Originally Posted by willwren
The battery has no effect on everyday driving.
Weeelllll... If the alternator is all but dead, the battery is what will sustain normal driving, for an impressively long time, until finally both the alternator and the battery poop out.
And I really have to take mild exception to the advice about always replacing the battery at the same time as the alternator: the battery can take a few deep cycles and bounce back (although if it goes to reverse polarity, it'* history), to the extent that it can recover for normal service and normal recharging from the alternator (i.e. it would exert the normal system load after it'* recharged).
I agree that when replacing the alternator, it'* definitely a good idea for the battery to get an external recharge at the same time, and what the heck, do a load test on it, too, but assuming it tests out good after that, then put it back in the car.