[Kenwood] TS-820S 12VDC Operation

[Terry]

I have a TS-820S radio that has an internal 120V power supply.  In 
reading the manual and talking to the factory rep I can configure 
another power cord that can attach to a 12V battery.  
When I tryed to do this a few years ago, the radio would power up 
and receive but no transmit. Found out later there is another 
module (internal??) that is needed to boost the power or make it 
transmit.

Does anyone have any more knowledge about this and where to find 
one of these modules?

Any help would be much appreciated as I enjoy taking this radio 
camping on field day and would like to use quiet batteries to 
operate.

Thanks
73