Testing Dual Boat Batteries: Multimeter Know-How

Testing the voltage of a boat battery is not a task that the average boat owner would undertake, but it's a useful skill to have. Using a multimeter, you can gain insights into the health of your battery, such as whether your ignition panel gauges are correct or how long you need to charge your battery until it's fully charged. Before testing, ensure you have exposed the battery terminals and that the battery has been sitting for at least an hour to get an accurate reading of the resting voltage. Now, let's go through the steps to test a dual boat battery with a multimeter.

Insert probes into multimeter

To test dual boat batteries with a multimeter, you will need to insert the probes into the multimeter correctly. The probes are the red and black test wires that come with your multimeter. It is important to attach these wires to the correct terminals on the multimeter. The black probe should be attached to the plug marked "COM", which stands for common terminal. The red probe should be attached to the plug marked with a symbol such as "V" or "Ω", which stands for volts and ohms, respectively.

On some multimeters, the red probe may be marked with a "+" symbol, indicating that it should be attached to the positive terminal. Additionally, the red probe may be plugged into the terminal marked with "V" and "Ω", as mentioned earlier. This terminal is typically marked with a "+" symbol as well.

It is important to ensure that the multimeter is set to the correct settings before testing. For measuring DC voltage, the dial on the multimeter should be set to "V DC" or the symbol for voltage (a dashed line and a solid line above the letter "V"). The dial should be set to "20" for measuring between 0-20 volts accurately.

Once the probes are inserted into the correct terminals and the multimeter is set to the appropriate settings, you can proceed to test the battery voltage by placing the probes on the respective battery terminals.

Open boat's battery box

Testing a dual boat battery with a multimeter is a valuable skill for boat owners to have. It can give you an insight into many things, such as whether the gauges on your ignition panel are correct or how long you need to charge your batteries until they are fully charged.

Now, to the battery box. A battery box or dedicated battery locker will ensure your batteries are secure and safe. A well-designed box will also help maintain crucial electrical connections. This is especially important on a boat, where constant movement can be detrimental to unsecured batteries.

When designing a battery box, you should consider the ease of removing and replacing the battery. Batteries are heavy, and handling them in a confined space could lead to physical injury. You may need to lift heavier batteries out sideways, so consider making the box with a removable side.

The box should be made from materials that can withstand the dangers of unsecured batteries. Acid spills, for example, are particularly undesirable on a wooden boat. The box should be sealed to prevent acid from spilling or escaping from a cracked or damaged battery case. All interior joints should be sealed with epoxy resin and a filler, and the external corners should be fibre-glassed. The box should then be lined with fibreglass.

While normal epoxy fibreglass should be sufficient, you can also use fluorocarbon resins, which are supposedly acid-resistant. Do not use cheaper polyester resins as the bonds will be weaker.

To hold the battery securely, you can fix two or three wooden slats to the inside surfaces of the box. Strips of neoprene glued to the slats will help dampen any vibration or knocks. However, ensure the battery is not held too tightly, as this will make removal difficult.

Finally, consider adding a lip at the bottom of the box to create a sealed tray to collect any spilled acid. The side panel will also need a gasket seal, such as neoprene, to contain any gases produced while charging. As hydrogen gas is produced, which is highly flammable, the battery box should vent to the outside. Provide a vent from the highest point of the box to the outside of the boat.

Attach probes to battery terminals

To attach the probes to the battery terminals, start by inserting the red probe into the terminal on the multimeter's face for volts, ohms, and diode testing. The terminal will be marked with a "V" and the Greek letter omega, which is the symbol for ohms. Next, attach the black probe to the common terminal, which is usually marked "COM".

Now, with the probes in place, it's time to make the connection to the battery. Ensure the battery terminals are exposed and be careful not to touch both terminals with anything metal, as this will cause a short circuit. Take the red probe and place it on the positive terminal of the battery. The positive terminal is typically marked in red and carries a "+" symbol. Then, take the black probe and place it on the negative terminal of the battery, which is usually marked in black and carries a "-" symbol.

