RE: batteries...
I have, for a long time now, been considering building my own state-of-the art standard. I have been watching these threads with great interest, in order to learn from those who know much more than I do.
I currently have the Voltage Standard VREF10-003 and the Awesome14 version. Both are stable enough for my current needs, but I will repeat, I cannot verify if it's any better than 100 ppm. I only have two 5-1/2 digit meters here, so these standards were good enough for my current needs.
But I always thought, if I ever built my own really, really top-shelf volt-standard based on the knowledge shared in these threads, then I would definitely do it with batteries only (along with all the other best-practices the forum has been discussing in the many threads). I thought about how to have a SLA battery running the reference, and charging the battery too, without getting any line noise into the reference circuit. I don't think it can be done, so I thought I'd need 2 batteries, one running the reference and the other one being charged, then switch over with a hard switch or relay (make before break, so the reference never loses power). I wanted to have the unit battery powered, but constantly plugged in being charged. Maybe doing that will still introduce noise, due to the proximity of the AC charging circuit to the reference, even if it's not actually in the reference circuit at all. Maybe the AC magnetic fields will have some ppm effect.. I don't know this yet, but I hope I will build it up one days and find out.