Thanks for the info guys. I will hopefully have a transmitter which will do a max 20 watt transmit. The third battery is a good idea. Now I just have to find a space to fit it.
As a side topic, has anyone seen these old military surplus reducers which are basically a long coil wrapped on a cermic insulator. The voltage is adjusted by moving the centre ring. This is what they use to use on the NATO spec trucks to run their 12 V tube radios. My neighbour had one he used on his 3B swapped tray-back to run his stereo and CB radio. They work fine as long as the load does not change. If you change the load then you have to adjust it again to get the correct voltage. Apparently they also get very hot.
