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It's actually both alternator and battery that's powering the car because they are connected in parallel. The higher-voltage device will contribute more power depending upon the balance. There's multiple conditions:
1. When starting, the battery's providing 100% of power to spin the engine. This drops its fully-charged 13-13.5v down to 12v or less depending upon condition of battery.
2. Right after starting, the battery is partially drained and the alternator's providing 100% of power used to run the car AND recharge the battery. Since the alternator's native output is 14-14.5v, current ONLY flows from alternator to the lower 11-12v battery.
3. After driving around for 10-15 minutes, the alternator charges the battery to 13.5-13.8v and current flow decreases. However, at no time does the alternator's output fall below battery voltage (unless regulator fails). At this point, the most that occurs is zero flow into or out of battery.
An amp-meter will confirm these states for you. Shows tonne of current flowing out when starting (-), then 10-20a flowing back into battery from alternator (+). This rate slows as battery is charge and reaches zero. It MAY pulse to use the battery (-) if you have sudden surges such as turning on lights & stereos, but quickly goes back to zero as the alternator catches up.
OP, what kind of battery problems are you having?
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