# At what internal resistance is a battery considered dead?

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?

• 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? Commented May 4, 2017 at 8:43
• 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. Commented May 4, 2017 at 8:54
• mR=milli-ohms?? Commented May 4, 2017 at 9:05
• 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.
– JRE
Commented May 4, 2017 at 11:34
• @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. Commented May 4, 2017 at 14:36