Spperflat alkaline cell.

[hamster_nz]

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....

[ogden]

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.

[hamster_nz]

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.

[ogden]

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