Bad Alternator vs Bad Battery (Which is to Blame?)

We have all been in situations with our cars when we try to start up the engine, but the car won’t start. It’s easy to misdiagnose a power problem by thinking we have a bad battery when we really have a faulty alternator, or vice versa.

So, how can you tell if it’s a bad alternator or bad battery and how do you not make the mistake of replacing the wrong part? Let’s look at a few ways that you can tell which components is giving you this problem.

battery voltage

Signs of a Dead Battery

When you notice that you’re having power problems, the battery is the first thing you should check. Fortunately, almost all newer cars and trucks have computer diagnostic systems built into them which will let you know if you have low battery power.

But you shouldn’t always rely on them because they don’t always give you the right information. For starters, have someone give you a jump-start to see if it gets your car running. If it does, this means you have a bad or drained battery.

Depending on the battery’s age, it may simply need to be recharged (for instance if you accidentally left the headlights on) or you may need to purchase a new car battery (if the current battery is a few years old).

weak car battery

Other things that my indicate a dead car battery include: an engine that is slow to crank but eventually starts, electrical systems such as headlights, power windows, and windshield wipers don’t work as well as they should, and the car battery warning light illuminates in your dashboard.

You should also make sure there is no corrosion where the battery cables connect to the battery and that everything is tight. Either one of these would indicated a bad battery even though it’s simply a connection issue.

Related: Symptoms of a Bad Car Battery

Signs of a Bad Alternator

If you have a failing alternator, your car will show symptoms like dimming lights, strange sounds, and even strange smells while you are driving. When you jump-start the vehicle, it typically won’t start up in this circumstance. If it does start up, the car will shut off after about a minute of being turned on.

You see when an alternator goes bad, the battery’s power begins to compensate for the power that is not being generated by the alternator. But this power compensation won’t last long because batteries are not meant to supply power for long periods of time.

alternator not charging battery

So, when a battery is forced to do that, it will wear out quickly and you’ll literally have minutes before the battery is fully drained and needs recharging. But in some cases, the opposite happens when the alternator begins overcharging and electrical components may start to fail.

Before troubleshooting the alternator itself, make sure it’s not simply a blown fuse.

Related: Causes of an Alternator Not Charging


A lot of people find out the hard way that even though they purchased a new battery, they actually have a bad alternator. They’ll put the new battery in and the car will start up just fine. But then when they pull out of their driveway, the car will conk out by the time they make it around the block.

Similarly, the vehicle owner may assume it’s a bad alternator since their battery isn’t too old and then spend a lot of money replacing or repairing the alternator which didn’t need fixing in the first place. Even though car batteries come with warranties (ie: 48 months, 60 months, etc.), it doesn’t mean they will actually last that long.

The best thing to do is to take your vehicle to a mechanic where diagnostic tests can be done to confirm whether the battery or alternator (or another component) is at fault so the part can be replaced.

Mark Stevens


  1. With my car, a blinking battery indicator light is usually a sign that there is something going on with the battery or the alternator. Get it done right away for you do not want to get stuck out in nowhere land. Towing it back home or to a shop may cost you a hefty amount.

    Considering that your wires are not the culprits, you can purchase a multimeter tool to test the voltage of your battery and alternator. Turn the dial to 20, usually at the 11 o clock position. Stick it on the battery terminal and with engine off, check the voltage of the battery. Should be 13 around there if the battery has full charge and is good. Then you start your engine and you should get a reading of 14 something as your alternator is charging your battery while the engine is on. Now turn on your headlights and radio and AC and notice how the voltage is now losing little power. This means that everything is working fine.

    With my car, I’m getting 13 for battery and 13.5 when the alternator is charging my battery while the engine is running. IT is barely charging the battery so my alternator needs to be replace.

  2. The battery light keeps coming on. The battery is 6 months old. Last night I had 3 miles to home and everything started going out e.g. inside dashlights and headlights very low.
    Got home turned car off and it would not even turn over again.

    1. Sounds like you’re lucky you got it home before the battery died. It typically takes more juice to start the car than it does to keep it running. This is because the starter requires a large current, which the battery cannot deliver when the voltage has dropped too much.

      Have you had the alternator tested?

      1. I had my battery tested which come back fine by a battery drop tester but then he used a multimeter and it read 13.0 slowly went up to 13.4 he said the alternator is to blame is he right ?

        1. That sounds like a solid case for a bad alternator. I don’t know if he’s right. Inspect the grounds in the charging system (especially the wire that runs from the battery to the alternator) to make sure those are clean and tight. If they look good, it probably is the alternator.

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