DIY Battery Charger with protection need help

[Nick C.]

I was thinking about building this charger. I've seen that some around here use it.

http://www.afrotechmods.com/reallycheap/batteries/batts.htm

I was thinking that maybe a voltage limitation on the battery could easily be fitted.
What about using a 9v zeener diode connected to ground?
Would something like this work and protect the battery from overheating?
How would you implement it into this circuit?

Thanks I advance

Nick