I have a 2008 Toyota Corolla and for about a week now the battery's been draining fast, even though I leave the car plugged in. The voltmeter shows 12V when the engine is off, but when I start it it barely reaches 13V. Could it be the alternator or could it be something else?
The first thing you need to check is whether the alternator is actually charging - 13V is way too low, it should be between 13.5V and 14.5V with the engine running, so it's almost definitely the alternator that's the problem. Before you replace it, make sure the battery cables are tight and free of corrosion, because sometimes that makes it look like a dead alternator when it's not. If you clean the connectors and it's still showing the same thing, you can pretty much confirm that the alternator needs to be replaced, because it means it's not regulating the charge it's sending to the battery properly.
Have you already checked if there's any weird noise in the engine when it starts, like a squealing sound? Because besides what they told you about the volts (and they're totally right that 13V is way too low), there's another pretty common culprit that people forget about: the alternator belt can be worn out or loose, and even if the alternator itself is fine, it's not spinning enough to charge. If the belt is broken or really slack, the alternator doesn't work, and your battery drains even though it's technically connected.
Before you spend money on a new alternator, what you can do is visually check that belt (pop the hood, look at the front of the engine) and see if it has cracks, is frayed, or looks really dirty. If it's loose, you'll probably see it moving a ton when you touch it carefully. You can also have someone start the engine while you watch to see if the belt is spinning smoothly without slipping. The belt is way cheaper than replacing the whole alternator 🚗
If the belt looks normal, then it's probably the alternator itself. But at least that way you rule out something that's relatively easy to check first.
Before you blame the alternator, check the connections - sometimes the battery terminals or alternator terminals are loose or corroded, and that drops the charge more than a broken alternator would. If the terminals are clean and tight but it's still reading 13V, then yeah, it's probably the alternator, but it could also be a problem with the voltage regulator that runs with the alternator.
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