How To Tell If Boat Alternator Is Bad? Bad Symptoms To Look For

The alternator plays a crucial role in your boat. However, alternators in your boat fail occasionally, and chances are low, but they eventually fail at the end. By knowing the symptoms of a bad boat alternator prior, you can extend their life or prevent a bad day in the waters.

Signs of a bad boat alternator are your battery keeps dying often, and the voltage drops below 13.5V while operating at around 1500+ RPMs. Even overheating and a bad smell from the alternator will indicate that there is something that needs to be fixed to prevent failure in the near future.

But generally, the alternator fails are rear. Mostly, the alternators can fail “partially,” where it just puts out a lower than normal voltage. If your alternator on the boat is dead, the most likely culprit can be the brushes. But now and again, the diodes will die, but it’s less likely.

This means (most of the time) you have to replace the diodes or brushes inside, and that just costs about less than $30 or so if you can do it yourself. Importantly look at the connections because fixing the connection between the battery and the alternator or others may be all you need to do after all.

However, putting excessive pressure on a smaller alternator will definitely cause damage, so the rule of thumb is to use one that accommodates your boat needs and don’t buy a smaller one due to less price. Without further talking, let’s go through the bad alternator symptoms in a detailed way.

What Are The Bad Boat Alternator Symptoms?

Boat Battery Often Dies (Discharges Deeply)

Wondering what the alternator has to do with the batter? Powering your boat’s electronics isn’t solely the job of the battery. The alternator provides the necessary electrical power whenever your boat’s engine is running. Plus, the alternator charges the battery at the same time.

The alternator takes the energy from the crankshaft and turns it into the electricity that is stored in the battery of your boat. Obviously, batteries sometimes fail on their own–but a bad alternator can actually cause the battery to drain because it’s failing to recharge the battery.

So If you find yourself jumpstarting your boat engine frequently or your battery dies (deeply discharges) often, be sure to have your alternator checked because it could be failing to recharge your battery. So if the alternator is starting to go bad, it cannot charge the battery as efficiently.

That is one of the main symptoms of the bad alternator in your boat. However, don’t replace it completely; as mentioned earlier, it could be a diode or brushes “failure” thing, and $30 can fix that problem. But you need to check the battery condition (load test it) because if the battery is bad, it will die.

Related post – How to tell if boat battery is bad? Check this article to know some of the core symptoms that a bad battery can show on your boat, along with some small things you can do to improve battery life dramatically.

Battery Voltage Drops Below 13.5V At Operating Temps

Checking the voltage will mostly be the confirmation test to check whether the alternator is working or not. You just have to check the voltage on the battery or the alternator. If you see a voltage drop, the alternator needs an inspection.

The procedure as follows

  1. Charge your battery fully (100%) and let it sit overnight unplugged, and then perform the test. Otherwise, it will give bad (inconclusive) results due to the charge fluctuations (low charge). Connect the battery back in the boat and then perform the test.
  2. Connect the multimeter’s negative wire (black wire) to the battery’s negative terminal and the multimeter’s positive wire (red wire) to the battery’s positive terminal. Set it in DC 20V. All the time, the multimeter needs to be connected to the battery, don’t remove it.
  3. First, you should calculate the voltage with the ignition on, not running, and no other loads. That gives us the baseline reading of the voltage. I assume you fully charged the battery; then, it should show around 12.5V. Let’s assume it is showing 12.6V.
  4. Raise the RPMs about 1500 or more, and you should see the voltage rise anywhere between 1.0V to 2.2V (even 2.5V) from the baseline, i.e., it should be around 13.5V to 14.7V (14.8V). If the voltage doesn’t rise, it implies the alt is bad, and if it rises close to 15V or more, that tells you that there is a problem with the voltage regulator.
  5. Let’s assume it showed 14.6V. Now you should turn on all the loads in your boat and then check the voltage. If the voltage drops by more than 0.5V, it implies the alternator needs to be inspected or replaced.
  6. Check the wire connections before replacing the alternator. Bad wire connections or corroded wires will decrease the voltage, so check the wires properly before replacing the alternator.

So, it’s good to carry a small multimeter in the boat. Check this Cernova digital multimeter (linked to Amazon) if you want a multimeter, and it does your job better than most of the premium ones.

Check this very helpful video on testing the alternator on your boat to get an illustrated view of the above procedure in a detailed way.

Overheating And Smells

Overheating need not necessarily be the symptom of a bad alternator, but if it is too hot to touch, you definitely need to take it near a mechanic. Excessive overheating of any device is a problem; all of them need to be inspected.

However, you might overlook the overheat unless you touch the alternator. But the series of belts that work with the alternator will give you a rubber-burning smell. If you smell anything like that, your alternator is overheating, and you need to inspect or replace it if needed.

The Key Takeaways From The Post

Your battery on the boat keeps dying often, and the voltage drops below 13.5V while running at 1500 RPMs or more are the symptoms of a bad boat alternator. The most likely culprit of a bad alternator can be the brushes. But now and again, the diodes will die, but it’s less likely.

This means (most of the time) you have to replace the diodes or brushes inside, and that just costs about less than $30 or so if you can do it yourself. But check the wire connections and battery condition before replacing the alternator directly if it bad.


My name is Mahidhar, and I am passionate about boating. Every day I learn some new things about boats and share them here on the site.

Recent Posts