Load testing the battery is fairly easy to do on your own with a digital voltmeter. Before you begin, make sure the battery is fully charged. Put the prongs of the voltmeter to the proper terminals on the battery, and try starting the boat. A good 12-volt battery can hold a voltage between 9.5 and 10.5 volts for at least 30 seconds. If the voltage starts to drop quickly and significantly, you have a problem with your battery.