***DANGER*** - cheap&easy small batteries for small, cheap 24" styro glider
Just found an interesting battery source for off-the-shelf batteries for a cheap, small glider, such as the 24" styrofoam glider I got for $4.95 from my LHS and threw a couple micro servos into.
The 9V battery in my Fluke meter was dead, so I tossed it and put in a fresh one. Old one was a Duracell, I replaced it with an Eveready Energizer 9V.
Then I fetched the dead one back from the trash can and cut it open. Guess what... it had six tiny batteries inside, wired in series. Smaller than AA cells, smaller than AAA. These, I believe are AAAA cells. 1.5V each. Probably not a whole lot of milliampere-hours, and so probably not suitable for running a motor. But if you're just running a teeny receiver and two micro servos, these just might fill the bill.
NOTE: Here's a source that says that an AAAA battery has about 695 mAh. Caveat emptor: http://wapedia.mobi/en/AAAA_battery
Hacked around on the net, and found out that someone else found the same thing I did. See: http://blag.xkcd.com/2007/08/20/tes...ult-on-battery/
This one was dead, of course, so I got an Energizer 9V battery (fresh) and cut that one open too. Voila, same thing. Six teeny batteries! Well, three of them are going into this 24" span styro glider, as soon as I get back from a shopping trip to get my 6th grade son's school supplies (plus maybe a Duracell 9V batery or two).
One useful difference between the Eveready 9V and the Duracell 9V: The Duracell's six teeny batteries, are connected together with thin metal strips, what I call solder tabs although here they are tack-welded to the batteries, not soldered. The Eveready's six teeny batteries, are just in there with the terminals touching the ends of the batteies, not soldered or tabbed in any way, just the pressure of the metal case and insulators holding the contacts in place. When I cut open the Energizer case, three of them slid out and rolled around on the bench. Well, I guess it works inside the 9V case, I've never head of an Energizer cutting out suddenly.
But this makes the Duracell's teenies more suitable for putting in a 24" styro glider. I CAN solder to those tabs, after carefully cutting them where I need to, much better for a homemade installation! In my dark, distant past, I've been known to solder directly to the metal cases (and the button on the end) of various AA batteies that didn't have solder tabs. That's tricky, it can ruin the battery, and possibly even make it leak or go BOOM. Finding teenies with solder tabs already factory-welded on, is a great boon!
Last edited by Little-Acorn; Aug 25, 2009 at 11:58 PM.
Here's Eveready's datasheet for the AAAA Energizer battery.
Basically says that, if you discharge it at 10mA, it has about 650 mAh.
At 100 mA, it has 450 mAh.
At 200 mA, it has 200 mAh.
At 300 mA, it has maybe 160 mAh.
Bottom line, it's better used for fairly low-drain applications, not for powering a motor in 3D flight.
That's what threw me. And I stupidly didn't check the teeny batteries I got out of the Duracell 9V, but just assumed they'd be the same as the larger batteries they resembled.
There's no reason why Duracell should make them like the AA's, of course. They're not selling them as cylindrical cells, in fact they're clamping them in a sealed steel box where there's no way you could know what they look like at all. They correctly wire the outside terminals of the 9V battery. Who could possibly care how the teeny sub-batteries inside are made, as long as the 9V works correctly?
Moral of the story is, when using products in ways the manufacturer never intended you to, don't assume ANYTHING. Check first.
If I'd had my head out, and had done a simple polarity check, I'd have two good receivers instead of being down to one. And it's no one's fault except my own.
My warning post seems to have disappeared, something about "While I'm editing it". Well, I'm not editing it!
Here it is again:
I just tried three of these teeny batteries from a Duracell 9V battery, on the radio in my 24" span cheapo styrofoam glider. And guess what I found out the hard way: THESE TEENY BATTERIES HAVE REVERSED POLARITY FROM "NORMAL" CELLS!!! ON THE TEENY BATTERIES, THE BUTTON END IS NEGATIVE, AND THE FLAT END IS POSITIVE, at least on the ones I just tried.
I arrogantly (and stupidly) assumed that the button end was positive, and the flat end (actually the metal case) was negative, as real AA batteries are. Put togther the battery, put in a wiring harness, plugged into the receiver and servos, and powered up. NOTHING. DEAD. I tested battery voltage, found a weird reading like 1.78 volts, where it should have been 4.5V. And then the battery started getting hot.
Ripped out the battery connector from the receiver. Unplugged the servos. Measured voltage of the battery at its plug. Yup, backward. Took apart the battery pack and examined it VERY closely. Yup, black wire was connected to the flat end, red wire to the button end, all solder tabs correctly in place. Tested voltage directly on the solder connections: red meter lead on the button, black lead on the flat end. And yes, the leads were plugged into the meter correctly. Result: negative voltage, now back up to -4.8V. Measured each individual cell, same result: The button is NEGATIVE, flat end is POSITIVE.
Son of a BITCH.
Then I got a known-good battery (the AA cells I've used with this receiver many times during testing) and a known-good servo, plugged them into the receiver, and powered up. Still dead, and a low battery voltage, even though I knew this time the polarity was correct. Unplugged the battery quickly, saying unpleasant things.
I had another receiver identical to this one, so I pulled the crystal and swapped it over, then plugged in the known-good battery and servo. Worked perfectly. I picked up the old receiver by its antenna wire (something I never do), and dropped it in the trash can.
Then plugged one of the two Blue Arrow 4.3g servos that had taken the brunt of the negative voltage, into the new receiver. It worked! Plugged in the other, it worked too! Those Blue Arrow 4.3g servos are tough little bastards, they took the worst I could give them, and kept right on ticking!
MORAL OF THE STORY: Don't assume ANYTHING about the polarity or voltage of mystery batteries like the ones you get out of a 9V or 12V battery. TEST THEM EVERY TIME, NO EXCEPTIONS. I just found some that were different from what I thought the "standard" was, and my stupidity cost me a perfectly good receiver, fried.
Don't do what I did. TEST THE BATTERIES FIRST.
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