We do a straight line fit 700=90%, 540 = 10% to find the remaining percentage P Lets say your scaling factor is 0=0V and 1023 = 2.096V you your reading N is 700 for 1.4V and 540 for 1.08V (I've chosen that to make the math numbers easy: 700-540 = 160) So the simplest and most realistic approach is to ignore the first and last 10% and use interpolation between the 10% and 90% points.Īt 10% the voltage is AROUND 1.4V (NOT 1.40000000000) (Also, a useful arduino circuit will generally draw a lot more than "like 1mA" as you said initially) (see also )ġ: the open-circuit voltage of a fresh cell is between 1.5 and 1.7VĢ: on load the voltage drops to about 1.4 when it is about 10% discharged.ģ: The voltage you measure will depend on the load current.Ĥ: when the battery has only about 10% charge left its voltage will start to drop much more sharply.Ħ: the graph is only representative of the discharge current used. Having done many battery discharge tests with my data logger I can state the following. But you dont know how the population was distributed. Secondly, you are taking readings from a graph that shows the behaviour of a population, and applying it to an individual case. Your voltage readings can not have more than 0.25% accuracy and may be a LOT worse depending on the reference voltage you use. 4 and 7 are given to single figure accuracy. Generally one is most interested in getting the approximation right near the "almost dead" end.Īs an aside, line 2 in your table probably shouldn't be there as the battery is not of any practical use long before it hits 0 volts and it will do bad things to any curve fit.įelic, you have made a common math error, called "extending the precision". This approximation gives 0% remaining at a shade under 1.1 Volts and claims 120% at 1.5 Volts, but it's probably close enough for most applications. Thusly, using, that is entries 3 and 11 in the table, one gets the formula y = 291.6x - 317.34 from the "equation finder" web page. Finally you want remaining charge as a function of voltage, so the X and Y columns in your data table need to be reversed. As a minimum one would use two points to get a linear fit as suggested in post #8. To make the equation less complex, one would use fewer data points for the curve fit. Unfortunately I have my doubts that the Arduino will be able to make use it this monster: I just found a website that can generate a mathematical function given a couple of coordinates. For instance, good Li-Ion battery have almost constant internal resistance, no matter of it's capacity and thus internal resistance value is used to determinate the battery is for usage or recycling bin.įor a phone battery this would be much more difficult though because of capacity degradation, temperature and load.įor a new AA alkaline battery under a constant load and temperature this should be much easier to calculate. It is another story with batteries have different chemistry. If measuring capacity of the battery outside the device, probably non of commercial testers use upper method, but measiring internal resistance of the alkaline battery. Since none of device use just one AA battery, upper method is way unprecise or complicated to implement inside a device which have several batteries in order to measure each battery separately. The difference is 0.4V and the formula is quite simple:Ġ.4V : 100 = d : p -> p = (d / 0.4) * 100 -> p = 250 * d Above (including) 1.5V is 100% full and under 1.1V is empty. To constantly monitor inside a device, it is quite simple, as the drop is a straight line and in such manner proportional in range of 1.5 to 1.1V from the graph for an alkaline AA battery. It will hardy drain it, as usually AA alkaline battery have capacity of between 17 mAh. For instance, with 5 Ohm resistor, current draw is about 300mA for a few seconds. With a low value power resistor, it will draw some significant current for that period of time depending on its value, however it will hardly discharge it. Measuring internal resistance does require few second for voltage to stabilize. Actually, is this a question regarding testing battery capacity externally and one time only or constantly inside some device? This question is crucial for decision which method to use.
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