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View Full Version : OT- off to the electric flying field



darryl
08-07-2011, 04:36 PM
Been looking lately for a source of single li-po cells. I have an idea to incorporate these into a power tool, and want to make charging a simple and clean process. I have it worked out electronically, but I need to use single cells and build the packs myself. I can buy 3 or 4 cell packs already made, but then I'd have to take them apart- I don't want to go that route. I also don't want to be committed to a charger that is covered with ports, has readouts, will do all battery types through myriad adapter harnesses, etc. I want to be able to build a pack that doesn't have wires hanging off it, hence the need to buy individual cells. And I want to be able to drop it into a charger, just like any cordless drill battery. I know all about the need for a discharge cutoff based on voltage, and the parameters of charging where no cell can tolerate overcharging or overdischarging.

I just remembered yesterday that we have an r/c club here, and today is electric only flying day. Going to check it out, talk to some of the guys, etc.

The local hobby shops don't sell individual cells, and they also are almost unanimous in saying that you 'don't want to build your own packs'. I've seen the videos where packs vent dramatically, go on fire, etc. One shows a nail put through a 2x4, then resting on the battery pack, and the dumbass cautiously hammers the nail right through the pack, then they get to say these cells are dangerous as it vents and goes on fire. Gimme a break- these guys are dangerous themselves, regardless of what technology they are playing with.

Anyway, I'm on my way to the flying field, should be interesting if nothing else. Maybe somebody will clue me in to a local source for these li-po cells.

rohart
08-07-2011, 04:55 PM
I'm in the middle of a similar project. My aging Nikon D1 takes a pack with six NiMh cels in it. New packs don't last long, and they'll stop being available soon.

I'm ripping old packs apart so I can either stuff in normal cells in an emergency, or fill them with single rechargeables. I'll then have to make my own charger, because I won't want to recharge them in singles.

Normal AA cells are a little longer and narrows than the similar industrial ones that are daisy chained in packs, so some new geometry is called for in my case.

This is one of those fits and starts projects. Another couple of starts and I should be almost there. If it all fits, that is !

sch
08-07-2011, 06:38 PM
A google on batteries will turn up a bunch of suppliers of Li rechargeable
batteries suitable for making packs. Be aware the cells EACH require a
little electronic protector to prevent over or improper charging and these
protectors will add a $1 or so to the price. Some cells are sold with
protectors built in or attached, some are not. Cells in the 18350/18650
sized range can be had for $7-10 ea and are about the right size to
replace C cell sized NiCd cells in rechargeable drill packs. They can be had with
solder tabs already attached and appropriate Li chargers are $15-20.
One site: http://www.batteryjunction.com/2600pcb-18650.html
but multiple others exist. The electronic protector ckt boards are very
small and available separately, and are independent of and in addition
to the charger circuitry.

darryl
08-07-2011, 07:09 PM
Just got back from the field. Nobody was flying because there was some wind. I talked to a few of the folks, and they were steering me towards li-ion instead of li-po. The usual things came up- li-pos are prone to exploding, are too easy to over-discharge and over-charge, etc. One fella brought out a li-ion pack which he claims is capable of a 100 amp discharge rate. The cells looked to be about the size of a C, maybe a bit longer but no wider. I'll have to see if this chemistry is available in a flat pack, which would be about the most usable for my application.

Someone mentioned lithium sulphate or sulphite- apparently not so dangerous and as capable as any high-output cell. I don't recall seeing any available anywhere, but maybe soon. Since the workhorse nicad, we've had nickel metal, lithium ion, lithium polymer, now lithium sulphite- Nickel metal seems to have fallen out of favor, with many manufacturers going li-ion now, and nicads being available mostly as replacement packs for the older cordless tools. So, I'm going to try finding li-ion in flat packages.

