Someone weigh in on this but I was told that if a battery went Tango Uniform the alternators would not be able to produce power direct to the busses. That is why I went with the 2 battery system. (Also primary and back up alternators). Someone with more competent electron knowledge than me please help out. Somehow having 2 batteries with independent BMS systems provides some degree of comfort.
The answer is "it depends". And the only way to know for sure is to test it. And if the test goes wrong it is quite possible to let the smoke out of a lot of devices with zeroes and commas attached to them.
So, a "good" alternator and regulator setup will (should) be able to control voltage fairly well without a battery attached to the bus, IF the current draw required is fairly constant. When the current draw is very low, then any change in the current draw represents a fairly large percentage swing, and the results of the regulator trying to "catch up" with it can be quite dramatic. If the current draw is large, then any small changes are relatively minor and can be absorbed.
The only way to test this is to start your entire aircraft setup, then isolate the battery with the alternator still online. This is not possible if you are running the standard Cessna/Piper type split master bus switch, as turning off the battery also turns off the alternator field switch. But for those that have a different switch, or have two alternators, it may be possible. As mentioned above, if you "test" this and you are wrong, it's entirely possible that your alternator may put out several dozens of volts onto your bus for many milliseconds, long enough to cook sensitive and expensive electronics, as the regulator hunts up and down on the field current trying to match the voltage without a storage battery to damp the swings.
In my case, I performed the test quite by accident - I have dual alternators, the primary on a Cessna/Piper style split master, and the backup on a completely separate switch. Many times in cruise I will turn on the backup and kill the primary for a while, to make sure both are working as desired with no weakness. One day while in some turbulence at cruise, I did this and turned on my backup alternator, then reached for the "ALT" side of the split bus just as a good jolt of turbulence hit, and my finger hit the "BATT" side of the split master, shutting both of them off. Now my primary alternator was dead, and my battery was offline - but all my electron-driven items stayed happy. I was operating, quite accidentally, on my backup alternator without my battery online. Voltage was stable, current draw was stable, and it took me a few seconds to figure out what I had done before I fixed it. I immediately turned the master back on again and began to assess, and then breathe again. Total "test" time was about 20 seconds.
I have a non-standard fuel system with electric pumps, as well as a full glass panel - which means that in standard VFR cruise I draw about 21 amps of power, and in IFR mode at night it climbs to 31 with all lights on. My primary alternator is good for 60 amps and the backup is good for 40 - which means that at the time I performed my unintentional "test" I was pulling about 50 % load on the backup alternator (B&C vacuum pad). Apparently that was enough to give it some stability and the regulator was able to hold the voltage relatively stable. My electric fuel pump did not sag, my fuel pressure did not fall, my engine computer and EFIS's did not blink. I pulled a datafile from my Dynons and looked at the voltage trace, it varied from 13.4 to 14.7 at the extremes but settled at 13.9 within a few seconds and stayed there for the duration. It's important to note here that inductive or switching loads (like EFIS's or LED lights) are not going to help this situation - they will pull only the amount of power needed, and will draw fewer amps at a high voltage, causing instability in this situation. Stable non-inductive direct-drive resistance type loads, like standard filament lights or strobe power packs or electric fuel pumps, will draw more amps as the the voltage climbs and will damp the upward swing. I had both electric fuel pumps and filament-type nav lights active at the time which would help lend stability to the voltage swing.
I do not recommend anyone do this intentionally - I did not - but the results showed that a storage battery being online is not necessarily required, at least under some conditions, with some equipment.
YMMV. You heard this story on the internet from a guy you don't know. Don't do this.