Monitoring Battery Voltage
If you are among the many sailors who will be headed south for the first time this season you may suddenly find that routines and systems that worked fine for short summer cruises may need some modification for extended voyages. Energy management is probably one the most difficult challenges that new long-term cruisers face, and to manage energy properly we need to accurately monitor battery state. Whether you use a sophisticated battery monitor or a simple hydrometer to check the state of your battery, knowing how and when to check your batteries voltage is important.
Battery voltage is about the closest you will come to having equivalent of a gauge for the charge level of your house bank. At-rest voltage is a pretty good indicator of charge level, but unfortunately the batteries on a boat in use are rarely at rest. If the battery is being discharged – say a refrigerator is running- it will register lower than true voltage. If it is being charged, the voltage reading will be huge. Even if neither is occurring at the moment, batteries “recover” for several hours after discharge. The voltage also rises for some time after charging stops. For an accurate assessment of charge level based on battery voltage, the batteries should have rested for 24 hours. In practice, you will get a fairly accurate reading of battery condition after a couple hours of rest. A good time to check battery voltage on a cruising boat is just before sunrise. The cycle time for electric refrigeration will be at its longest, there are likely to be few other loads, and any fixed solar panels will still be dormant.
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