My understanding is a car should never be driven without a battery
connected. It can damage the alternator which, I guess, needs the
resistive load. But I have never heard of a bad battery causing the
car to stall. I guess it could if an internal cell failed
and was shorting out. At idle, a car might be running off
the battery since the alternator may not be providing enough power
at idle speed. You can see that at night when a car's lights dim a
bit as it comes to a stop (mostly on much older cars).

It would also be dangerous to disconnect a battery once the engine was
running (and you need it to start the car). You can arc weld using
a car battery and you don't want your body to be the conduit that
completes a circuit.

Also, there are two sizes of battery listed for the 850/70 series.
the larger one has a bit more cranking power. It also weighs more
(weight is the only real downside other then it costs a bit more,
too).