Uneven discharge and polarity reversal in remote control batteries

[Luu]

Hi,

I'm just a hobbyist and just witnessed something I'm having a hard time explaining but figured somebody with EE experience might be able to help me.

The remote of my Amazon FireTV gen 2. stopped working the other day (after only occasional use since January, i.e. three months) and I figured I'd check the battery status before trashing them.

I removed the two AAA batteries from the remote and measured their voltage. The one was at 1.17V which seems normal the other one however was at -0.17V which I found odd. I also checked a fresh battery for reference and it showed 1.61V as one would expect. At first I thought that the remote doesn't put the batteries in series as they are both inserted the same direction. However there is continuity between the positive terminal of the left and the negative terminal of the right battery. So the same direction thing seems to be just aesthetics.

Why would one of the two batteries be so much more discharged and even have it's polarity reversed? Would it be something the remote is doing? Was the battery faulty to begin with? What kinds of faults would cause this?

Thanks!

[Twoflower]

Remotes don't protect individual cells for reversing (e.g. with an diode). and they don't stop until the batteries are extremely discharged. The reason is as they are designed to operate with normal batteries and not rechargeable batteries. Not sure how low they can discharge the batteries before the remote don't work any more. Probably 1V (0.5V per cell) or even lover.

What you see is caused by continuous discharge. The cell voltage drops to 0V and the current is driven from the remaining cell(s). This current works as charge current. But unfortunately it is by design the wrong direction. And causing to charge the weak cell in the opposite direction.

The result you're showing is that one cell was already a bit weaker than the other and got revered. But even the other one is so far discharged that there is a damage in the cell. You should re-charge such batteries regularly before they are charged below about 1V. So probably it's good idea to re-charge them for example once every year.

EDIT: Should have read the original a bit better.

The short time that happened is indeed a sign that the one cell could have had some trouble from the beginning. Or the one cell wasn't fully charged. Hard to say without knowing how the cell voltage was from the beginning.

[SeanB]

One cell had, probably either due to being tested in the factory, being filled with less of the active ingredients or because of other factors, a lower capacity than the other, despite them coming off the same production line. Thus when it depleted, either due to use or because it had an internal leak path that allowed it to discharge faster, it went low, and then the other cell started to charge it in reverse. Very common with primary cells, as they all have a nominal capacity, and being a mass manufactured item the tolerances can vary from unit to unit quite a bit, and you just had two cells that sat at either end of the curve.

[drussell]

Most likely just a defective battery that had some internal leakage and discharged itself significantly more rapidly than the other.  Put two new, matched cells in it and you should be fine. 

[Audioguru]

Cheap batteries from "over there" always produce problems because they are full of rice. Use good quality Western batteries instead. Don't try to charge a non-rechargeable battery.

[Twoflower]

The Cheap batteries from "over there" are Amazon batteries ;-)

[helius]

This negative cell voltage phenomenon even happens with single cells that are not used in series or kept on the shelf. It is from corrosion between the cell contents and its steel case and is a sign that leakage is imminent.

[Kleinstein]

The other cell is also empty at 1.17 V so the two cells were not that different. Often the cells are closer, but there is some scattering in capacitance and self discharge.