I suppose this particular post belongs to the “everyone should have done this at one point in their lives” bin, but strangely, they probably haven’t. Just the other day, one of my multimeters decided to let me know that it needed a battery change. Out came the old 9v battery, and in went a new one. So, what do we do with the old battery? Throw it away … right?
Well, that’s only one possibility. Another is to take it apart to understand what’s inside a 9V carbon zinc cell. After all, there’s nothing really chemically dangerous in these cells as long as you don’t eat it, so why not?
This particular cell comes from Thumbcells, a Chinese company whose logo is a little scary …
The battery is fairly recent, so it complies with RoHS regulations for the protection of the environment. The first step is to locate the seam that runs down the side of the battery – this is where the metal “casing” for the battery is crimped together. This particular casing is only for mechanical rigidity and doesn’t serve any electrical function. Once the seam has been located, take a small flat-bladed screwdriver and begin prying along the seam.
The way the seam has been made, the can is actually folded in on itself in an interlocking way, which explains why it is so difficult to take apart. A pair of pliers could help. Already, we can see some of the structure – lets peel away the outer can.
This reveals the internal structure of the battery. The 9v battery is composed of six 1.5v carbon zinc cells connected in series. The six cells are seen here, looking suspiciously like cellophane-wrapped Werther’s Original candy … but trust me, they’re not for eating. The stack is wrapped in clear plastic to hold it together and insulate it. The plastic top card holds the terminals, whereas the bottom plastic card just completes the cell. The folded edge of the outer casing applies mechanical pressure to the end plastic cards to “squeeze” the cells together to make contact.
The terminals themselves are only little pieces of metal with “spikes” to bite into and make contact with the cells. The positive terminal is connected to the bottom of the stack and the metal contact is further insulated with paper to avoid short circuiting under mechanical stress (e.g. crushing). Since these cells give so little current due to high internal resistance, even a short circuit is unlikely to cause major issues aside from a dead battery.
Here we can see the “bite” marks from the top and bottom terminals into the cells themselves. The end cells seem to be constructed specially, with soft zinc plate contacts for the terminals to bite into. The stack of cells can be further disassembled by scoring the outside plastic shrink wrap and peeling it away.
It’s now when you realize the “gooey”, almost plasticine like nature of the cells. As they’re made of electrolyte-soaked carbon powder and what appears to be cardboard, you’re acutely aware not to squeeze them too much or you’ll get a mess … so out comes the outer layer of clear plastic.
This allows us to separate cells from the stack itself, and we can already see the difference with the cells inside the stack – their ends are not nice and shiny and are instead very much dark and carbon-coloured. Also, surprisingly, we find that we’re not free from the clear plastic shrink wrap just yet, as each individual cell is also plastic wrapped.
Squeezing the cell, it becomes evident that it’s not all soft. There’s something inside …
There’s lots of carbon soaked in some electrolyte, and that makes a pretty nice mess if you stake it apart. It will also rust some metals as it’s slightly corrosive too. But removing the wrap entirely …
… reveals an embedded thin zinc plate electrode. So it seems that there is no carbon rod in the cell, as it might not be required due to the small cell size itself.
After making a ripe old mess, we managed to get a good look at the lozenge-like cells that make up a carbon zinc 9V battery, and look into those cells too. Unlike alkaline 9v cells, there are no AAAA-sized cells inside … so don’t buy carbon zinc cells expecting to extract AAAA cells! Maybe I’ll break one of those apart in the future just to show the difference.