OK only five flights in the morning and only about thirty minutes. Did you have any trouble starting the engine a time or two maybe, bet you included nothing of all the down time between those flights that the radio was on and control checks so forth. Now you go home and later in the day go back for three more flights with no charging? Lets see did I get that right and am I correct that your rx pack was a stock pack (typically 5/700 mah). Now when did you plug that Rx pack in to be recharged and was it a full night before?

This is no mystery, you dumped the Rx pack while you were in the air. That makes the airplane do funny things in the air and usually crashs. No gadget or widget such as that failsafe is gonna help in this situation.

Now as far as a second flight pack battery, my opinion bad idea for you. First you are gonna have more batterys to charge and more stuff to worry about properly charging and "thinking about" and remember Nimh and Nicd batteries loose their charge substancially even over just one day so don,t even think about depending on a battery that was not charged the 'Night Before' .

So yes the solution would appear on the surface to be to purchase higher capacity batteries but that not gonna help if you do not have a capable charger as noted by the fellows . But remember you have to also have a capable charger for the transmitter battery.

So maybe the best solution of all if there is two of you flying the same airplane then you need to each get your own period. Two folks sharing the same airplane especially if both are trying to teach themselves how to fly usually never works. That's a fact.

John