both batteries liked show they are 2 cell=7.4v why go backwards to fm? when 2.4 now adays is less for entire new radio and rx that just the rx in a fm then you gotta make sure you have correct crystal
after adding up new bat charger radio or fm rx I'm thinking a entire nib rc car will be...
unplug main battery 1st
do you have a charger to charge that battery ?
if you do charge it
do you have a volt meter also called a vom .voltage ohm meter.
if you do id check esc to see if bec is working... you will need to set vom to dc on like 10 volt range. does it read voltage when turned...