the info. I am sure the pager and siren are my future!

 

very easy to install with the instructions and the pager is great-ride safe