Remote Car Alarm Won`t Work

Contact your local Porsche dealership and talk to them. My Mitsubishi had a similar problem. I rang them and they told me a "trick" to do with the key in the ignition and now all works fine.
Each brand has a different "trick" tho so you'll need specialist info me thinks.

I think its the battery but afraid to open car with key in case alarm goes off

If battery has no problem and you have recently changed it, then you need to check the wiring of the alarm system. If there is a short circuit or the wires are loose, it can also cause probelm with the alarm system.