7.4v 1400mAh lipoly battery, ships from Hong Kong, took 14 days to arrive. $4.96 each shipped. Flat shape, should fit in M4 fore-ends or similar small spaces, normally for R/C cars. Mine even shipped with deans connectors - but subsequent inquiries from the seller, I was told that was an error and should not have deans - you decide. Still cheap for a lipo. Check out their eBay store for different batteries.