Starting issue
 

My 04 RX8 with a nearly new engine works perfectly in the summer time (warm) but when temperatures drop it wont crank! - until I warm up the battery/ pcm area with a fan heater for 30mins. Battery fully charged and tested. Can anyone suggest what is going on here, please?

How was the battery tested? Load test? If not, have a load test.

If that passed, clean and snug down the battery terminals? What happens when you try to crank? Are you getting the relay click?

How cold is cold where you are?

Did you ever have the alternator tested? What thickness of oil are you using? What is the CCA rating of your battery? How old is it? Only other thing it could be is frozen water in the fuel lines but I cant see that being the issue since it starts once you warm it up with the heater.

How old is the battery? If it's more than 4 or 5 years old buy a new one.....

...what Loki says...Load test? If not, have a load test.

Take the battery to AutoStore... they load test for free (Autozone/Orielly's). I seem to go through more batteries than any other car... make sure the battery is back snug! You would be surprised at how a tight corner with a little dip moves things around!

side note... AGM's charge differently. Your not supposed to put high amps from a regular charger on it. They make special chargers if you go AGM or have an AGM battery.

Battery Tender makes a fine AGM and regular charger - I've been using one for about a year now for another car's AGM battery and my KLR650 with a battery relocation with the AGM in a sideways configuration (military Diesel KLR650 design).