How do you take the battery cover off a BE M9A G36 Metal Gearbox Electric Airsoft AEG?

Question

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Answer 1

Remove the flash hider, then tap out the pin (from the skinny side, located where the rail meets the handguard) and remove the handguard over the barrel.  

Install the battery between the grooves, reinstall the handguard (may have to hold the battery & wiring in place), place the pin back in (may have to tap the pin slightly with a small hammer) and reinstall the flash hider.

cheers