The 
Airtech Studios BEU Battery Extension Unit AM-013-014-015 - Black
 is an airsoft accessory that provides extra space for larger batteries in specific Ares Amoeba models
 
Product Details & Reviews
Function: The unit offers approximately 60% additional space in the stock compartment, allowing for the use of larger LiPo batteries and eliminating the need to constantly charge or switch batteries during a game.
Compatibility: It is designed specifically for the Ares Amoeba AM-013, AM-014, and AM-015 PDW stocks, including the Nuprol branded models.
Construction: It is made of strong, flexible nylon that resists BB impacts and rough handling.
Installation: The unit attaches seamlessly to the original design with no modifications required for installation.
User Experience: Reviewers on Amazon UK praise it as a "perfect fit" that is "well made" and allows them to use a wider range of batteries. One user noted that it feels like a 3D print build, but it effectively serves its purpose.
Additional Benefits: The added space helps protect internal components like the MOSFET, fuse, and wires from stress, overheating, and damage. It also helps eliminate the rattling of the butt-stock by locking down the two metal rails.