My car battery for example is rated at 80AH and 275A for 10 seconds, my watch battery is probably rated at 100uA for 1000 hours.Before explaining how to use this calculator, first, we will give some definitions.
In case 2, the battery would be incapable of delivering anything like as much as 10A so even a theoretical calculation like "0.05 seconds" would be meaningless.Īccordingly, the test currents are set to be useful in terms of typical battery usage. In case 1, 1mA is insignificant to a battery that might be rated at 100AH and even differences in one battery to the next on the same production line would make more difference than the 1mA load. would it be sensible to measure a watch battery discharge at 10A? would it be sensible to measure a car battery discharge time at 1mA load?Ģ. The difference comes when you try to draw more current, the output voltage approximates to "(perfect battery voltage) - (internal resistance * current being drawn)" so as the internal resistance increases, the voltage proportionately decreases.Īgain, I stress the internal resistance is really a reluctance of the chemicals to react, it is not a physical component.Īs for why different currents are specified, let me quote two extremes,ġ. Measuring an old and discharged 9V battery, while drawing negligible current will also drop almost nothing across the internal resistance so it might still say 9V. So a new 9V battery would have a low resistance and measuring the voltage across the terminals, bearing in mind the meter draws negligible current, would be 9V. Again, according to Ohms law, the higher the resistance the more voltage per mA will be dropped across it. On a new battery the resistance appears to be low and as it ages or discharges the resistance appears to increase. We call this imaginary resistor the batteries "internal resistance" for obvious reasons.
So the model of a perfect battery with a resistance is series with it shows the same effect, more current = less voltage.
Likewise, the voltage across a battery drops when current is drawn from it. According to Ohms law, when a current passes through a resistance, a voltage is dropped across it. I stress this is NOT a physical component, there is no resistor inside the battery packaging, it is just a concept that produces a close approximation to how a real battery works. We IMAGINE the battery is perfect but it has a resistor in series with it. Trying to do the complex chemical math to work out how much current could be produced would be a near impossible task so we do it a much simpler way. They can bring the voltage up to normal (or near normal) but not keep it there under load. The real reason is that the chemicals within the battery have exhausted their ability to react quickly, but they can still react slowly. How much it can release depends on the rate the chemical reaction can take place.Īt risk of another million questions from you, I will try to explain how we MODEL the performance of a battery:Īs you have already noticed, the voltage from a discharged battery may still be high, what changed when it discharged was it's ability to maintain that voltage when current was drawn from it. It is a chemical reservoir which releases current on demand. Why are they testing a 1.5 volt battery current output at 4.0mA?ĤmA current drawn will give maximum time of discharge, so we can find out the battery life.Ī battery does not 'hold' or 'store' current. Why are they testing a 9 volt batterys current output at 25ma?Ģ5mA current drawn will give maximum time of discharge, so we can find out the battery life. No, 4mA is the safe current for maximum efficiency of the battery. No, 25mA is the safe current for maximum efficiency of the battery.ġ.5 volts is 4.0mA, is 4.0mA the max current out of a 1.5 volt battery? Other wise you can draw 25mA for 24 hours at rated voltageĩ volts is at 25 mA, is 25mA the max current out of a 9 volt battery? In the data sheet showing 600mAh, It means you can drawn 600mA current for one hour at 9V (Practically it is not possible, you can drawn 50% to 60% stored charge at maximum current).
Battery storage capacity is mentioned in mAh, it means how much current drawn from a battery within one hour at rated voltage.