Is that spread acceptable?
I also use an Alinco. It supplies constant voltage as the current goes up from 1 amp or so when the radio is idling, to about 15 or so when transmitting. Measured at the PS, the voltage is constant. Measured at the radio, the IR drop takes about 1/2 a volt off the delivered voltage. I recently boosted the output voltage a little to take up this slack. The PS is capable of 32 amps, so it's running at it's mid-point.
Anyway, I shortened some of the power cable runs to my rig runner and from there to the 6700. That helped a tiny bit, but not much. I also have a RS-70M power supply, and can confirm that there is not voltage drop from it on transmit at the supply, the thing is a beast. I do see a voltage drop at the radio however, I was reading 13.11V idle and 12.23 during transmit using K9DUR's meters. I was reading 13.85 out at the posts on the back of the RS-70M with a Fluke multimeter.
A quick google search reveals a trim wheel on the circuit board inside the RS-70M. Most Astron supplies have this. I disconnected it from all power draws, and put the Fluke back on it. I adjusted the trim wheel to get 14.6V out at the posts with no load. Now I am 13.8V idle and 13.4V using K9DUR's meters.
So, depending on what all you have running off the power supply, you could turn up the voltage a bit and probably resolve your issue. This worked really well for me. At least for now, and of course YMMV.
Best of luck!
You do not want to exceed 15 VDC st the radio.
My first 6300 Flex was not happy with 10" of heavy cable and rig runner in middle. Would only do about 85 watts.
A 15A PS can maintain a 7Ah - 150Ah battery except in the heaviest draw (e.g. a 12 hour 100W RTTY contesting)
Only condition is the internal resistance of the battery
Really depends on the chemistry of the battery to understand voltage and current (power) available at the input terminals of the Flex .The typical lead acid battery under load will stabilize at 12.5 Vdc which will not support 13.8 Vdc at the Flex terminals. Output power of the Flex will be less than the rated output because of this. NiCad's and Lithium Ion are a different chemistry for charging and load voltage/current.
In the end the best bet is the largest gauge wire, good terminations and shortest lengths of wire from the power source to the Flex.