I have a Kenwood TH-D72 and I just got an old Radio Shack HTA-20 2m amplifier to use with it. They're both powered from a sizeable 3S2P Li-Ion battery pack and they work great together for the most part, but I recently noticed something odd. I connected a meter to measure the current draw of the amp, and with it turned off it still seemed to be drawing about 100mA. After a bit of head scratching as the amp has a physical power switch and so shouldn't draw anything when off, I concluded that when they're both connected to the same battery the radio must be getting a much lower resistance connection to the power supply ground/negative through the coax braid and the 12 gauge wire I've run to the amp than through its own comparatively dinky power lead; when I unplug the radio from the shared external battery or disconnect the coax from the radio to the amp, the current draw at the powered-off amp goes away.
Using the coax as a power supply lead just doesn't seem like a good thing to me, but is this really a big deal? The amp starts acting up when the battery gets below about 10.7V in this configuration, which is what got me hooking up meters and testing things in the first place, but it does the same thing when the battery gets a bit lower even when the radio is running on its internal battery, so I think this amp just doesn't handle undervoltage well and the extra voltage drop to the amp when its cabling is also partly powering the radio (which itself pulls a couple of amps on TX) causes it to misbehave sooner.
So, do I need to stop the radio from using the coax braid as its primary negative power supply connection and if so, how? Some sort of balun/unun/other transformerish isolator?