AA Ni-MH rechargeable battery showing VERY high voltage

[Bazzaman]

Hi,

I have been using 2500 Ni-MH AA rechargeable batteries in a battery operated doorbell chime for some time. Had been working fine and recharged OK as and when needed.

A few days ago the doorbell chime no longer responded to the doorbell push button (and the push button battery was definitely OK). So time for a recharge (or so I thought). Took both batteries out of the chime unit and placed them correctly in the charger that I've used for the last 3 years. The instant I plugged the charger in, there was a "poof", a small flash and a burning smell (from the charger, not the batteries) . So I immediately switched off the mains power at the socket.

Decided to check the battery voltages. First one showed 0.654 volts. Second one was....... wait for it....... 107 volts (yes that's one hundred and seven volts).

I checked the duff battery again today. Now shows as 113 volts.

Can anyone shed any light on what might have happened?

Obviously, at least at this stage, I can't be sure if the chime unit somehow affected the battery (or if the battery just went haywire of its own accord) or if it was the charger that screwed the battery (BTW the charger is now completely dead).

Cheers, Bazzaman

[Phil1977]

Sell your invention to Tesla or some other carmaker and get billionaire!

Some very cheap chargers just use capacitive droppers as a power supply. That means, if the capacitor shorts out, you could get mains voltage onto the battery. Did you measure the voltage while the battery still was in the charger?

If you want to know more then please post a picture of the charger. If it´s already dead, then try to brake open the housing and take a pic of the PCB. I´m quite sure that then the theories what happened will pop up...

[Bazzaman]

Thanks for quick response.

Photos attached.

Only checked battery voltages out of the charger.

If the capacitor shorted, would both batteries have been affected?

PS. maybe already billionaire

[Jeroen3]

Did you measure this high voltage while it was still plugged in?

[Bazzaman]

As I've said "Only checked battery voltages out of the charger." So whether the charger was still plugged in is immaterial as the batteries were not in the charger.

[VinzC]

If the capacitor shorted, would both batteries have been affected?

The capacitor was just a hypothesis until you shared pictures of the charger. By the looks of it that thing embeds an AC-DC converter (aka a low power SMPS) so that "capacitor" is no longer relevant. I'd personally suspect a failure in the charger that may have damaged one of the batteries (but then I'm stepping out of my knowledge and field of experience). I see no reason for a NiCd battery to ever reach the hundred volts though. I'd say it's physically impossible so there's a reasonable explanation for that and I do not know the internal structure of these batteries enough to provide any sensible explanation.

However due investigation on the charger itself is needed as there may be a potential risk for fire or electrical shock. Try locate the component that smoked, knowing that it is most probably not the one that failed in the first place, probably a consequence of the actual failure.

[BILLPOD]

Good Morning Bazzaman,  What meter were you using to read these voltages

[magic]

... and are you sure you aren't missing a "milli" somewhere?

[madires]

That was also my first thought, i.e. autoranging to mV.

[tunk]

Think I would first check the fuses and diodes in the
upper left of the first photo. There could be some
burns at the top of the transformer - maybe check the
continuity of the windings?

[Bazzaman]

OK, massive egg on face.

Was using a newish meter (previous one was many decades old). Not familiar with "autoranging". Make note to myself to RTFM I suppose!!! But having looked at the manual it doesn't even cover this.

Anyways, apologies to all.

But I don't really understand with the meter autoranging, why does 654 mV display as 0.654V but 107mv display as 107mV. As the significant digit for both is in the 100s of mV, I would have thought  it would make sense to either display both as V or both as mV.

Now demoted to simple millionaire? Sob, sob.

But I'm now still left with the puzzle of why the charger blew. Could it have been the battery at 107mV was just too low and caused an overload conditioon?

[tunk]

If it's a 6000/4000/2000 count multimeter, then it's using the 6/4/2V range for 0.654V.
107mV is in the 600/400/200mV range.

[MK14]

But I'm now still left with the puzzle of why the charger blew. Could it have been the battery at 107mV was just too low and caused an overload conditioon?

