Unless it's radically different from all the other G36C's that I've seen...you can't fit a large battery in the handguard. With the stock body/setup, you're stuck to 8.4v mini batteries...or with a little mod (you cut out part of the gas tube) you can get a 9.6v mini.
Other guys will do the following to get more power:
1. Change the front end to a G36K...still stuck with mini's but there's more room
2. Swap to a G&P G36 folding sniper stock (that will hold a large battery)...need to rewire the mechbox (easy to do)
3. Reroute wiring and use an external battery bag...either strapped to the buttstock or sling.
4. You could go with LiPo batteries.
Out of those options...I'd personally go with #4...but if LiPo batteries aren't your thing, #2 is probably your best bet.
Best of luck,