How to Test a Marine Battery

by Will Charpentier

One of the last things you do before the beginning of the boating season is make sure your boat's starting and deep-cycle batteries are fully charged. They were fully charged at the end of the previous season, before you sent your boat into its winter hibernation and the battery charger won't overcharge them, but how did they hold up over the winter? If you check your battery regularly -- every other week, whether you're boating or not -- you may learn something about your battery's condition.

Open the boat's battery box. Use a 5/16-inch box- or open-end wrench to remove the black ground cable from the battery's negative post.

Insert the red probe into the terminal on the face of the multimeter for volts, ohms and diode testing, marked with the letter "V" and the Greek letter omega, the symbol for ohms. Insert the black probe into the common terminal marked, "COM." Turn the selector dial on the face of the multimeter to the "V DC" function for measuring DC volts. (See Reference 1)

Press and hold the button in the center of the rotary dial of the multimeter and turn the multimeter on. Continue to hold the button in the center of the dial for two seconds, putting the meter into the "Automatic Touch Hold Mode."

Touch the red probe to the positive post of the battery and the black probe to the negative post of the battery. The meter will beep and 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 12.8 to 12.9 volts; a voltage reading of 10.5 volts or lower indicates a battery cell has shorted out.

Items you will need

  • 3/8- and 5/16-inch wrenches, box- or open-end
  • Multimeter


  • Your boat's battery has two different size posts: the positive post requires a 3/8-inch wrench; the negative post takes a 5/16-inch wrench.


  • Always disconnect the negative battery lead of your boat's battery first.


About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.