there are many batteries that can't be used as starting batteries. They have maybe enough CCA/CA but if the electrical reserve is not high enough you can damage the battery when you pull to much from it. Deepcycle AGM batteries shouldn't be discharged more then 50%. So if it's below 0 outside, you have a little bit weak CCA and you use the car a lot during the winter to do small trips to the store. Then you'll probably discharge it below 50% if you do not use a battery charger, this will shorten the lifespan of your battery. I have used a Kinetik HC800 as a starting battery on my 1.8l engine, it works ok, a bigger battery would be better. Especially if you have a big engine. you can read a little bit about battery types here: Deep Cycle Battery FAQ