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

HydrometerWith wet cell batteries the most accurate determination of the state of charge is with a hydrometer. The hydrometer determines the amount of sulphuric acid in the electrolyte which translates to the state of charge of the battery. Because this concentration varies with temperature the measurement has to be carried out at the standard temperature of 80 degrees F or calculations have to be made to correct the reading to the actual temperature. A fully charged cell has a specific gravity of around 1.27 to 1.28 depending on the battery manufacturer. A discharged cell will have a specific gravity of about 1.1 To be fully accurate the battery has to have been at rest for a period of 24 hours before the reading is takes to ensure homogeneity of the electrolyte. The reading has to be taken for each cell in the battery. Cells with a deviation of 0.2 from each other indicate a failure of the battery as a whole. For sealed batteries it is not possible to do this procedure and anyway it is not one that can be done practically on a daily basis even for wet cell batteries. It is obvious that another method is needed for day to day operations. 

Using a voltmeter to measure state of charge 

VoltmeterA voltmeter can be used to get a quick snapshot of the state of charge of any battery type. A fully charged 12 volt battery will have an open circuit voltage of around 12.6 volts depending on the type, and a battery is considered fully discharged when the voltage drops to 10.5 volts at the 20 hour discharge rate. A 50% state of charge is around 12.2 volts. The problem with using a voltmeter to measure state of charge is that the battery has to be at rest for 24 hours to get an accurate reading. This precludes using the method for day to day operations. When the voltmeter is used without waiting 24 hours a false reading is produced by the surface charge on the plates. Although the voltmeter appears to give an indication of state of charge it really doesn’t do it accurately enough for most purposes. If it is the only option then consider an expanded range voltmeter as shown in this picture, or a digital one.

Using an Amp Hour Meter to measure state of charge 

Amp Hour MeterAn amp hour meter counts the rate and time of current flow from a battery being discharged and does the same when the battery is being charged back up. It needs to modify the count when charging to allow for battery inefficiency. Once it determines the full point, the capacity and the efficiency of the battery the amp hour meter is in a position to give an instantaneous readout of the state of charge in amp hours or percent of total at any time in the cycle. This is the only way to get accurate readings of charge from a battery in constant use. Quite a bit of computer power is needed to do the above calculations because battery capacity is a moveable target. The full point of the battery is not exactly defined, it is always possible to put more current in but once the battery is full this additional current will just be converted to heat and will not be available on discharge. The amount of current available to be drawn from the battery will depend on the temperature of the battery and the rate of discharge. Fuzzy logic is required to perform these calculations but amp hour meters rely on periodic and frequent resets which are automatically performed every time the battery reaches full capacity. The longer a battery remains without getting fully charged the more inaccurate the amp hour meter reading will be. The meter has to calculate the efficiency of the battery to be accurate and this is accomplished by comparing the charge to discharge total from one cycle to the next. The calculation has to be averaged over a number of cycles so the more the battery gets cycled the more accurate this reading will become. The picture shown here is of the Victron Energy BMV 712 Amp Hour Meter 

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. 


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. 


We have a lot of options here at PKYS