Actually the Tower Hobbies checker mentioned above won't tell you if your batteries are failing. Only an actual capacity test can do that. I cycle my batteries with a capable charger through several charge/discharge cycles and average the discharge mah that the battery can supply. If that number isn't at least 75% of the battery's rated capacity, I trash can it. For more battery info than you really want to know, go here http://www.hangtimes.com/redsbatteryclinic.html