Join Date: Aug 2006
Location: Arlington, VA
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There are advantages and disadvantages to each.
An analog meter with an expanded scale (like 10V to 16V) will tell you at a glance the approximate voltage of the battery being measured. Analog meters read instantly; you don't have to wait for them to "settle down". Also, analog meters don't create RFI problems for your radio.
You can easily wire in a switch to switch from one bank to another. If you have only two banks to measure, the simplest way is to buy a single-pole double-through (SPDT) toggle switch and run a positive wire from each bank to the end terminals on the switch, and the center terminal to the meter.
The downside of these meters is that they cannot give you the highly accurate measurement you need to really assess the state of your batteries. A change of only 0.4 volts is the difference between a fully charged (12.6 volt) battery and a battery which is 50% or more depleted (12.2 volts).
To measure voltage with some accuracy, you need a digital meter and, IMHO, one which has been calibrated against a known voltage. Once you have such a meter, you can tell with good accuracy the actual voltage of the battery bank and -- providing you remove all loads and all charging for 12 hours or more -- you can measure the voltage in a "resting state" and convert that directly to state of charge. The voltages cited above are "resting voltages", e.g., 12.6V = fully charged, 12.4V = 25% depleted, 12.2V = 50% depleted, etc.
The downside of many digital meters, including those from reputable manufacturers of digital meters for marine use, is that they often emit signals which can interfere with your AM radio reception and your SSB reception on HF frequencies as well. I have one which is very noisy, and have to turn it off when using the radio.
One alternative is to buy a good analog meter to install on the panel and also a good digital multimeter for more accurate measurements taken periodically. Every boat ought to have such a meter anyway!