In reply to:
I’m not a battery expert, but I would be concerned that routinely taxiing/running up with alternators off might reduce the service of the battery as it is not designed for deep-discharge cycling. I think this type of battery, designed for starting, is happier when kept charged and can potentially be damaged by long duration high charge rates like you will get if you routinely discharge it substantially before turning on the alternators.
Bat #1 has a capacity of only 10 amp-hours at a 1-hour discharge rate…less at higher discharge rates. After a normal start I normally see a charge rate of nearly 30 amps for a while, and as I recall the normal load on the battery is in the 15-20 amp range.
With a battery in perfect condition, a normal load with alternators off might be expected to fully discharge the battery in 30-40 minutes, not factoring in the substantial drain from engine start. If your taxi and runup takes 10-15 minutes (?) you might be routinely discharging the battery by as much as 40-50%.
Just my 2 cents worth.
Beyond the battery life issue, I can think of another reason to not do this. After a start off Batt 1 and running avionics off batt 1 and 2 only (no alt) your going to leave the ground with something less than full batteries.
That is your reserve in case of alternator failure. So, in the interest of keeping the hobbs meter accurate, you will unwittingly launch without your max backup reserve in the batteries. Same is true after landing. Your going to store the plane with less than full batteries and when you need max battery output to turn the starter, you have something less than that.
My take is even if it didn’t harm the batteries (I tend to think its not good for them at a minimum), it is just not a prudent way to operate your system. BTW, think about the charge load on the entire system too. The alternators will try like crazy to quickly charge the batteries. That’s a lot more stress on everything to operate them at that load.