This is a really phenominal task if you think about it. There is no one kind of "battle system". There aren't even a couple of types, and within any types you can define, there are tonnes of variables/differences/etc in the way they work. This is why Hotspot said, "Coding". The only way to do it without using a completely generic battle system is to sit down, think about how you want it to work, and then work out how to code it yourself.
The other thing you have to keep in mind is that this is an adventure game engine, and we can't expect CJ to go to such an extent for people who want to try their hand at RPGs.
If you don't mind using a generic battle system then, your only option is to wait for someone else to release a template with their own, really.
Coding a battle system is frustrating (I'm doing one myself) but it's really worth it to see it working and know it's your own. For example, I'm trying to make mine taking all the individual components of other battle systems I've come across that I liked best and putting them all together. Hopefully, it'll work out well.