How to Test a Boat Battery with a Multimeter

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Did you know that a car battery can be utilized on a bat, but the battery types utilized on boats are normally not car batteries? The battery load requirements of a boat are very much unique to that of a car. Its electrical system is, for the most part, a battery-run system.

The battery may or may not be charged by the boat motor. The bigger boats do have AC systems and parts, but for this guide, we will limit to a DC battery system on small boats. Remember that the type of battery used for marine applications is very different from the type used in trucks and cars. 

What Makes Boat Batteries Different from Car Batteries?

Yes, they are. The electrical system of a boat begins with a battery supplying electricity for the boat. The system is normally twelve-volt DC but can be six volts, twelve volts, or twenty-four volts, depending on how much and the type of batteries the system is created for. 

Keep in mind that the wiring system of a boat is a 2-wire one. One wire goes from the battery to the light or instrument to be utilized, and the second wire returns to the battery from the light or instrument to finish the circuit.

In a DC system, the electricity flows in one direction only. Further, the electricity flows from the battery to the light and returns to the battery. Every item utilized will have two wires, one to get power to it and one to revert the electricity. That’s a basic explanation of how a boat is typically wired.

What are the Batteries Used in a Boat? 

The batteries utilized on a boat are composed of three basic types:

  • wet cell battery to the type used in a car  
  • gel cell battery 
  • absorbed-glass mat or AGM battery

These three types are all rechargeable. How much electricity it can produce or its battery capacity is given by the voltage and amps specified on the battery. Further, the group size is the battery’s physical size, width, length, and height. 

That enables you to receive the correct size, which will fit in the space you have for the battery. Take note that the battery designation will be cranking or deep cycle.

A deep cycle battery will put out a stable current over a long period. On the other hand, a cranking battery could put out a great amount of current for a short period to crank a motor over to start it. However, it won’t last under constant use as a deep cycle can. 

Other batteries such as AGM batteries are often created as both and are considered to be dual-purpose batteries. Reserve Capacity, Cold Cranking Amps, and Marine Cranking Amps data are also often presented. Such numbers tell you they react under a load condition and allows you to compare batteries of the same size with one another. You see, the boat motor on the boat will identify what cranking amps are needed to begin the motor. 

Meanwhile, a wet cell battery normally has cells you can open and add water to them. Water evaporates from them when the battery heats up underuse. The cells should have the water acid liquid in them covering the cells; otherwise, the battery dies.

It’s worth mentioning as well that the acid doesn’t evaporate but only the water. Hence, the water should be added regularly. Such wet cell types should be level, or the acid water mixture will run out. The acid can ruin a lot of things when it leaks out. 

You can get sealed, leak-proof good cell batteries to prevent having to add water. AGM and gel cell batteries are always sealed and leak-proof as well. Moreover, an AGM battery can be stored in any position and its discharge rate—when it sits unused—is much better than the gel cell batteries and wet cell batteries.

The battery’s load will identify the type of boat battery needed. If it is used to start a motor and run a few electric items, a cranking battery will just work fine. However, what about if it will be used to power trolling motors or other electronics? Then making a continuous drain for a longer period of time will need a deep cycle battery. 

Remember that AGM dual-purpose batteries are becoming more common as they can deal with both starting and loads well. Another reason is that they’re sealed and might be stored and utilized in any position. That’s why AGM batteries are the top option in the marine industry these days. 

How Should You Test a Boat Battery?

You can test your boat’s battery using a digital multimeter. Here’s what you need to do:

  1. Open the battery box of the boat.
  1. Insert the red probe into the terminal of the multimeter’s face for volts, ohms, and diode testing. Attach the black probe into the COM terminal. Turn the selector dial to the V DC function for you to measure DC volts.
  1. Press and hold the button in the middle of the rotary dial and turn your multimeter on. Continue holding it for two seconds. It will put your digital multimeter into the “Automatic Touch Hold Mode.” 
  1. Attach the red probe to the positive terminal of the battery and the black probe to the negative terminal. Your DMM will create a sound and show the battery voltage on the screen. 

For a wet cell battery, you should have a 12.4V or higher readings. For a Gel Cell or AGM battery, your meter must show a 12.8V to 12.9V reading. A reading of 10.4V and below means the battery is shorted. 

Final Thoughts

A car battery can be utilized on a boat, but a battery designed for the marine industry can offer you a much better service. They are designed to match the load requirements of your bat, normally leak-proof and sealed, as well as a dual-purpose high-load deep cycle type. 

AGM batteries are the choice of the boaters. They offer greater power with low internal resistance than the traditional batteries, not to mention they have two times the life of standard batteries.

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