Take a sample of your Bio in a jar or water bottle and put it in the fridge and see where it gels at. I have noticed that each bath of Bio I make is a little different (maybe + or _ a few deg). Of course the source oil plays a big part in the gel temp too. PHO or HO will gel at a much higher temp. so it all depends on your Bio.
Right now I have a 20 oz bottle in the truck of b100 and it seems to be happy to about 45 deg, then it gets ugly. at the moment I am running d100 since I have no time to make bio at the moment
My bio stuff is also outside and its been in the 30s to 40s at night and 50 to 60 during the day. takes a LONG time to get hot!