A voltmeter isn’t the best gauge of your boat battery’s condition. (Pun intended.) But a voltmeter can provide information useful to savvy boaters. Here’s how to use yours.
Learning how to read a voltmeter is easy. Follow these simple tips:
1. You can use a voltmeter to measure the voltage at the battery terminals. This is called the, “open circuit voltage.” Make sure all accessories are switched off and only conduct the test after the battery has had 24 hours without any draw on it or charge put into it. If you do not wait 24 hours, surface charge on the plates will give you an erroneous reading.
A fully charged 12-volt battery shows an open circuit voltage of about 12.6 volts. The same battery is considered fully discharged when the voltage drops to 10.5 volts. A fifty-percent state of charge is around 12.2 volts.
2. You can indirectly get a bead on your battery’s condition by paying attention to the voltmeter when you are underway. With the engine running at cruise speed, the voltmeter is showing the charging voltage. This should be between about 13.2 and 13.4 volts for batteries in good shape near a full charge. If the meter reads any higher, the batteries are really drawn down, and may be “dead” or dying. If the voltmeter reads more than 14.5 volts, you may have a problem with your charging system.
Pay attention to the voltage displayed underway at your cruise speed with the minimal number of appliances energized. Write them down and get an average while your batteries and charging system are healthy. Doing so will provide you with a “target” to look for to ensure strong batteries and a robust charging system.
Remember: A difference in reading as small as one-half a volt can mean in a big difference in your boat battery's state of charge.