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Can I measure the current of a battery by shorting the circuit and by ohms-law divide the voltage of the battery by the current measured to get internal resistance?

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  • \$\begingroup\$ You can't get the current without providing a resistance for voltage to drop over, at which point you may as well just get the voltage drop with a reasonable resistance and do that math instead. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 14 '18 at 10:53
  • \$\begingroup\$ I’m sorry if I’m so wrong as this is not my field of studies, but doesn’t voltage drop over the internal resistance as well? \$\endgroup\$ – AznBoyStride Jul 14 '18 at 10:56
  • \$\begingroup\$ Yes, proportional to the ratio created by the potential divider created by the two resistances. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 14 '18 at 10:57
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    \$\begingroup\$ If you're using a meter to measure current then the resistance is inside the meter, and is affecting your measurement. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 14 '18 at 11:31
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    \$\begingroup\$ it depends on the battery and the meter if you can measure the short circuit current, if the battery is an AA alkaline and the meter doesn't have a fuse on the 10A range, you might be lucky. also short circuit current may not be representative of the resistance of the battery at lower currents. \$\endgroup\$ – Jasen Jul 14 '18 at 11:43
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I would like to direct you here BU-902: How to Measure Internal Resistance

For a quick idea, measuring open circuit voltage and again with a suitable load (so that battery's internal resistance will be comparable in ration with this load) should give an idea of internal resistance.

With the load, the voltage read will be less than 1.5 V. Say, it is 1.3 V. Then the remaining 0.2 V is dropped somewhere other than the load. The drop will be across internal resistance of the battery.

You know the load resistance value(Rload), supply value (1.5V) and drop across load that is 1.3 V. Internal resistance can be calculated easily.

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Please take care of wattage of resistors being used and the allowed safe limit of current of the battery.

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  • \$\begingroup\$ ohmslawcalculator.com/voltage-divider-calculator may come handy.. You just need ohms law alone to solve \$\endgroup\$ – Umar Jul 14 '18 at 11:34
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    \$\begingroup\$ However, and this is important, the internal resistance is not ohmic, in the sense that it does not apply a single value across a broad range of voltages and currents. Internal resistance, as measured here, varies with current, state of charge and age of battery. Not to mention, of course, battery chemistry and construction. \$\endgroup\$ – WhatRoughBeast Jul 14 '18 at 12:42
  • \$\begingroup\$ Generally internal resistance will also increase with time if the load is heavy due to "polarization". This is because the electro-chemical reaction slows. \$\endgroup\$ – Spehro Pefhany Jul 14 '18 at 16:43

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