Checking A Boat Battery: Multimeter Know-How

Testing a boat battery with a multimeter is a great way to get an insight into the health of your battery and electrical system. A multimeter is a handheld tester that can measure voltage, amperage, and electrical continuity. It is an important tool for boat owners to keep in their onboard toolkit, as electrical faults can be hard to trace without one. By testing your boat battery with a multimeter, you can check the voltage and ensure that your battery is fully charged and in good condition. This is especially important before the start of the boating season, as batteries can lose charge over the winter.

Preparing the multimeter

To prepare the multimeter, start by attaching the test wires to the device. The black test wire should be attached to the plug marked "COM", while the red test wire should be attached to the plug with symbols, including something similar to an omega (Ω). Make sure the red test wire is not attached to a plug marked with an "A".

Next, set the multimeter to the correct settings. All testers can read volts, and some will do so automatically. If your multimeter does not have an automatic function, select the one marked "V" followed by two lines, with the bottom line broken. This stands for voltage (direct current). Then, choose the sub-setting that matches your boat's voltage. For a 12V system, select the sub-setting "20", and for a 24V system, select "200".

Once you have attached the probes and set the multimeter to the correct settings, you are ready to test the battery voltage.

Preparing the battery

To prepare your boat battery for testing, you'll need to locate the battery and ensure it is accessible. Many boat batteries are located in the engine bay, but some manufacturers may have placed them elsewhere. If you're unsure, consult the owner's manual. The battery may be encased in a plastic box, shroud, or cover, which you'll need to remove carefully.

Once the battery is exposed, be cautious not to touch both terminals with any metal object, as this will cause a short circuit. Before testing, it is recommended to let the battery rest for at least an hour, or even overnight, to obtain an accurate reading of the resting voltage. Ensure that the battery is not being charged or discharged during this time.

Now, you can set up your multimeter. First, ensure it is correctly set to measure DC voltage. This is indicated by a dashed line and a solid line above the letter 'V'. Set the dial to '20' to measure between 0-20 volts accurately. If the reading blinks when nothing is attached, you may need to replace the multimeter's internal battery.

Next, attach the test wires to the multimeter. Connect the black test wire to the 'COM' plug, and the red test wire to the plug marked with symbols, including something similar to the omega symbol (Ω).

With the multimeter and battery prepared, you are now ready to begin testing the battery.

Testing the battery voltage

Step 1: Prepare the Multimeter

• Attach the test wires to the multimeter. The black test wire attaches to the plug marked COM, and the red test wire attaches to the plug with symbols, including something similar to Ω (Omega).
• Select the function to read volts. Most testers can do this automatically. If your multimeter doesn't have this function, select the one marked V followed by two lines (the bottom line will be broken). This stands for voltage (direct current). Then, choose the sub-setting that matches your boat's voltage. For a 12v system, select the sub-setting 20.

Step 2: Prepare the Battery

• Before testing, remove any covers from the battery and let it stand for an hour.
• Turn off the isolator switches to isolate the battery.
• If your boat has multiple batteries, make sure the battery switches are set for the battery you are checking.
• It is recommended to discharge the battery slightly by running some accessories for 15-30 minutes before testing.

Step 3: Test the Battery Voltage

• Place the red probe on the + (positive) terminal of the battery and the black probe on the - (negative) terminal.
• The multimeter should now show a reading.

Step 4: Interpret the Results

• A fully charged marine battery should register between 12.6 to 12.8 volts.
• A battery reading of 12.4 volts is half-charged, and a reading of 12.2 volts is flat.
• A reading below 12 volts indicates that the battery is discharged, and deep discharge and sulphation will occur at lower voltages.
• For a wet cell battery, the reading should be 12.4V or higher. For a Gel Cell or AGM battery, the reading should be 12.8V to 12.9V. A reading of 10.4V and below means the battery is shorted.

Interpreting the results

The voltage reading on your multimeter will give you an indication of the health of your battery. A healthy marine battery should register between 12.6 to 12.7 volts. A fully charged battery will read 12.6 to 12.7 volts, and a battery reading of 12.4 volts is considered half-charged. A reading of 12.2 volts indicates a flat battery, and anything below 12 volts is considered discharged. If your battery is reading below 12.2 volts, it is likely that your boat will not start, and you risk being stranded on the water.

If you are testing a wet cell battery, a reading of 12.4V or higher is normal. For a Gel Cell or AGM battery, a healthy reading is between 12.8V and 12.9V. A reading of 10.4V or below indicates that the battery is shorted.

If you are testing the alternator or charging system, a healthy reading should be between 13.6 volts and 14.4 volts at idle tick-over. A reading below 13.6 volts indicates that your charging system is not supplying enough power, and a reading above 14.4 volts means that your batteries are being over-charged, which will reduce their lifespan.

If you are load testing your battery, a reading of over 11 volts is necessary to crank the boat. If the reading is below 11 volts, the battery will need to be charged.

It is important to test your boat battery regularly, especially if your boat is used infrequently or for short journeys. This will help you avoid being stranded with a dead battery and ensure the longevity of your battery.

Taking action based on results

Now that you have the readings from the multimeter, you can take action based on the results.

Fully Charged Battery

If your multimeter reading is between 12.6V and 12.7V, your marine battery is fully charged. This is the resting voltage of a healthy marine battery.

Half-Charged Battery

If your multimeter reading is around 12.4V, your battery is half-charged. While this voltage level is not ideal, it does not pose an immediate danger. However, it is recommended to consider recharging the battery to ensure optimal performance.

Flat Battery

A reading of approximately 12.2V indicates a flat battery. At this level, your boat may suddenly fail to start, which can be dangerous, especially if you are already out on the water. Take immediate action to recharge the battery.

Discharged Battery

A reading below 12V indicates a discharged battery. This is a critical level, and the battery will need to be recharged as soon as possible.

Deep Discharge

When the voltage drops below the discharged level, it enters a deep discharge state, and sulphation will occur. This can cause permanent damage to the battery, and you may need to replace it.

Overcharged Battery

If your multimeter reading is above 14.4V, your battery is being overcharged. This can be just as detrimental as an undercharged battery, as it can "cook" the battery, causing the electrolyte to boil away. Adjust your charging system to prevent further damage.

Undercharged Battery

A reading below 13.6V indicates that your charging system is not supplying enough power to keep your vessel's electrical systems running and your batteries charged. This can lead to flat or discharged batteries, and simply replacing the batteries will not solve the underlying issue.

Shorted Battery

A reading of 10.4V or below suggests that one of the battery cells has shorted out. This battery will need to be replaced, as it can no longer reliably hold a charge.

Preventative Measures

To avoid finding yourself in a situation with a dead battery, it is recommended to regularly check your battery's charge, especially before heading out on the water. Additionally, consider keeping a reserve battery on your boat at all times, and ensure it is also fully charged and maintained.

By understanding how to interpret the results from a multimeter, you can take appropriate action to maintain your boat's battery and ensure reliable performance.

Frequently asked questions