Can somebody help explain how one of two AA batteries in series is being changed to emit a negative charge?

Question

This, I have never seen before and I can not explain, and I have no idea how this can even be.

I have one of those outside temperature sensors. La Crosse model TX37U-IT https://www.lacrossetechnology.com/products/tx37u-it

The sensor thing is outside in the cold and it transmits to the base unit in the house.

The outside sensor unit takes 2 AA batteries.

I've had this thing for probably 10 years now. and it's worked fine. A few months ago, maybe a year or so by now, it's started chewing through batteries.

I usually use rechargeables because that's how I roll, and they usually last a while and I recharge them and all is well.

But here's what's started happening recently. Usually a charge will last many months if not a year or two. now a charge will last a few months tops.

And Here's The Weird Part.

When I test the 2 AA batteries from the sensor after it stops working, one of them reads a bit low like 1.2 volts, and the other one reads NEGATIVE .1 or so volts.

And it's always the one in the same position in the device.

The first time I saw this, I figured it was a bad rechargeable battery. I threw them out. The second time, the same thing happened, seemed a bit fishy.

So most recently, I put in two fresh brand new AA duracell batteries, not rechargeable. They should last YEARS in this thing. And just now after maybe a month, the thing isn't working again, one battery is at 1.2v and the other is at -0.2

How is this even possible?

Okay, so now that it's been sitting here for a few minutes, the -0.2 has gone positive to 0.1 So something must be forcibly backfeeding it?

Except it's in series with the other battery. How can whatever's going on possibly affect one battery and not the other?

Yet it keeps happening.

I'm stumped, but I know nothing about electricity so that's to be expected, but is there some obvious or rational explanation for this? I've never seen anything behave this way with batteries.

Answer 1

And just now after maybe a month, the thing isn't working again, one battery is at 1.2v and the other is at -0.2. How is this even possible?

Okay, so now that it's been sitting here for a few minutes, the -0.2 has gone positive to 0.1 So something must be forcibly backfeeding it? Except it's in series with the other battery. How can whatever's going on possibly affect one battery and not the other?

Because the thing that's "forcibly backfeeding it" is the other battery. Think about it. Let's say that the "bad" battery is the one attached to the device's positive terminal. The bad battery's negative terminal is directly connected to the good battery's positive terminal, and the bad battery's positive terminal is indirectly (through the electronics of the temperature sensor) connected to the good battery's negative terminal.

This is always happening when you have a series connection. When the two batteries have an equal state of charge then no current flows (except whatever the device uses) and nothing bad happens. But when one battery is dead and the other still has some charge, it results in the dead one "reverse charging".

So nothing freaky weird is happening except that one battery is dying a little bit before the other. Why is that? Well, maybe it's just coincidence that every time you checked, it's a certain one that died. Maybe one is a little bit warmer or colder than the other. Or maybe there's some crud in the battery compartment that is short-circuiting the one battery (not a short in the sense of 0 ohms, but a short in the sense that it provides a path between the two terminals of the battery that doesn't go through the device that it's supposed to be powering). We can't really say. You haven't found a smoking gun.

Answer 2

I am pretty sure , the thing happening is polarity reversal, as evident from the name , in this the polarity of a cell gets reversed , a very common phenomenon for batteries in series.

I am unaware of the circuit, the device you mention is using , but it is tending to nearly deplete one of your batteries before the other one..

now consider one battery as X and other as Y , Y is nearly depleted fully , but X still has potential energy so the circuit will still use that potential and current will pass through the depleted battery Y

the scenario is

-[0000]+  -[0000]+ _______circuit 
   x          y

   -[0000]+
      Y
<------------  flow of electrons 

this leads to opposite charge accumulation and thus the battery starts getting reversed polarized...it is common when new batteries are used with old , one gets depleted before the other , in your case it might have been a manufacture defect from the company who made the cells.