You may be suffering from crappy batteries, crappy charger, or simply overworking your batteries (or all three). In my opinion LiPos below 2000mAh are rarely adequate for the velocity and trigger response demands that we're subjecting them to in this country, especially if you're attending games and not just plinking.
As SuperHog says, if you overdischarge these things (i.e. keep shooting once your battery is past the "voltage cliff") you'll perma-damage the battery. This is usually super obvious and easy to avoid even without a monitor, though if you're using a particularly weak LiPo you can sag into the dangerzone pretty easily. If you are actually running out of juice mid-game or sagging that badly, it's a strong
sign you should seek a battery with a lot more mAh.
If you suffer from low-grade batteries and/or a low-grade charger unit and want to reboot your setup, get your next battery and charger from Hobbyking. Also, spend some time finding the absolute biggest battery you can find that will fit in your G36. It may be tricky to do this at 11.1V, so you may want to stepping down to a 7.4V battery, which will give you more C and more mAh for the same amount of physical space.
If you miss the snappy trigger and high rate of fire, you can always throw in a high torque motor and speed gears to compensate.
: I should mention that it's unlikely your batteries are dying from overcharging as long as your charger knows how many cells your battery has, and I assume you're charging them with a balance plug, (see picture below). If you are charging using this plug, even a budget charger shouldn't cause any issues. Most low end LiPo chargers I have seen, if they do one thing well, it's knowing when to stop.