Switch off radio prior to start the engine
The problem is not the current of the battery, it is the voltage!
You are not only supposed to switch off the radio, you should switch off all avionics for the engine start of a small airplane!
While your battery is delivering the current to the starter, battery voltage drops and depending on the airplane,
the system voltage regulator is increasing the voltage which may lead to voltage spikes, which the aircraft electronic does not like!
You may be 100 times lucky, but one day it has killed your avionics.
Some airplanes have voltage surge limiters installed to protect the aircraft electronics during engine start.
Cheers
easaman