I have a FT7800 too. I use it with a 12V 7.2Ah lead acid battery and a Diamond NR-770R. The lead acid battery is a gel battery which means that it can be used in every orientation. It can take cycling to the point where the transceiver has to be shut off.
The antenna is a dual band antenna for 2m and 70cm and rated for 100W. It can be taken down to three parts (I usually make it two). With 5W PEP I had QSOs with reports of 57 to 59+ at a distance of up to 30km.
One important lesson was that itīs important to have a sufficient amount of cable to the antenna. A change of antenna placement sometimes yielded 1 or 2 points for the signal strength for both receiving and transmitting (bear in mind that yielding 1 point in signal strength means a factor of 4). Proper antenna placement may help you to save a lot of precious battery power and weight.
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If it isnīt broken, it doesnīt have enough features yet.