RE: When is it time to change a Tx battery ?
I started to use lipo packs (5000mah 10cell) for power, and 480mah/790mah TP rx packs for receiver/servos, on my pattern airplanes this year. The frequency is use is pretty high (about 50 flights @ 7 minutes each per week).
I feel Lipo batteries are pretty safe to use. These batteries do require special attentions/precautions. I strictly stick to the following practices: balanced charge, pull them out of plane to charge, store in room temperature in a fire-proof case, never drain them to zero volts, never drop them, and charged to 50% during off seasons.
I do not let them sit in a car parked outside in hot weather for long time either.