Sunday, June 04, 2006

The Frugal Hacker

Harvesting Cells for Economy


What do you do when your favorite little flashlight wants oddball AAAA sized cells that are going to empty your wallet faster than you'd like? There's always buying "wholesale"...

The flashlight I carry with me everywhere to aid my old eyes uses some very unusual, and very expensive, AAAA cells. (Yes, that's four A's). At about $4.00 USD for a set of three to fill the flashlight finding a better way to get power to the LED was certainly on the agenda. Fortunately there is a way to get a substantial discount, if you're willing to do a little work.

Technically, a "battery" is a set of cells—usually connected in a series, which raises the voltage. Each cell has a characteristic voltage, based on its chemistry. The common alkaline primary cell with its zinc and manganese construction, produces 1.5V when fully charged. (As a contrast, a Nickel Cadmium secondary ("rechargeable") cell produces 1.2V and a Lithium Ion secondary cell produces 3.8V)

So, the common cells—AAA, AA, C, D—differ not in voltage but in total current capacity; picture them as different sized "tanks" of voltage producing chemicals. However, the alkaline 9V battery is a real battery. It is composed of cells that are 1.5V alkaline cells and stuffed into that little square can. Back in my childhood tearing open a "transistor battery" (so-called because it was designed for the new transistor radios which needed the 9V potential in a small package) yielded a set of six flat little carbon cells. If you are old enough, I know you have seen these.

Today, the much more difficult (the cans are made a lot better) equivalent act yields six very thin cylindrical alkaline cells. By this time you have guessed, these are AAAA cells. At under $2.00 USD each for the 9V "host", we just got a 50% discount. However, in addition to the labor of opening the can (watch for sharp metal!), a little effort is needed to use the batteries in the flashlight.

The cells in the 9V battery are missing the caps that the individual AAAA cells sport on top and bottom. One forms the "nipple" a the positive end, the other the dimple at the negative end. The flashlight's battery compartment length is designed for additional length of these so if you just drop the liberated 9V cells in there, no light will result.

There are a few tricks that will make these cheap alternatives operational. First, beware of appearances. The naked cells are confusingly reversed in appearance with a "nipple" on the negative side! This is a result of the internal geometry of the cell. If your flashlight isn't working double check this first.

My favorite way to make these cells work takes advantage of the fact the cells were wired in a series circuit inside the can. The manufacturer spotwelds a small metal strap between cells as the electrical connect. By preserving two of these straps, and carefully folding over the cells so that they will fit in the flashlight, we have not only added to the overall length but improved the electrical connection. These straps will probably not add enough length, though, so a bit more "padding" will probably be required.

I use a small aluminum foil ball, about 3 mm or so, dropped into the flashlight. When the cap is screwed down tight, the ball is crushed flat and becomes a contact. The flashlight's contact on that end is one lead of the LED, it is the positive side. Note that this means you will be putting the flat side of the cell stack down the tube. A bit of playing around will get you results, this isn't rocket science, be creative!

An alternative method I previously used, and have abandoned in favor of the easier method above, was to put a blob of solder on the post extending from the negative side of the cell. This works, but be careful not to overheat the cells in process. I have some sympathy for those who find this method aesthetically pleasing.

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4 Comments:

Anonymous Anonymous said...

Great write-up on retrofitting a Streamlight Stylus with AAAA cells from a 9V battery! Would be nice, though, if you had a picture or two of the process. So are these cells installed upside-down, compared to retail AAAA cells?
By the way, found your article from the "Insert Coin" digg'd article.

Fri Oct 19, 02:13:00 AM EDT  
Blogger Yaakov said...

Thanks. I agree about the photos and had planned to add some but put it off. I think I will do that now.

Yes, the batteries look upside down.

Fri Oct 19, 05:17:00 AM EDT  
Anonymous tahrey said...

Word of warning: Only SOME 9v batteries may be like that, if indeed you're not just pranking us.

Just caught all kinds of hell at work after idly picking a dead one apart... only to find it was made up of two metal plates (and a connecting wire) sandwiching a stack of crumbly lozenge-like things that stained my fingers like printer toner (I guess it was a zinc-carbon battery, though it didn't say exactly what it was on the casing?). No normal cylindrical-type cells at all.

Don't take it for granted.

Thu Sep 16, 11:37:00 AM EDT  
Blogger Yaakov said...

tahrey:

What you took apart was a carbon zinc battery. It must have been a very cheap one. Today, any battery worth using is alkaline and uses the construction I described. What they do, today, with the cheap ones is to put "heavy duty" on them, which is really a claim of nothing at all.

In the case of 9V batteries, the carbon versions are really very pathetic since the size of the cells is quite tiny. Carbon 9V batteries go flat very quickly.

Look for "alkaline" on the can before you tear it open.

Yaakov

Mon Sep 20, 08:52:00 AM EDT  

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