I have been a Icom user for years and most of the SCS TNC's have the ability to draw power from the Rig via the communication cord. This means that I have not had to run many power cables for the SCS modems while using Icom rigs.
Now that I have a Yaesu FT-897D radio for portable operation, I have had to rethink powering TNC's in a portable environment. Unfortunately the Yaesu doesn't offer the option of using the internal batteries for powering external TNC's except through the CAT port which is being used to power the dedicated tuner.
All of the TNC's that I own have power requirements in the 200-300 mA range at 10-20V except during firmware updates where power requirements can get up to 500 mA, but that is the exception rather than the rule.
I am trying to minimize the size and weight of batteries for my portable station, so the idea of lugging around a 40 AH wet cell doesn't sit well with me, and that much energy is a bit over the top for the project.
I have formed two solutions to this problem, each to be used depending on how much bulk I am willing to carry with me.
For maximum portability, I have wired two AA cells along with a 9V battery in series. This gives a nominal 12V and runs my SCS tracker easily due to the low current draw. While the 9V battery can't be expected to supply much current for long, it seems to provide enough power for low power use, and is a very light package. I did try this with the DR-7800 modem, but the Dragon's thirst was a bit much for this power supply since this modem needs 300-400 mA to operate and the loaded voltage out of the battery pack was 6 V, too low to run the modem.
To handle the increased load of the Pactor modems, I went to step two, a 7 AH wet cell housed in my Celestron Power Tank. This device is a portable light and power supply that has a small flashlight, a 800,000 candle power spot light, two 10V/10A cigarette lighter plugs, 3, 6 or 9V/1A coax plugs, and posts that come directly out of the battery for high loads. I ran 12G wire with rig lugs from the posts to supply a RigRunner 4005 which supplies up to 5 outlets using Anderson PowerPoles for connectors. This supply will have enough power to run the TNC's and my portable antenna coils for a prolonged time. This configuration also allows me to have some lighting, run my HT’s through the mobile cigarette power supplies.
Power to the HF rig is still supplied by the internal batteries, but the 7 AH battery has enough energy to supply the designed load for as long as the rig batteries will last in a package that is still reasonable to carry.