Hi!
The two diode volt watcher is a commercial unit made here in Sweden and available in every hobby store over here...cost is around 10 dollars.
It works like this: When the green diode lights ...everyting is fine!...When both the red and the green diod lights ..watch out! ...When only the red diode lights...don't fly, Go home and charge!
A friend made a copy electrical circuit diagram for it a couple of yeras ago.
...I have not heard of anyone not using one of these in a plane for the last 10 years or so.

Regards!
Jan K
Sweden