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. 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. In two cases I have installed these side by side with an amp hour meter and It has made for an interesting comparison. Comments so far have been very positive, one professional review was very keen. Hookup is extremely easy, just two wires attach to the battery. It just displays battery voltage and percentage charged. It measures battery impedance and by referring to stored data uses the impedance to produce a reading of how full the battery is. 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. The picture shown here is of the Victron Energy BMV 712 Amp Hour Meter 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 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 longer a battery remains without getting fully charged the more inaccurate the amp hour meter reading will be. 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 amount of current available to be drawn from the battery will depend on the temperature of the battery and the rate of discharge. 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. Quite a bit of computer power is needed to do the above calculations because battery capacity is a moveable target. This is the only way to get accurate readings of charge from a battery in constant use. 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. It needs to modify the count when charging to allow for battery inefficiency. It is obvious that another method is needed for day to day operations.Īn 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. 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. Cells with a deviation of 0.2 from each other indicate a failure of the battery as a whole. The reading has to be taken for each cell in the battery. 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. A fully charged cell has a specific gravity of around 1.27 to 1.28 depending on the battery manufacturer. 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. The hydrometer determines the amount of sulphuric acid in the electrolyte which translates to the state of charge of the battery. With wet cell batteries the most accurate determination of the state of charge is with a hydrometer.
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