This morning I received an answer from Tim regarding yesterday's inquiry into wiring the LM-3 to a keep-alive or a battery, and my question about when he thought Ring's keep-alive would be available.
On running on battery:
"To connect a battery to a LM-3 it would be best to connect the negative
battery wire to the 'negative terminal' on the 6 pin connector and the
battery positive to the red wire on the 9 pin connector."
For me at least, since I have a steamer that I plan to run fully on battery power, and given Tim's answer, I think I'll stick with connecting the battery leads to the rail leads (red and black on the 9-pin) and not mess with the new negative lead. That way all I am doing is replacing the DC supply that normally comes through the rails with an onboard DC supply. I get that connecting it according to his suggestion probably eliminates one forward voltage drop (approx 0.65V) across the internal rectifier, but I wonder what else it might bypass (see next quote).
On using an existing DCC keep-alive:
"We do not recommend connecting other manufactures products to ours.
You could damage either our product or theirs and the worst case would be
that you cause a fire. However, at your own risk, you could connect a
DCC keep alive to a LM-3s since it has a negative terminal. If the DCC
keep alive can withstand track voltage you could connect the most
negative wire to the 'negative' terminal on the LM-3 and the positive
wire to the blue wire on the 9 pin connector. Connecting to the blue
wire will bypass some of the LM-3 internal circuit protection and can
cause more damage to the LM-3 if there is an internal short (not likely
- but not impossible either)."
This leads me to the conclusion that I will wait for Ring's keep-alive product. Using theirs, unforeseen problems will be covered by Ring. I can't afford to replace LM-3S modules because I ignored the risks.
And on the subject of Ring's keep-alive:
"We do plan to make a RailPro keep alive but it is not expected to be
available real soon. "
I am bummed that the keep-alive still doesn't have a target release date, and the release of said product seems to be getting pushed off further and further. Any hope I had of it coming out soon is pretty well dashed. But, they say patience is a virtue. Sometimes, I'm not terribly virtuous.