Yes, you will. Now you will make claims and the answer is
I tried it with a 7Ah gell that was well down. The 10A fuse
(in line with the gell) lasted maybe 50 milliseconds. It might
have lasted less if the engine was running and instead of
the standing 12.7V trucks battery. The battery was sitting
at 11.5V (at least 80% DOD if not more).
Nominally if the battery isn't too far down I'd agree. However
when the difference is more than 2V and the battery internal
resistances are low whats to limit current... a few hundred
milliohms of wire, unsafe.
The problem is once the target battery is just a bit charged,
like you say not enough potential difference to do anything,
and it will only charge to the local voltage which is maybe 80%
given enough time (with the engine running you have a chance
for more).
Parallel feeding a load was not in the discussion from where I was.
Only charging the target battery. Hence the suggestion of a low
power inverter and a smart charger. Efficient, not, but safe and will
work. If you driving home after an outing that makes workable
situation.
As to running a radio at high power.... I've run my Eagle at full bore
for a few minutes (5 minutes SSB to be exact). That's all I needed,
wanted or expected. It did eat most of the 90WH in the battery and
I could see it at 12.2V and dropping during RX (2A load). Good
idea, no, efficient no, I needed time to bridge to a different source.
It did that.
Generally the battery in my mind should be at least 10X the RX power
needs (for the Eagle that's 20AH roughly, for my 817 thats 4.5aH). That
only gets to a reasonable 1/10thC discharge rate for RX. For a little
Rockmite that is more like 12V worth of Alkaline AAA cells.
The problem is still the how much weight does one what to deal with,
to run XXX radio at YYY power for how long. For that we still need
to know:
RX/TX operating ratio.
Total time desired
Power needs of the radio for RX and TX(average).
What the usual question is that starts all this is reads like:
I want to run portable. What battery should I get to run my KX3
from a picnic table and also the IC7200 for field day?
For that only one answer, the biggest thing you can carry times two.
Gotta go with the worst case. The other no one even read.
Allison