The best way to know the condition of your radio and E.C.U. batteries is to cycle them on a charger such as the Schulze Chamaleon.
We use this unit (or it predecessor the Graupner McUltra Duo Plus II) exclusively for all of our charging/cycling needs. It is a smart charger that will give you fair warning of a weakening battery and the current used per flight. It is best not to charge through a switch harness but rather directly into the battery connector.
The simple overnight charger supplied with most radio systems yields little information about battery condition. All you really know is that the red light was still on in the morning.

Twin Schulze "Chamaleon" chargers are handy for quick turnarounds every 2 flights. A 26amp 12v gel cell powers both units allowing us to charge the airborne receiver pack and engine ECU Nicads simultaneously.
The charger box is available (in kit form), complete with aluminum handle and computer cut parts.
Various insignia's are from Pro-Mark.com.