Author Topic: Spperflat alkaline cell.  (Read 435 times)

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Online hamster_nz

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Spperflat alkaline cell.
« on: November 27, 2017, 09:02:52 am »
I had a project outdoors running on 3 x AA batteries, and noticed that it hadn't checked in on the WiFi for a while. The batteries were flat (due to corrosion on the sensor at first guess). Drat - it was supposed to last all summer.  :(

But what was interesting was the three AA batteries. One metered 1.1V, the next 0.9V and the last was -1.0V (yes + was -1.0V relative to the - end).

I didn't realize you could discharge a 1.5V cell so low that it would hold -1.0V charge! After shorting for a few seconds it was still reading -0.375V, slowly getting more negative as the cell recovered.

Very odd - after about 30 minutes it is now down to -0.25V. I might leave it to the morning and see what happens....
Gaze not into the abyss, lest you become recognized as an abyss domain expert, and they expect you keep gazing into the damn thing.
 

Offline ogden

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Re: Spperflat alkaline cell.
« Reply #1 on: November 27, 2017, 09:17:46 am »
But what was interesting was the three AA batteries. One metered 1.1V, the next 0.9V and the last was -1.0V (yes + was -1.0V relative to the - end).

I didn't realize you could discharge a 1.5V cell so low that it would hold -1.0V charge!

It's called battery reversal. Nothing unusual.

https://en.wikipedia.org/wiki/Rechargeable_battery#Damage_from_cell_reversal

That's why you always shall use identical batteries in pack - fresh, branded, from same batch/package. For instance when I use NIMH in my photoflash, I sort them by capacity and ESR so there is no "weak link" in battery pack and they all discharge evenly.
 

Online hamster_nz

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Re: Spperflat alkaline cell.
« Reply #2 on: November 27, 2017, 09:28:11 am »
But what was interesting was the three AA batteries. One metered 1.1V, the next 0.9V and the last was -1.0V (yes + was -1.0V relative to the - end).

I didn't realize you could discharge a 1.5V cell so low that it would hold -1.0V charge!

It's called battery reversal. Nothing unusual.

https://en.wikipedia.org/wiki/Rechargeable_battery#Damage_from_cell_reversal

It seems it was unusual enough that somebody gave it a name, and a couple of paragraphs on a Wikipedia page.... ;)

I was just surprised at the magnitude of the Voltage, and that it was relatively stable until I shorted it out.
Gaze not into the abyss, lest you become recognized as an abyss domain expert, and they expect you keep gazing into the damn thing.
 

Offline ogden

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Re: Spperflat alkaline cell.
« Reply #3 on: November 27, 2017, 09:58:29 am »
I was just surprised at the magnitude of the Voltage, and that it was relatively stable until I shorted it out.

Voltage alone does not tell much about magnitude of the reversal. Stored energy, capacity (mAh) does. I bet that it was so tiny that it is not even worth electronic ink of internet we are using to talk about it :)
 


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