Battery terminal connection corrosion

Ah the dreaded black wire corrosion

"Key Causes and Indicators:

- Electrochemical Action: Moisture + voltage causes rapid oxidation.

- Battery Vapours: Common in RC hobbies/electronics with old NiCad batteries.

- Environmental Factors: High humidity, storage in damp, unheated areas (sheds/garages).

- Wire Quality/Insulation: Poor quality copper or insulation containing sulphur can cause this.

Symptoms: Wire turns black, becomes brittle, and causes high resistance or intermittent failures. "

Clean the area - it's got some battery material in it somehow. Or some other chemical

Then consider using lithium iron sulphide non-rechargeable batteries or Eneloop nickel metal hydride rechargeables. Neither of these types appear to be as likely to leak as alkaline manganese batteries.

The lithium iron sulphide batteries were originally exclusive to Energizer, but the patent has expired and there are now alternative suppliers. The initial voltage is about 1.8V, dropping to about 1.4V when they are nearly exhausted. The shelf life is 25 years! John

short answer = "no" - you are not imagining it

Also I have noticed that a significant number of rechargeable AA cells seem to be just a tiny bit larger than they used to be - sometimes making it difficult to get them in and out of some devices, most notably remote controls.

Or maybe I am imagining that too :-\

No. I've noticed that too. Sometimes it's a tight fit lengthways, sometimes it's the width. There's a bit of variation allowed. For example, for AA size:

I bought a battery holder for 3xAA - even the dry cells don't fit

I have fairly often suffered the black wire syndrome with NiCads, I had not realised the implication that eg. NiMH does not give rise to this. Makes some sense though, I have not seen the issue too much in recent years.

Does anyone know what the 'battery vapours' were?

IIRC it is some kind of alkali. NiCds are famous for this and it takes out old motherboards that used them to store non volatile memory and keep the RTC clock running

Capacitors too. And 'alkaline' dry cells

Now NiCd contains potassium hydroxide in high concentrations, as do 'alkaline' batteries

Aluminium capacitors tend to use acids like boric acid

IIRC potassium hydroxide alone does not attack copper, but if there is an aqueous solution of it and and electrical potential, then it does. Which is why it attacks battery wires in particular

Boric acid tends to form a green corrosion product instead of a black one

I have also noticed that that with some cells (so far almost all of them being AAA) the terminal button at the top is not long enough to reach its intended contact - theres some sort of locating / alignment shoulder thing getting in the way (remote control again !!)

It occurs to me that the spec for the terminal button on AA cells "minimum 1 mm high and a maximum 5.5 mm" is insanely sloppy - I couldnt find a spec for the AAA cell

And something organic, maybe an amide. One infallible diagnostic was the customer saying that the device smelled of fish when it got warm.

Generally the dodgy cylindrical SMT AL capacitors developed a white or yellow powder deposit around the negative pin, and if they got hot enough to give off liquid electrolyte it dissolved copper with initially a black surface oxidation. The liquid was of course conductive, and I once saw a layer of it under a cap across the 5V rail, drawing half an amp and smelling very fishy.

Yes. I had forgot the fishy smell.

That rings a bell. Cells intended specifically for DECT-type phones (basically on float duty) had longer positive buttons, and the phones were made to require this type of cell. Google seems to know almost nothing about this, but there is a hint that it was only NiCd which this applied to, and these days everything is NiMH. I know that I ran into the problem many years ago, having bought standard rechargeables before finding that they wouldn't work.

Yes, it's a common problem I've found too. I think the intent behind the 'shoulder' is that it prevents a connection if the battery is inserted the wrong way round.

Thats all sounds logical to me

I am assuning that "That rings a bell" is some sort of DECT related pun ?

Maybe, but it does require non-standard cells. I definitely remember needing to buy them.

Should have looked first: