Monitoring Battery Capacity
Keeping accurate track of battery state of charge is essential for the high performance marine electrical system. Today’s expensive heavy duty batteries are too valuable to leave without proper monitoring. Traditional ways of assessing state of charge don’t always work with these batteries and more sophisticated measuring systems are now available at relatively low cost compared to the cost of the batteries they are looking after. It makes good economic sense to upgrade to a modern metering system.
Using a hydrometer to measure state of charge for wet cell batteries

Using a voltmeter to measure state of charge

Using an Amp Hour Meter to measure state of charge

Using an impedance based meter to measure state of charge
When this article was first written this option wasn't available, but now we have the Balmar Smart Gauge and assess its success at measuring battery capacity. It measures battery impedance and by referring to stored data uses the impedance to produce a reading of how full the battery is. It just displays battery voltage and percentage charged. It doesn't show amps or amp hours. Hookup is extremely easy, just two wires attach to the battery. That's it. Comments so far have been very positive, one professional review was very keen. In two cases I have installed these side by side with an amp hour meter and It has made for an interesting comparison. Because the two meters come up with their readings in completely different ways they are never going to come up with exactly the same answer, but the readings have been fairly consistent between the two meters. The bottom line is that the amount of charge in a battery is not fully defined until you actually go to use it and depends on temperature and the rate at which you draw the power. This is just a reminder that the whole science of measuring battery capacity is not an exact one.
Why is it important to be so accurate in monitoring batteries
The life of a battery is determined by the number of cycles it has to perform and the depth of the discharge. In general the optimum life to utility ratio will occur if the battery is not discharged lower than 50%. With an expensive battery bank it is worth spending some money on monitoring to prevent excessive discharge and premature battery failure. The only effective way to do this is with an amp hour meter. When charging batteries the top 20% of charge will normally go too slowly to be worth running an engine for. Being aware of what is going on will allow the operator to avoid running the engine for extended periods when it is not being effective, and will reduce unnecessary wear and tear on the engine. A further benefit of an amp hour meter is that it allows the boat owner to keep a check on charging equipment to monitor if it is working correctly, and also makes it possible to be aware of equipment that is using an excessive amount of power so that it can be turned off or reduced. Most amp hour meters will show a calculation of battery efficiency which can be used to give advanced warning of battery failure. Most also record historic data which can be very useful in troubleshooting. Finally, having an amp hour meter will allow the full use of the batteries for the comfort of the boats occupants, ending once and for all the mystery of whether there is enough power.
Conclusion
I hope I have made a satisfactory case for the use of some sort of advanced battery monitoring equipment on any battery installation. I believe that the modest expense incurred will repay itself several times over throughout the life of the installation.
Further reading
I have an entire section of my Blog devoted to Battery Monitoring.
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