Okay. Ordered it. I wouldn’t even know how to interpret the reading, but I ordered one...
From Yuasa website on how to use voltmeter to determine battery state.
"Unlike Conventional batteries, AGM types cannot be tested using a hydrometer because they are sealed. Instead a voltmeter can be used to perform an open circuit voltage test. The test can be used for both Conventional and AGM batteries. The test is used to determine the following: battery state-of charge, ability to hold a charge and shorted or open battery cells. Before performing an open circuit voltage test the battery must be fully charged.
Charging a battery using the vehicle’s charging system or a battery charger creates a “surface” charge across the battery’s cells. The surface charge needs to be removed before an accurate test can be performed. To remove surface charge, turn on the ignition for about three minutes then turn it off. Let the battery sit for about 10 minutes before completing a test.
Open circuit Voltage indicates what percent of charge the battery has reached after charging. Open circuit voltage for a fully, 100% charged AGM battery is 12.8 to 13.0 Volts. AGM batteries that are 75% to 100% charged will measure 12.5 to 12.8 volts. Conventional batteries have slightly lower open circuit Voltages: 12.6 volts (12.8 volts with Sulfate Stop) for 100% charge and 12.4 for 75% charge.
If after charging the open circuit voltage indicates that the battery is less than 75% charged, the battery is probably no good and should be replaced. Before the battery is condemned, try charging it again. If the battery is still not close to 100% charged it needs to be replaced. The open circuit voltage test is not conclusive. It is possible to have a 100% charged battery as indicated by the open circuit voltage test that will not start a powersport vehicle reliably. In such cases a load tester is recommended."