Why is my electric trolling motor running out of power so fast?

When you take your boat out on the water for a day of fishing the last thing you want is a problem. When you do have a common question it would be nice to find a straightforward answer.

One of the most common questions is, why is my electric trolling motor running out of power so fast? Weak or worn-out batteries are the most common cause of trolling motor problems, including running out of power fast, autopilot failure, and failure to maintain spot lock.

There are a few other possible causes that we will talk about in today’s article.

Batteries:

If your batteries are having issues it can cut your fishing day short so here are a few tips for testing your batteries. First, you should always charge your batteries completely and let them rest at least one hour after charging but twenty-four hours is better.

Learn more about batteries with our article A Beginner’s Guide to Marine Batteries

Battery voltage test:

After your batteries are rested test each battery with a voltmeter, a cell in a standard battery should read above 2 volts each. Your battery should read 12.5 to 12.7 volts for Flooded Lead Acid or AGM (Gel should run about 12.3 to 12.5). If your battery is reading under 11 volts, then it is most likely a bad cell.

Battery electrolyte test:

Another test that you can perform on Flooded Lead Acid batteries is to check the specific gravity of the electrolyte in the battery cell with a hydrometer. You can buy a hydrometer very inexpensively.  There are safety warnings on all the packaging I have seen, but I going to mention gloves and eye protection before you start.

The hydrometer is like a turkey baster with a built-in gauge. Remove the caps on your battery cells and use the hydrometer to sample each cell one at a time. Draw the electrolyte into the hydrometer until it reaches the full line or as instructed on the packaging then read the gauge.

Most of the gauges have specific gravity numbers, but some gauges are as simple low, fair, and good or red, blue, and green. If you are in the green then usually that would indicate a good battery cell that as taken a full charge.

Battery load test:

Battery load tests are harder to perform at home because most home users don’t have the equipment to do a proper test. Typically, you would need to take your battery to a shop or auto parts store, and they can connect a battery test machine that will perform a load test.

I have a hack that you can try at home if you have a 12-volt headlight with a known amperage. You should be able to calculate how long your light should run with a simple formula by how many amp hours your battery is rated.

Connect your fully charged battery to your light with proper size wire, fuse, and a switch. Put on your voltmeter and switch on your light. Stop your test when the battery reaches 10.5 volts and use that time for 50% of the amp hour rating on your battery. This test is a rough method and can take time but will give you an idea of your battery performance. If your battery goes dead in fifteen minutes, you can conclude that you have a battery load issue.

If you have a bad battery check out our article The Best Marine Battery for Trolling Motor

Your prop shaft is fouled:

If your prop shaft is fouled, it can drain your batteries faster than average. Many types of fishing line will float to the top of the water and can quickly become tangled around the prop shaft. Fishing line can shut down your motor completely, which will have you searching for the problem until you find it and hopefully get back on the water.

If fishing line wraps up on the shaft behind the propeller and your motor keeps running, you might not even know you have a problem.  Fishing line can work its way up the shaft and damage the seals on your motor. Also, the fishing line can cause resistance in the motor, increasing the amount of current your motor draws. When your motor draws more current that decreases your time on the water, plus this can be bad for the wiring. Checking for hidden fishing line is always a good idea.

Water in the motor:

If you have fresh water in your motor, it can cause the motor to have a small amount of ground resistance. Simply put the water in the motor completes a circuit to ground that only draws a small amount of power. This small amount of power could only be a 10% increase in current draw, but in three to four hours would make a significant difference in battery power.

You can try to test for motor ground with the ohms setting on your multimeter. It can be challenging to detect a motor ground because it will ground to the lake. Try taking the motor head off while the boat is on the trailer and testing the power wire going to the motor for resistance.

Clip one probe to the power wire, and you will need to find (or create) some exposed metal on the motor housing to touch the other probe for an ohm reading. If the motor shows no resistance, that is a good sign, then spin the propeller. If again you show no resistance then your motor is not shorting to ground, and you can look for your problem somewhere else.

Over time any water inside your motor will cause it to rust, and then you will be left with junk. I am not a big fan of taking apart your motor because of how well the factory seals motors. If you are sure that a draw in your motor is the problem then before you buy a new lower unit, you might as well open it up and take a look.

If you find water, can see where it is coming from, and it is not rusted, then you need to dry it out. Put all the parts on a cookie sheet and place them in your oven on low for two to three hours. I was a boat racer in a past life, and when we flipped a boat with an electric start motor, we always dried out our starters in the oven because any water left inside the motor will rust sooner or later. Finally, you replace the bad seals and reassemble the motor, taking great care to make it watertight.

Note: your motor could have a short inside for other reasons, so if you don’t find water inside your motor carefully inspect all parts for warn or lose connection.

Wiring:

It is essential to check the system for a short to ground. To test for a short, disconnect your wires from the battery(ies), your motor and tape the ends of each circuit. Set your meter on ohms and take a reading for both system cables. If your meter changes at all, you have a short with some amount of resistance. To find the short go from junction point to junction point while testing for ohms then disconnect the last section and test again until the short is gone. Once it is gone, replace that defective part.

The last thing I would check is the boat wiring even though it is not the common cause of running your batteries dead. I perform my wiring tests on the water while running my motor. When I test the wiring, I am testing for two things heat and voltage, I also have an ammeter, so I test amps.

After running the motor at full speed for ten minutes, I lower the speed down to 70% and give my remote to my buddy. Starting from the battery, I take a volt reading, temperature, and amps. Battery voltage should still be around 12.3 volts per battery at this point of 24.6 on a 24-volt system. All of the wires, terminals, and connection points should be cool to the touch. Amp readings are the last test I perform. I am looking for no greater than 70% of the max draw. Since my motor’s max draw is 56 amps, 39.2 amps or less is good. (56 x .70 = 39.2)

I repeat this test at each junction point to the motor. If at any point I have a major change in my numbers or find a hot point in the system, then I know I have found a problem.

For replacement, tips check out our article Best gauge wire for a 24 volt trolling motor? This will also help for any voltage motor as long as you make sure you follow the maximum amp rating.

Tip:

If you want to get an idea of how many amps your motor is drawing and can’t get or use an ammeter, try buying a few fuses smaller than normal for your main power feed. My fuses are 60 amp. I would buy 50, 40, and 30-amp fuses so I could get a 10-amp range test. Switch your fuse to the next size down and run your motor on full power for five minutes. Repeat this process until you blow a fuse then you will have a good idea how many amps your motor is drawing.

Conclusion:

If your batteries are running out too fast on your electric trolling motor, there are three main things to check and then don’t try number four if you have not found the problem.

  1. Bad batteries
  2. Propeller shaft
  3. Water in your motor
  4. Short in your wiring

Check these things in this order, and you should cover all your bases.

If you would like to know how long your batteries should last click here to read my article on choosing the proper battery size.