you need a "smart charger" The good RC batt. chargers will have a discharge feature as well as a timer and a trickle feature. Repeated charging without the batteries being fully discharged in not recommend for batteries of this type..... Also depending on your charger leaving them plugged in too long can damage them also if your charger does not switch to a trickle feature or auto shut off. Those batteries will eventually die and show the symptoms that you describe, but after only 20 uses that seems a bit premature. I suspect overcharging and not properly discharging caused that problem. Also beware, those cheap RC batteries found on ebay and what-not are exactly that Cheap, and will not last nearly as long as a good high quality battery. I might suggest going to your local hobby shop where they sell R/C stuff and talking with the guys there, they should be able to steer you toward a good charger and some good quality batteries. I used to be into the R/C world pretty heavy (mostly gas though) and I know the battery operated car guys say the battery is everything and the most expensive part of their setup. I will look at the charger I have here and get back to you with the brand and model, It has all the features listed above and charges/discharges in 15 min. and can be powered by AC power or a 12V plug from your car. Great for festy recharges. Maybe that get you in the right direction. I am sure there are others here that have had good success with diff. brands and such of batteries and could give you better advice as to what works well there as I have been out of the R/C game for a little while now. Hope this helps a little.
-Kiel