I've a pair of NRF24L01+PA+LNA & Arduino Pro Mini(5V) soldered in a veroboard. I've used tmrh20 version of RF24 library.
In my project I use one device to send a small integer value to the other device. If the value is received by the other device, It flashes a led. I've been using this pair of devices for a month with 9 volt batteries and they've been working great.
But one day, I was testing these devices outdoors and it was hot outside. When I started the devices, it worked for first 1~2 seconds, after that Transmitter device stopped working.
After some moments, I realized my battery is hot and the nrf24l01 radio is even hotter. I turned off the circuit, let it cool, swapped the battery with the receiver device and tried again, but the same thing happened. After failure, I swapped the nrf24l01 radio with the receiver device. After attempting this, both radio started to become really hot and so are the batteries. I checked my AMS1117 3.3v voltage regulator it was fine.
I went back home and tested both the 9v batteries. They showed 7v reading and their current supply was gradually decreasing (0.9mA to 0.3mA).
I used two fresh radios with fresh batteries and everything started working again.
My question is, how can I know why the radio became so hot? What are the probable reasons a nrf24l01 radio can become hot and start to malfunction? Is it related to environment or a malfunctional battery?