Stupid 9-Volt Tricks by XlogicX ( If I was put in a situation where I could only use one kind of battery, it would be the 9-volt. Sure AAs and AAAs may be more common, but the 9-volt is so much more flexible, especially as a hobbyist. Most of these hacks are no secret, although one of them is a personal trick of mine (the clip). I will go into some detail on why some of these more known tricks actually do work. The 9-Volt Clip When building devices powered by a 9-volt, you want a 9-volt holder and a clip to attach the battery to. The typical clip that you can find just about anywhere, including RadioShack and Fry's Electronics is very flimsy in my opinion; the inside contacts seem to break with enough repetition of removing the battery from the clip. An even cheaper (almost free) and more reliable trick is to take apart a dead 9-volt battery and use the top cap of it as a connector. I just solder a black wire to the male (small circle) part of the clip, and then solder a red wire to the female (larger hexagon) part of the clip. (Note that this is reverse of power and ground, due to the clip being connected in complement.) The color of wire doesn't matter, however, red and black are a standard for positive and ground. I find the hard plastic generally used for 9-volt caps turns out to be much more durable. Quadruple-A Batteries As you may have discovered while disassembling your 9-volt, most common batteries such as Duracell and Energizer actually have six AAAA batteries inside the 9-volt shell. Not all 9-volts are designed this way, however. Some 9-volts have a stack of flat carbon-zinc cells. AA, AAA, and AAAA are all generally 1.5 volts. Batteries hooked up in series are additive with voltage. So being that a 9-volt is typically just six AAAA batteries hooked up in series, the math works out (6 multiplied by 1.5 equals 9). So if you're ever in a bind and need those very common AAAA batteries but only have a 9-volt, you have an option. Triple-A Battery Replacement OK, so AAAA batteries aren't really that common, but AAAs are. There are many videos out on the YouTubes saying that you can practically take apart any 9-volt battery and use the AAAAs inside instead of AAAs. Due to some complications (such as the flat cell 9-volts and obvious size difference), there is a lot of skepticism and question of whether this trick is a hoax. Let me explain why it is not. First, realize that not all batteries have six quad-As in them - if you find a flat cell, don't assume that all of them will be like this (as some have assumed). Also, keep in mind that AAAAs are smaller The claim that they can be immediately be used in place of a AAA without modification is usually incorrect. However, modification is usually very simple; some 9-volts have small metal clips used to connect the AAAAs in series. You can bend one of these clips in half and use it as a conductive expander. The main rule-of-thumb is to find anything conductive that will extend the length of the battery. So, what about voltage, current output, and battery life (the main relevant points of a power source)? We already know that the voltages for the "A" batteries are generally 1.5 volts. But can a quad-A handle the current load? For perspective, a typical average/high load for a double-A is about 50 mA (milliamps). A triple-A is typically around 10 mA. Quadruple-A batteries typically handle a load at around 1O mA-15 mA. Therefore, load should not be a concern when using AAAA batteries in place of AAA. However, with the smaller size of the AAAA, there must be a catch: capacity. A typical AAA has a capacity of 1150 mAh (milliamp hours). This means if you were to put a 1.15 amp load on a AAA battery, it would last for only one hour (in theory, not in practice; higher load drops capacity). Likewise, running half the load (575 mAh) would last for two hours. A quadruple-A battery has a typical capacity of 595 mAh, so AAAAs have about half of the lifetime of a AAA. So, when using a AAAA as a replacement for a AAA, know that it should work, but will only last about half as long. More Current, More Voltage A typical 9-volt is designed for 15 mA at a 595 mAh capacity. You sure could push one past 15 mA, but the capacity starts to tank when the load gets higher than the optimal 15 mA. In other words, running at 30 mA will last much less than half as long as running at 15 mA. But with one 9-volt battery, you could run at 90 mA at the same capacity, but at 1.5 volts. To do this, find a way to connect the internal AAAA batteries in parallel, instead of the default series. Or, for a quick and dirty high voltage hack, just daisy chain a bunch of 9-volt batteries in series. Their connectors are perfect for pulling this off with no extra hardware. Current and capacity will remain the same though. USB Charger This is a fairly popular trick. I'll describe the no frills version. You can build a 9-volt USB power charger with some wire, a soldering iron (and some solder), a battery clip (homemade even), female USB plug, and a 30 cent 5-volt regulator. For the 5-volt regulator, I recommend the 7805T. You can pick one of these up from Jameco, Digi-Key, or Mouser (among many other vendors). This regulator in particular can take an input of up to 35 volts and output up to 1 amp. If you're afraid of soldering, just tape it all together and it might still "work" (I'm sure a local hackerspace can get you up to speed on soldering though). For simplicity, I will say "positive" = red wire, 9-volts, and 5-volts. Then "ground" = negative, black wire, and 0-volts. The 5-volt regulator looks like a typical transistor. If you orient it to where you can read the label and the pins are pointing straight down, I will refer to left, center, and right pins. USB connections are simple; there are two data pins, one power pin, and one ground pin. Power is pin 1 and ground is pin 4. The pins should go from 4-1 left to right on the male plug. Connect ground of battery to middle pin of regulator with solder/wire. Connect positive of battery to left pin of regulator. Connect positive pin of female USB plug to right pin of regulator. Finally, connect ground pin of female USB plug to either middle pin of regulator, or ground of battery plug (it is the same connection either way). All you have to do now is plug a battery into the clip, and plug your USB device into the female USB plug. For people who need visuals, I'm sure there are some good write-ups on Instructables with some more frills (such as an on/off switch); I didn't invent this trick, I just understand it and am merely reporting it. Resources