AAA battery negative voltage

[Antonio90]

Hi.

I just picked up my HP 50g after a few weeks unused, and found it doesn't turn on. Changed the batteries, nothing, connected it to the PC via mini-USB, nothing.
Out of curiosity, I measured the batteries, I-TECH, Carrefour brand, and 3 of them measured around 0.8V, so dead enough, except one, which measures -0.3V.

So, why could that happen? I've never seen a negative voltage in such a battery. The calculator is pristine externally, no battery leakage, stays in the living room, climate is quite dry in center Spain.

On another line, ¿Any chance of a reasonably easy repair? By easy I mean basic components and no unobtanium IC damage. I can get another calc for 25 quid or so, but I'd rather repair this.

[BeBuLamar]

Are the new batteries measured good? The HP-50G doesn't work with USB power just fo communication. Remove the lithium backup battery and let the calculator in that state for may be a day. Then install all good batteries in the calculator.

[Gyro]

It's because, as the batteries reach the end of life, the weakest battery becomes exhausted first (no 4 batteries will be exactly the same capacity, there will always be a weakest one). When it becomes exhausted, it gets reverse charged by the other three batteries through the load. [Edit: even primary cells can be charged or reverse charged to some extent.]

This same effect can cause big problems in series strings of rechargeable batteries. Again, the weakest cell gets reverse charged and damaged. Because of this, there is always a minimum 'safe' discharge voltage for a battery pack.

[Antonio90]

Thanks, will try that. I have a backup 50g, and the new batteries work on that one. It also turns on when connected to the laptop's USB even without batteries installed.

If that doesn't work I guess I'll have to buy a suction cup and test for PSU damage.

[Antonio90]

It's because, as the batteries reach the end of life, the weakest battery becomes exhausted first (no 4 batteries will be exactly the same capacity, there will always be a weakest one). When it becomes exhausted, it gets reverse charged by the other three batteries through the load. [Edit: even primary cells can be charged or reverse charged to some extent.]

This same effect can cause big problems in series strings of rechargeable batteries. Again, the weakest cell gets reverse charged and damaged. Because of this, there is always a minimum 'safe' discharge voltage for a battery pack.

That makes sense, I'll keep an eye on the batteries from now, the 50g is a bit of a power hog. SwissMicros really look tempting, but are really quite expensive.

[rsjsouza]

As Gyro said, I have seen this many times as well, including my kids' toys.

Interestingly enough, my HP48SX (also using three AAA batteries) was very prone to do that, especially in the middle cell. So much so that, at certain point, I was starting to suspect there was a connection only on the middle cell that could be powering its memory while the calculator was off - it turns out that all three cells power the calculator permanently and it might have been a coincidence or perhaps something physical that would deplete it faster.

[inse]

The reversing also boosts leakage