1
\$\begingroup\$

My kid has a battery-operated toy, it runs on three AAA batteries, connected in serial. Once, the toy suddenly stopped working, but all batteries were good. I decided to disassemble the toy and look for internal damage or some loose wire. Found nothing and reassembled it. The next day, on a last attempt before dumping it, I placed the batteries back on, just to confirm, and it just started working again. Have no idea what changed. Humidity maybe?

Now the issue. Since then, the toy is killing one of the batteries in the pack, in a week or so. It stops working, I check the batteries, the one on the left is completely dead with 0 volts, the other ones are ok. I replace the dead battery, it gets back working ok. Another week or so, the toy stops working again, and the newly replaced battery is dead. It's always on the same deck position.

Seems a short, like @Frog stated, but the plates and springs seem to be far apart. Could be damage to the wires, a pinch, or something like @Kyle B stated. But by looking inside I can't spot anything suspicious. Can't be faulty battery, because replacing just the damaged one by a new one, and the new one gets dead as well, proves the issue cannot be on this battery, I think. Every time this happens, I now remove the dead battery, place one of the others on this place, and place the new one elsewhere.

I have now placed photos.

Can anyone help?

(this is a re-post as the other one got closed. This time I tried to be more clear, and added photos)

\$\endgroup\$
8
  • 2
    \$\begingroup\$ Replace all 3 batteries at the same time with matching batteries, all brand new from the same pack. \$\endgroup\$
    – Justin
    Mar 4, 2021 at 18:31
  • 1
    \$\begingroup\$ why are you posting this again? ... you were supposed to add pictures to your previous post and improve it, so that it would get reopened \$\endgroup\$
    – jsotola
    Mar 4, 2021 at 18:31
  • \$\begingroup\$ Indeed it sounds like the left cell is not exactly the same as the others (lower current rating) so the middle and right cell force it to completely discharge. Are the brands different? Not supposed to mix battery types / brands - this is why. \$\endgroup\$
    – rdtsc
    Mar 4, 2021 at 18:54
  • \$\begingroup\$ Hartwin - As already commented, you should not have posted a whole new question, and should instead have added the photos to the previous one, then flagged it to join the queue for voting to be re-opened. However, since your previous question didn't yet have answers, and this one now has some, it seems pragmatic to allow this question to "live" and the previous one should remain closed. Please don't open duplicate questions again in the future. Thanks! \$\endgroup\$
    – SamGibson
    Mar 4, 2021 at 19:00
  • 1
    \$\begingroup\$ Is there any possibility that there are three conductors connecting the battery holder to the inside of the toy, instead of two? \$\endgroup\$ Jun 7, 2021 at 19:56

5 Answers 5

3
\$\begingroup\$

You have bad luck with batteries.

Since all three batteries are in series (there's only two contacts to the battery compartement) the toy can only drain all batteries, or none. There's no physical way this toy could just empty one battery, and not the others.

\$\endgroup\$
2
  • \$\begingroup\$ True. That is why this is so odd. It can only be a short, yet it seems physically impossible. \$\endgroup\$
    – Hartwin
    Mar 5, 2021 at 20:21
  • \$\begingroup\$ or it's simply bad luck with batteries :) \$\endgroup\$ Mar 5, 2021 at 20:38
3
\$\begingroup\$

Any series connected cells will stress the weakest cell fastest, so if you keep swapping another old cell into the dead cell location, it will keep failing. The random result is to replace all with matched cell voltages at the same time. This is true for any battery technology.

My cheap CO detectors only last 6mos on 3 alkaline AA instead of expected 1 or 3 years. Your toy is intended for lower minutes of use per day. Buy Panasonic Enerlec cells in bulk if you want longer use.

\$\endgroup\$
1
  • \$\begingroup\$ It might be true. But the first time it happened, all three were alkaline, that one died, other two were in good shape, I placed one of these two on the "bad" slot, and a rechargeable one is the free slot. As per the logic, the rechargeable battery had to be killed first, but it didn't.The alkaline from the "bad" slot died after a week, and both the other alkaline and the rechargeable one were in good shape. Before this started to happen, all batteries would last over a month, and I was ok with that. \$\endgroup\$
    – Hartwin
    Mar 5, 2021 at 20:19
1
\$\begingroup\$

Evil Magnetic Forces.

If the kid really likes the toy, open it up every few days and shuffle the batteries. If for some reason it really does have one cursed slot, that'll distribute the effect.

OTOH -- if you're not getting high quality name-brand batteries (Energizer, Rayovac, etc.), then give those a try. Store-brand batteries are not to be trusted.

\$\endgroup\$
2
  • \$\begingroup\$ Shuffling is what I was trying now. Others say do not. Anyway, it won't solve. I may try a fresh set of good batteries, but I don't expect it to do any difference, as different configs of batteries have been tried and the battery sitting on that slot is always the one that gets killed. \$\endgroup\$
    – Hartwin
    Mar 5, 2021 at 20:25
  • 1
    \$\begingroup\$ If you're convinced that it's the slot you could take all the batteries out and measure the resistance just at that slot. They may be tapping 1.5V off of the pack for some reason -- but if they are, it's show as an extra wire coming off of one of the "two battery" bridge strips. \$\endgroup\$
    – TimWescott
    Mar 5, 2021 at 21:10
0
\$\begingroup\$

Just to update you on this issue. I can't reproduce it anymore, it seems it is no longer happening, the event that one specific battery slot seemed to be killing every battery I'd sit there. Seemed a short, but it is visible that the plates are far apart. Besides, I've tested it with a multimeter and no short was found.

