That is not a very accurate method of peaking your batteries.
You can use a volt meter to watch for your batteries peak.
Here the thread where I talk about it.
http://tinyrc.com/forums/showthread....&threadid=7511
I get killer punch, and really long run times, and the batteries only seem to get more consistant.
My older Zip snaps right on the clone charger base that I charge my bits on, but my newer Zip Zap is lacking the notches on the side of the chassis to hold it on. So If you have a newer one you might have to rig something up.
If you don't want all the hassle, I've found that with a fresh set of batteries it takes 3-4 min. for the ZZ's 100mah cell to fill, so you could safely get away with 4-5 consecutive charges, and be filling your battery nearly to capacity.