This is the VoltWatch 2 Receiver Battery Monitor for 4.8v and 6v NiCd and NiMH receiver packs. This mounts on the outside of the airplane, so you can see at a glance the status of the receiver battery mounted inside the airplane.

Features: 7 LEDs the provide instant, accurate battery voltage status of low, safe, or max. A small switch to select 4.8v or 6v battery. Lightweight! The .15 oz unit can be mounted on the side of the fuselage, or under the plane's canopy. Draws very low current, from 4-34 mA depending on the battery condition. Plugs into any open receiver channel using a Futaba J connector.