Yes, it can.

Good quality, decent chargers,  shouldn't be hurt by such things.  They could either attempt to recharge them, while still protecting the charger and (ideally) warn the user, depending on what Leds, displays and buzzers etc, the charger has.

[IanB]

But I'm now still left with the puzzle of why the charger blew. Could it have been the battery at 107mV was just too low and caused an overload conditioon?

Anything is possible.

But it is more likely just a coincidence. Sometimes electronic items fail, and the moment of plugging them into a power outlet is one of the times that a failure might occur, since that is when a sudden surge of inlet power happens. For example, incandescent bulbs almost always fail with a "ping" at the moment you turn them on.

[MK14]

It can also be a chicken and egg thing.  Maybe the batteries were ok, as you put them into the charger (did you check the original voltages, just before trying to charge them)?

But the faulty/failed charger, damaged the batteries (or at least made their voltage drop too low).

[madires]

But I don't really understand with the meter autoranging, why does 654 mV display as 0.654V but 107mv display as 107mV. As the significant digit for both is in the 100s of mV, I would have thought  it would make sense to either display both as V or both as mV.

Based on the number of digits DMMs have a upper/lower limit for each range. For example, 200 mV up to 2V. The DMM would display 0.200 up to 1.999 V. Below 200 mV the DMM switches to the next lower range, e.g. 0.0 up to 199.0 mV.

But I'm now still left with the puzzle of why the charger blew. Could it have been the battery at 107mV was just too low and caused an overload conditioon?

Shouldn't (unless the charger is cheap junk). Could be just pure coincidence.

[magic]

Probably the batteries are simply completely depleted, and the charger simply blew for unrelated reasons. If it blew on the primary side of the transformer, which is quite likely, the batteries shouldn't be affected much.

As for what happened to the charger, I don't know, but the usual places to start is to check if the fuse is blown, check the rectifying diodes and check if the primary power transistor isn't shorted.

[MK14]

It seems, at a quick glance, you have a blown up component, on the charger.

The underside (surface mount side, second picture), (just below, and a bit to the right of R10) near the bottom middle, a small SOT23 transistor, seems to have blown its lid off (I could easily be wrong, pictures can lead to confusion).

EDIT: Removed section, as I've decided I'm not sure, if I have damaged/broken a charger(s), with over-discharged batteries, or not.  Probably NOT, I think it broke, through natural causes.

[RJSV]

   You'll need a new or functioning charger regardless.   Once you get that, you could try to charge that 'low boy' (the one with 106 mV).
Then, you've only to deal with the broken charger, and perhaps that had drained that battery (anyway).

   Many older folks, of long-time great wealth display a category of 'helpless', so you maybe need to cultivate that...(For more believable image).
   "He has to be vacuumed, after he eats...".
...helps to present that 'filthy rich' stereotype.

....But THEN;   I gleaned this knowledge from a movie I saw.

[factory]

But I'm now still left with the puzzle of why the charger blew. Could it have been the battery at 107mV was just too low and caused an overload conditioon?

Anything is possible.

But it is more likely just a coincidence. Sometimes electronic items fail, and the moment of plugging them into a power outlet is one of the times that a failure might occur, since that is when a sudden surge of inlet power happens. For example, incandescent bulbs almost always fail with a "ping" at the moment you turn them on.

Charger seems to be quite old, datasheet at RS suggests at least 10 years old, some things don't last very long.

https://docs.rs-online.com/5e87/0900766b80d2b546.pdf

David

[BeBuLamar]

If it's indeed 107V you have invented something very impotant whether it useful or not. But yeah like the other said it's likely that you read 107mV.

[VinzC]

OK, massive egg on face.

We've all (well, most of us) been there, worry not .

But I don't really understand with the meter autoranging, why does 654 mV display as 0.654V but 107mv display as 107mV.

That really is not important. At all. What does matter is now you know. Everything on a DMM display is important; quick/careless eyeballing may result in missing important information, as in this case, three orders of magnitude . You'll get used to it in the long run, trust yourself.