I've got a Spektrum DX8 transmitter that normally would run over 7 hours on its internal battery. A few weeks ago, that DX8 undervoltage battery alarm went off after only three 6 minute flights.
A quick test on my Western Mountain CBAII battery analyzer showed the battery only had 70% of original capacity. Worse, that battery dropped to 1.05 Volts per cell at the load value of the transmitter. A new battery fixed things.
But, much worse, today a fellow club member lost his giant scale 50 cc gasser one minute after take off. The resulting crash wrecked the 50 cc engine, most of the receiver and servos. The transmitter LCD display was flashing on and off, and absolutely no control of the model was present.
That was the members first flight of the year on that JR radio. The radio was also about a year and a half old.
Another quick check on the battery pack in that JR transmitter showed what happened. The battery voltage measured 10.8 volts at no load. But with a load test of 400 milliamperes, that voltage dropped below 8 Volts DC in less than 5 seconds. Every cell in that pack was
That battery died so fast, the transmitter low voltage alarm didn't have enough time to warn the pilot. Very strange.
So, what to do
Nice thing about transmitters, they present a constant current load to its internal battery. So, after a transmitter has sat idle for a long period of time, charge it up.
Then turn it on, and do something else for four or five hours. If your transmitter quits in 4 or 5 hours, time for a new transmitter battery.
IMHO, that's a good thing to do once or twice a year. If you have a 72 Mhz transmitter, be sure to do this test with the antenna fully extended.
If this thread saves one model from disaster, its been worth while.