Back to the flying field- there was one guy who flew while I was there. Like most now, he was flying a styrofoam whatever- they look good and fly well, but are so light including the motor and battery that the wind just takes them. He had a hard time bringing his plane in to land anywhere near. When he did land, the plane was basically hanging there in front of him, airborne but with no forward speed. He basically porpoised a few times and dropped it straight down to the runway. I thought it had no motor until I saw the prop stop rotating. You couldn't even hear it. Wind had to be about 5-8 kph or so.

Had to laugh- reading here about wives wanting to post- on the field the wives were there, all facing away from the field, sunning and reading books. The guys all facing the field, bs'n and waiting for the wind to stop.

johnnyd
08-08-2011, 06:07 AM
Here is a link to some battery info that you may find of interest.
http://www.lithiumaviationbattery.com/:)

batt-man
08-08-2011, 08:14 AM
Have a look at http://www,dealextreme.com

They sell individual li-ion cells pretty cheaply in various styles...

Bob_W
08-08-2011, 09:30 AM
Darryl:

You may want to check out Hobby King or Hobby Partz for Li-Po's and chargers. The Hobby King Nano Tech series have very high current discharge and charge ratings.

The Hobby King site can be hard to browse for specific items but if you select the category "Li-Po, Li-Fe..." > "Li-poly (all brands)" and then click ANY battery and then select the yellow COMPARE button which shows up near the price you should see a listing with "Li-Po Finder" tool near the top of the page.

Good prices so you should be able to buy some and experiment without spending to much. You may have trouble finding single cells in the capacity you need however. I have ordered a lot from Hobby King without a problem, just watch you shipping weight and options.

http://www.hobbyking.com/hobbycity/store/index.rc
http://www.hobbypartz.com/

Found this 5000mAHr cell for ~$12:
http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=18562

or a packaged 5600mAHr cell for ~$25:
http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=17267

Bob

saltmine
08-08-2011, 12:43 PM
Something to remember; Each type of rechargeable battery requires a different type of charging procedure, or sometimes a different charger.

Using an old constant current NiCad charger to charge Li-po or li-ion packs can be extremely dangerous.

If the original pack in your Nikon D1 was Nickel metal hydride, you may not get a replacement NiCad pack to charge properly, using the Nikon charger, mainly because of the extremely high internal resistance of nickel metal hydride cells.

I went through this with cordless drills, when I replaced the NiCad pack in one with a Li-ion pack. The charger that it originally used almost blew up the new pack.(too much current, too fast)

Flying electric R/C planes has given me a whole new outlook on replacement battery pack technology, and charging requirements.

It's also worthy of noting that each pack has a different nominal charged voltage. NiCad-1.2V per cell, Li-ion 3.7V per cell, Alkaline 1.5V per cell

saltmine
08-08-2011, 01:09 PM
I've been flying R/C electrics almost as long as they have been available.

When I first started, we were fortunate to get one off of the ground, and make one circuit of the field before the NiCad packs of the time were depleted. My first experiments usually resulted in some pretty fast winged cars, before I finally found the formula to get a plane airborne.
(BTW, I have been flying R/C for over 30 years) Now, with the advent of more efficient motors and lightweight battery packs (I fly li-po packs almost exclusively) I find myself going to smaller motors and packs, and building larger planes to carry them. Yes, you read that right, smaller motors and packs, in bigger planes. I have one sport model that will fly for 45 minutes on one charge. It's a 53" wingspan scale model of the famous Piper Cub.

I used to fly and charge the packs at the flying field, but with that kind of duration, I seldom have to hook the charger up to my car battery.

My scale model of a De Havilland DH 82 Tiger Moth spans 44" and is powered by a tiny 250watt brushless motor. When flying it, I rarely use more than half-throttle. And yes, it will fly for about 30 minutes on a single charge. I also fly a 118" wingspan powered sailplane that usually requires about 1/4 throttle to climb nearly out of sight. I plan eventually to replace the motor in it with a smaller, lighter unit with an equally smaller battery pack. I haven't done it yet because adding a smaller power unit would mean adding ballast, which is mostly useless.

Of course, we have guys in the club that fly those foam 3D models that can't make up their minds if they want to be airplanes or helicopters...