With the probes attached correctly, your multimeter is now ready to test the battery voltage and provide valuable insights into its condition.

Check for correct voltage

Checking the voltage of your boat battery is a good way to test its health and charge level. A healthy marine battery should register between 12.6 to 12.7 volts. It is important to test the batteries after they have been sitting for at least an hour to get an accurate reading of the resting voltage.

To check the voltage of your boat battery with a multimeter, first, attach the test wires to the multimeter. Attach the black test wire to the plug marked "COM" and the red test wire to the plug marked with a "+" or "V" and "Ω". If your multimeter does not have an automatic function, you will need to select the setting marked "V" followed by two lines, with the bottom line broken. This stands for voltage (direct current). Then, choose the sub-setting "20" for 12v. The number reflects the maximum volt reading, so for 24v, choose the sub-setting "200".

Once you have selected the correct settings, you are ready to test the battery voltage. Place the red probe on the positive terminal of the battery, which is usually marked in red and with a "+" symbol. Place the black probe on the negative terminal of the battery, which is usually marked in black and with a "-" symbol. The multimeter will then display the battery voltage on the screen.

For a wet cell battery, the reading should be 12.4 volts or higher. For an AGM or Gel Cell battery, the multimeter should show a reading of 12.8 to 12.9 volts. A reading of 10.5 volts or lower indicates that a battery cell has shorted out. A fully charged battery should read 12.6 to 12.7 volts, and some AGM batteries can be 12.8 volts. If you have a reading of -12.6 or -12.7, you may have the probes the wrong way around.

As a guide, a battery reading of 11.8 volts or below is completely flat, 12.2 volts is 50% capacity, and 12.6-12.8 volts is fully charged. It is important to note that a boat or watercraft's starting battery should not start at 12.2 volts, as this is a dangerous level where the boat may suddenly not start. Below 12 volts is considered discharged, and deep discharge and sulphation will occur at lower voltage levels.

Compare with battery type

Testing a boat battery with a multimeter is a great way to ensure optimal performance and longevity. The process involves measuring the voltage and current of the battery to identify any faults.

Now, when it comes to boat batteries, there are three basic types: starter, deep cycle, and dual-purpose.

Starter Battery

Also called a cranking battery, this type of battery delivers a large amount of power in a short duration to start your boat's motor. However, it cannot keep the motor running, which is where the next type of battery comes in.

Deep Cycle Battery

Deep cycle batteries have thicker plates than starter batteries, allowing them to provide steady and consistent power over a long period. They can be charged and discharged multiple times and are ideal for powering items such as trolling motors, lights, GPS, and fish finders.

Dual-Purpose Battery

As the name suggests, this type of battery can function as both a starter and a deep cycle battery. However, they may not provide sufficient power to start certain engines or last as long as typical deep cycle batteries. An exception to this is lithium batteries, which can serve as true dual-purpose batteries.

Battery Chemistry

Marine batteries can also be categorised by their chemistry or composition. The four main types are lithium batteries, gel batteries, AGM (Absorbed Glass Mat) batteries, and wet cell batteries. All are lead-acid batteries, except for lithium batteries.

Lead-Acid Marine Batteries

Lead-acid batteries are one of the oldest types, typically consisting of lead plates and acid. They are bulky, heavy, and have a slow charging time. Additionally, they require regular maintenance and have a shorter lifespan compared to other types.

Ionic Lithium (LiFePO4) Marine Batteries

Ionic lithium batteries, on the other hand, offer a range of advantages. They are smaller, lighter, long-lasting, and faster charging. They are also safer, maintenance-free, and can be discharged past 50% without damage.

In summary, while lead-acid batteries may be more affordable upfront, lithium batteries offer superior performance and longevity, making them a more cost-effective choice in the long run.

Frequently asked questions