This is the VoltWatch2 on-board 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.
 

FEATURES: 7-LEDs provide instant, accurate battery voltage status of Low, Safe,
            or Max. Only one LED glows at a time
          Small switch to select 4.8V or 6V battery
          Very lightweight, only .15oz (4.3g) unit can mount on the side
           of the fuselage, or under the plane's canopy
          Draws amazingly low current - from 4-34mA depending on the battery
           voltage
          Plugs into any open receiver channel using a Futaba J connector
          One year warranty
 

INCLUDES: One VoltWatch2 On-Board Receiver Battery Monitor w/Futaba J plug
          One 1.5"x1" piece of double sided mounting tape
 

REQUIRES: Plugging into any unused receiver port
          Mounting onto the airplane with double-sided tape
 

SPECS:    Length: 1.65" (42mm)
          Width:   .63" (16mm)
          Height:  .16"  (4mm)
          Weight: .15oz (4.3 grams)
          Current Draw: approx 4-34mA
          Lead Length: 198mm (7.79")
 

COMMENTS: This has a Futaba J connector pre-installed. For radios other than
            Futaba, you will have to change or modify the plug.
          Because of reverse polarity, if you would like to use this with the
           older Airtronics batteries, you must also purchase AIR4400