Battery Monitoring

When I originally wrote this article back in 2015 I discussed measuring state of charge of lead acid wet cell batteries with a hydrometer and then went on to explain why trying to measure battery state of charge with a voltmeter didn't work.  All that seems rather quaint now that its almost 2024 and everyone is switching over to Lithium batteries.  You can't measure state of charge of lithium batteries with a hydrometer or with a voltmeter.  The voltage of lithium batteries stays remarkably constant throughout their charge cycle. In fact that is one of their principal advantages.  Its only at the extremities of the charge cycle that lithium batteries show much change in voltage.  However the need to measure state of charge is still there, and it is even more important now that the battery itself gives no clue as to whether it is about to run out, sometimes with little or no warning.  If you have ever tried to drill a hole with a battery drill and had it quit half way through the hole you know the concept.  

Today’s expensive batteries are too valuable to leave without proper monitoring. Traditional ways of assessing state of charge don’t 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. 

An 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 

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.

I have an entire section of my Blog devoted to Battery Monitoring. 

We have a lot of options here at PKYS