Backup Battery Recommendations?

I've just about completed installing an Alpine SEC-8048 alarm in my 1991 318is, and I would like to put a backup battery in with the system.  I've not been able to locate an Alpine BUB, not that I've looked very hard, as I suspect it would be a bit more pricey than I am willing to go.

I've seen the DEI-520T a few places, but it seems to cost a lot for a 0.7Ah lead-acid battery.  The battery will only power the alarm main unit and the siren, I've isolated the parking lights with a relay.  I'm looking for something that will provide more than 5 minutes of power to the alarm.  Any recommendations for a backup battery?  Is the 520T sufficient or should I go to a larger battery, say 7 or 8Ah?

I know I'll have to isolate the BUB from the main electrical system - how large a diode should I use for this?

Thanks much!

Try a 12V 7.0Ah UPS battery. They're sealed lead-acid and are about 6" x 4" x 2.5". They're available for approximately damn cheap (give or take) on Ebay.

Get a diode rated for double what you expect to draw from it. If you expect to draw much, use multiple diodes in parallel. The current capability will stack.