There is no substitute for dual batts and dual alts on dual buses. That’s what I have been flying for 9 years and 1200 hours of mostly IFR cross country flights. It’s a belts and suspenders approach and it provides a very high level of confidence. (That’s a Z-14 in Aeroelectric parlance)
With that said, the sweet spot on the RV10 in my opinion is dual alternators and a single battery. That is certainly case for a VFR cruiser but I’d extend that to IFR cruisers.
Why is that the sweet spot? Alternators, their controllers and mechanicals, will fail. Having a second one, even a little one, will not just give you backup but will get you to your original destination and back if desired. Batteries don’t suddenly fail. They definitely go bad over time and can go bad ahead of time if abused and improperly managed, but otherwise they are pretty much bullet proof and easily managed for high reliability.
The great benefit of dual everything is that one can ‘experiment’ with battery abuse and poor management without effecting one’s dispatch reliability. In other words, I’ve screwed up managing 3-4 Odyssey batteries before I learned to just read the owners manual and follow its direction. I now have total confidence in Odyssey AGM batteries and would not hesitate to run with only one well managed and maintained battery.
Battery management and maintenance: Old style lead acid, AGM, and Lithium batteries are all different and what is good for one can be deadly to another. Pick your technology, your brand and then follow the manufacturers direction to the letter. For Odyssey AGM it means:
Keeping it fully charged when not in use which in my case meant not connecting it to an EFIS with a clock that required a few milliamps to run 24/7. That minor, slow discharge meant that one of my batteries was always being slowly drawn down when not flying. Even for a plane flown 3+ times a month, this would slowly kill the battery (which I would then swap with the good battery and ruin that one). It doesn’t mean it requires a trickle charger unless not used for an extended period (many months-year) as specified in the manual.
Make sure your charging system charges the battery at 14.1 to 14.7 volts - not 13.9 which is fine for old style batteries. I had one controller set at 13.9 and the other at 14.5 for a variety of dumb reasons. The 13.9 charged battery would last for 2 years, the 14.5 for at least 4 years. It took a long time to figure that one out since I was swapping the good and bad batteries for other dumb reasons.
If you have to use a charger, use a charger designed for the battery chemistry or better yet, a charger specified in the battery owners manual. In particular, old trickle chargers will kill an Odyssey AGM pretty quickly.
You don’t need a battery load tester or other exotic equipment to check the status of an AGM battery. A simple volt meter will do the trick. First, charge the battery on a proper charger, let it sit off charger for 24 hours, then check the voltage with no load. If memory serves me, 12.8 volts means 100% okay, 12.5 means 75% and it’s time to change it out.. There is a chart in the owners manual with various voltages and battery capacity. There is also data on temperature adjustments to the voltage. But that voltage check, specifically on AGM batteries, is a reliable check on battery condition. I’m getting 4 years plus out of my small (790?) Odyssey batteries and I can run them down to 75% . A single battery might be best swapped out at 80 to 90% capacity
There’s a good case to be made for minimizing or eliminating backup batteries for specific avionic units. To be reliable, they need to be properly maintained. At best, just another maintenance item. At worse a maintenance item easy to ignore until after it fails. I’ve eliminated all of my backup batteries and rely on my main batteries. I would not hesitate to do the same with a single battery.