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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 14th Jun 2019, 12:04 pm   #1
Radio1950
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Default Alkaline Dry Cells Outer Tube Is Positive

Am I the only person on the planet to not know that the the outer metal tube case of AA, AAA, C and D alkaline cells is electrically connected to the positive terminal?

Which means that the whole outer of an alkaline dry cell is positive, except for that tiny metal disc at the bottom of the cell, which is negative.

Found accidentally today whilst fault finding a wireless thermometer, with Varta and Duracell cells.

I need to take a "brisk walk" as prescribed by another forum member to relieve certain afflictions.

I have been working with dry cells for about 60 yrs; the learning continues.
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Old 14th Jun 2019, 12:45 pm   #2
G6Tanuki
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

I was reminded of this some years back when I had a 4-D-cell flashlight: after a relatively short period clipped horizontally to the dashboard of my Land Rover the alkalines would go flat - the vibration was wearing-through the thin plastic insulation on the cases of the cells, allowing the +ve electrode to then short against the earthed inner part of the aluminium Maglite.
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Old 14th Jun 2019, 12:59 pm   #3
paulsherwin
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

I never realised this until now.
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Old 14th Jun 2019, 12:59 pm   #4
ionburn
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

I had noticed the bottom disc being separate but never really thought. An oversight which could trip up the unwary with some project or other. Thanks for the info.
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Old 14th Jun 2019, 3:59 pm   #5
Dave Moll
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

I checked this to be so with a Duracell I had handy, but is this true of all alkaline cells?

Most of the cells I have to hand are cheap zinc/carbon cells where this doesn't seem to be the case. One I tested (Kodak) had a case electrically continuous with the negative end, and another (Wilko) was insulated from both terminals.

Because one has to breach the paint or whatever to make electrical contact with the metal case, the non-isolation of the case isn't something I had noticed before.
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Old 14th Jun 2019, 4:40 pm   #6
Martin G7MRV
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

Just pulled the sleeve off of a couple of industrial AAA's, a Varta and an nx-power, just to prove it to myself!

I sort of already knew that the positive end and the case were one and the same, from having to dismantle a cell for some reason years ago, but my brain never actually made the connection (no pun intended)

These are alkaline types. As Dave says though, cheap zinc-carbon types, the negative is the case (just one sheet of stamped zinc) and the positive a cap on the carbon rod electrode.
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Old 14th Jun 2019, 4:58 pm   #7
emeritus
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

I only knew this from an "Allbatteries" catalogue I picked up at an exhibition in the early 1980's. It has a detailed description of each of the different primary and secondary battery technologies they stocked, and includes detailed sectioned drawings of the cells. I hadn't seen it anywhere else.
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Old 14th Jun 2019, 5:35 pm   #8
Paul JD
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

Apparently in an alkaline battery the metal case is the cathode so will be connected to the positive terminal. In a zinc carbon battery the case is the anode and will be connected to the negative end.

https://en.wikipedia.org/wiki/Alkaline_battery

https://en.wikipedia.org/wiki/Zinc%E2%80%93carbon_battery
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Old 14th Jun 2019, 6:10 pm   #9
Techman
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

Yes, I came across this for the first time about eighteen years ago at work, where someone complained that a piece of equipment was getting hot and fizzing. When I checked it I found that the industrial Duracell 'D' cell had been roughly fitted by someone into a battery holder which, to be fair, had a very sharp edge to the negative connector, which had deeply cut into the insulation between the disc and the positive wrap around side of the cell and caused it to short circuit. I removed the battery but it was getting hotter and hotter, so I put it into a bucket of water and put it outside to burn itself out - ended up with a nice bucket of warm water!

I'm surprised that there haven't been more reported accidents due to this not very good design.

Edit to add: The moral of this story is be very careful when installing these cells/batteries, as the wafer thin insulation between the positive case and the negative disc end can easily be damaged by rough insertion of the cells into certain battery holders.

Last edited by Techman; 14th Jun 2019 at 6:19 pm.
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Old 14th Jun 2019, 6:20 pm   #10
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

Quote:
Originally Posted by Martin G7MRV View Post
Just pulled the sleeve off of a couple of industrial AAA's, a Varta and an nx-power, just to prove it to myself!

I sort of already knew that the positive end and the case were one and the same, from having to dismantle a cell for some reason years ago, but my brain never actually made the connection (no pun intended)

These are alkaline types. As Dave says though, cheap zinc-carbon types, the negative is the case (just one sheet of stamped zinc) and the positive a cap on the carbon rod electrode.
I had understood, ages ago that Alkaline batteries were almost the same technology as the earlier Mercury cells.
All the US transistor radio instructions showed the Mercury cells to be installed the opposite of carbon-zinc types.
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Old 14th Jun 2019, 7:19 pm   #11
The Philpott
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Default Re: Alkaline Dry Cells Outer Tube Is Positive

As Techman points out the (shrunk on?) insulation is generally 'wafer' thin, a trap for the unwary. I recently had to sleeve a well made homebrew D holder to accept a C cell- to avoid this sort of incident.

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