The battery cannot reverse polarity like that. In order to see the 15.5 volts on the screen the battery had to either be plugged in backwards and turned on or shorted out while being on. If you have a battery cycler/charger you can hook the battery up it and test it to see if it's still good. On the transmitter, we'll have to take a look at it and see if it can be fixed by just replacing the burnt out IC's or if more serious damage has been done and the main pc board needs to be replaced.