Can't find a logical explanation, other than the dying batteries were just bad, or got bad, and happened to be sitting in the same slot by chance, and it all happened sequentially. The present battery set has been working for over a month and all batteries seem to be holding.

Thank you all for your thoughts and opinions.

\$\endgroup\$
2
  • \$\begingroup\$ Can't find a logical explanation If you replace only one battery, and you happen to have dud batteries (valid assumption since you didn't properly test them before replacing), then what else do you expect? The replacements are duds and will keep failing, while the two other batteries that were decent will keep on doing their job. That is the simplest explanation. The next simplest one is that the toy is perhaps failing and places high pulse loads on the batteries, which almost always kills one cell very well from among any 3 cells. \$\endgroup\$ Jun 7, 2021 at 19:37
  • 2
    \$\begingroup\$ @Kuba But Hartwin has, on some occasions, taken the battery out of the "bad" slot and replaced it with a new battery, and on other occasions, taken a "known good" battery from a different slot, put that one in the "bad" slot, and replaced the "known good" battery with a new battery. In all cases, the battery which discharges is the one placed in the "bad" slot, regardless of where that battery was taken from. That seems to rule out that hypothesis. \$\endgroup\$ Jun 7, 2021 at 19:51
0
\$\begingroup\$

This is very typical: when batteries discharge, they almost never discharge equally. That's why rechargeable battery packs have cell balancing circuits, and why those without such circuits suck in comparison. If you measure the batteries, you'll almost always see that one has discharged way more then the others. But the others likely aren't all that far behind.

Where you erred was replacing just one battery: you're suppose to replace all of them, all at once, and never mix old and new. Those admonitions in the manuals are, for once, actually something worth following :)

In other words, what you experience is normal, but you chose to be measuring things and not replacing all the batteries :)

But the first time it happened, all three were alkaline, that one died, other two were in good shape

That may be the case - and then you have your answer: a dud battery. I mean: what else were you implying or hoping for? Yes, some devices have taps in the battery packs, causing unequal drain, but that's not the case according to the photos of the device you've provided.

To measure battery "shape" in a way that would be acceptable in anything but a back-of-the-napkin calculation, you need to have a coulomb counter (charge meter) attached to the battery pack, and read that out. The cell voltage will mislead you almost every time - precisely because there are dud batteries due to bad luck, fake or otherwise sub-par product on the market, or just awful loads that kill battery life (yes, those do exist).

Each alkaline battery has a certain capacity, a ballpark: every battery's ultimate capacity may be slightly different.

The batteries are all connected in series, so the charge flowing though them will be exactly equal - that's a physical necessity. It just so happens that if one battery's capacity is 90% of each of the other two, it may appear "flat", in spite of the other batteries being mostly spent at that point.

This will be exacerbated by higher load devices that raise internal cell temperature significantly enough (perhaps in a localized hot spot) to affect internal resistance. This is a positive feedback loop: the cell most depleted has highest internal resistance, which makes it run hottest among the cells, further reducing its useful capacity by wasting energy and evaporating the electrolyte. In alkaline batteries, this almost always leads to runaway of the weakest cell to its premature end.

So:

  1. Replace all three batteries at once, and
  2. Before making any proclamations about battery state, use a coulomb counter to measure actual charge extracted from the batteries, to be able to quantitatively tell where the batteries should be on their discharge curve (assuming they were not duds/fakes), and
  3. Accept that some battery batches can be complete and utter junk. Nothing new there.

If you want to know anything about what's going on, you must investigate those batteries on their own - run a discharge cycle on each and see what the variation in usable capacity is, and do such discharges at the currents representative of the device acting as the load. Don't test just a couple cells. Test dozens. Ergo, also characterize the device acting as the load. Yeah, no free lunch there, and no, using a multimeter is probably not enough - you need a current transducer (even if rudimentary), and an oscilloscope.

It may be that due to some design shortcoming, the toy is placing low impedance across the batteries for short periods of time, causing high current flow. The average discharge current may still be "reasonable", but most batteries' usable capacities will be rather affected by such treatment, and it always exacerbates the inherent process spread - the original source of slight variation between each cell.

Beyond a certain I^2*t product in pulsed operation mode, even small alkaline battery packs with just three cells always run away to one cell being "very dead" with others having a reasonable cell voltage. Thus, what you observe is not unexpected at all, and I'd be more surprised if you did not observe something similar in 3+ cell alkaline packs in a variety of high load devices, or even moderate load but high ambient temperatures (e.g. toys left in sunlight or inside a car - bad news for alkaline batteries!). They are supposed to act that way, or else the Universe has gone bonkers. Also, this is a good test for the Universe going bonkers. Obviously. :)

\$\endgroup\$
1
  • \$\begingroup\$ Sounds like a project to me! If only I had the time, and the gear...Thank you for your post, you sure know a lot more than me about the subject. But even though, it is still interesting that the killing was always on the same slot, even when I scrambled and placed the new battery elsewhere, and again that specific slot killed the sitting battery. I'm just glad it stopped. \$\endgroup\$
    – Hartwin
    Jun 9, 2021 at 19:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.