I did not know what battery is in the Barra 400E so I searched. It is a Lithium Ion Polymer. The nominal voltage is 3.7 volt / cell, fully charged = 4.2v/c, (Some RC batteries can go to 4.3v/c.), fully drained = 3.2v/c and storage charge = around 3.6, preferably in a cool place. I used to store it in a small fridge set at 5 deg. Celsius in my hobby room.
For other batteries: Lithium Ion (without polymer separator) Max charge is 4.1v/c. Storage also around 3.6v/c.
Lithium Iron Phosphate (LiFePO4) Nominal volt 3.2v/c, max charge 3.65v/c, min. charge 2.8v/c (Some higher, some lower) Storage, 3.2v/c.
Some older airsoft used AA rechargeable batteries I think. Store fully charged.