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I got me a new fancy battery charger that measures the internal resistance (if that's the right term) of the battery and I am trying to learn how this works.

My current understand is a higher internal resistance means the battery will still work but it might not be able to provide enough power if the device draws more power. The fire alarm might look fine for example, since it needs little power, but it might not sound in case of a fire since the battery's internal resistance prevents it from providing enough power.

I am testing some old and new rechargeable batteries and indeed, my new AA battery has a 27mR (new eneloop) and the old one shows 127mR (years old Energizer). Meaning the old on has a higher internal resistance. Correct?

Now, I suppose the old one is so old I should throw it out but how do I know where to draw the line? 30mR might be really good but is 90mR? What about 100mR?

In short: how do I know at what threshold a battery should be considered "dead"?

Update:

I looked at the datasheet of the Energizer as proposed in the comments. It gives a "Internal Resistance" of 100 milliohms for a charged battery and an "Impedance" of 35 milliohms for the same charged battery. Which one is the the number I need here?

If I take the 35 milliohms the battery would be good if it read between 28 and 42 milliohms (+ and - 20% as shown in the sheet). Do I get this right?

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    \$\begingroup\$ You should check the datasheet of what the nominal impedance should be, although it is only dead when it isn't fit for your purpose anymore. Besides, what unit is mR? \$\endgroup\$ – PlasmaHH May 4 '17 at 8:43
  • \$\begingroup\$ smoke detectors in particular have to verify there's enough available current to beep and perform a "reminder chirp" weeks before that limit is reached. \$\endgroup\$ – dandavis May 4 '17 at 8:54
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    \$\begingroup\$ mR=milli-ohms?? \$\endgroup\$ – Leon Heller May 4 '17 at 9:05
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    \$\begingroup\$ Internal resistance of the battery is important. A high internal resistance will keep you from drawing high current when needed. Consider a two way radio. With high internal resistance, it can run in stand by for a long time since the radio isn't drawing much current. Then, you hitbthe transmit button and the radio shuts off because the voltage dropped at high current because of the internal resistance of the battery. So, the internal resistance is a necessary indicator of battery health. NiMH batteries tend to die this way. \$\endgroup\$ – JRE May 4 '17 at 11:34
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    \$\begingroup\$ @MarkoBuršič Please explain? As the answer has explained, internal resistance is fundamental to whether a battery is suitable for a particular application, and internal resistance varies with State Of Charge (differently according to chemistry), and life-cycle/age, and temperature. \$\endgroup\$ – Techydude May 4 '17 at 14:36
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High internal resistance doesn't mean the battery is 'dead', just that it cannot maintain the voltage at high current that it could when new. The highest acceptable internal resistance is entirely dependent on the application.

Rather than throw old batteries away I reuse them in devices that draw less current. I am using 10 year old 'worn out' rc model heli Lipos in devices that draw less than 1A. Even after hundreds of cycles under extreme conditions (10-15 minute discharge time) they still have plenty of capacity at lower currents.

Old Nicad and NIMH batteries tend to suffer from capacity loss and high leakage as well as high resistance. When a battery has greatly reduced capacity and won't hold its charge or can't sustain low current drain then it is time to throw it away.

"Internal Resistance" and "impedance" refers to the DC and AC characteristics. Internal resistance is traditionally measured by applying a load and reading the voltage drop, whereas impedance is obtained by applying a small high frequency AC signal (eg. 1KHz sine wave) and measuring its attenuation. Impedance is generally lower because the battery acts like a capacitor at high frequencies.

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