Compact rechargeable power for LED array

Hi all,

I'm working on a little project for my wife to make an LED hula hoop, controllable via Arduino for a little bit more flare. Now, I've assembled a few shields before, and can program the thing, but I'm getting stuck when getting down to hardware level (i'm a software developer and a newbie in electronics...), especially when needing to power this new project.

I'm planning to use Arduino Pro Mini, with about 20-25 of these controllable LEDs http://www.bliptronics.com/item.aspx?ItemID=113 According to specs, each one draws max 60mA. Ideally I'd like to use a few of AA rechargeables to run the thing for 2-3 hours, and expose some kind of plug to charge them - but that's where my knowledge ends. I assume I'd need some kind of 5V regulator since I can't as run many LEDs off Arduino itself, and how would I charge batteries without disassembling the entire thing?

Any help is really appreciated!

-- Sergey

Should you drive all 25 leds continuously at full power, they'll draw a current of 25 x 60 = 1500 milliAmperes, or 1.5 amperes an hour.

Quite a lot of NIMH-AA batteries can deliver up to 2700-3000 milliamperes. The arduino itself does take a few mA as well, but that's probably not worth counting.

So, you could drive a hoop up to 2 hours and probably even 2-3 times as long more since you won't drive all leds continuously, nor always at full power. (You should still be able see your spouse as well :D)

As far as I understand you won't need a special regulator, each led(-board) has a Plus and GND connection you directly connect to a 5 volt supply.

Recharging without taking the batteries out will probably require an special charging unit. Most commercial ones charge each batteries individually. You'll have to find/build one charging with more as 4.8 volts and preferably one that monitors the charging as good as possible. Bad charging = short lifetime and since the batteries are being locked up inside... changing them will be difficult.

Using "Already charged" batteries will probably best, after nicad-batteries NIMH batteries have slowly replaced the market. The early models NIMH discharges within a month or two by itself, in the long term it doesn't as charge good. The "already charged" ones discharge much slower and after having charged them a lot of times they keep behaving much better in my camera.

Should you drive all 25 leds continuously at full power, they'll draw a current of 25 x 60 = 1500 milliAmperes, or 1.5 amperes an hour.

Quite a lot of NIMH-AA batteries can deliver up to 2700-3000 milliamperes. The arduino itself does take a few mA as well, but that's probably not worth counting.

I think this is confusion between current and capacity. A batteries capacity is measured in mAh or Ah, which is really a measure of charge, not current. A 2700mA hour NiMH AA cell can provide 270mA for 10h (capacities are usually quoted at the 'ten hour rate' - run down a battery really hard and its capacity is reduced (and it may be damaged permanently). It does not follow it can provide 2.7A for 1 hour. It does not follow that it can provide 2.7A at all (it probably can, but at reduced voltage and generating a lot of heat in its internal resistance).

For completeness 1mAh = 3.6C (coulombs - one amp for one second = one coulomb)

Also good luck finding a battery holder that can take that sort of current - cheap ones use rivets and steel springs and can both lose a lot of voltage and be unreliable at high currents. There are more expensive rechargeable cells with solder tags and proper data sheets for industrial use - might be worth considering.

Won't having batteries in the hoop make it swing really funny around your wife's hips?

Only if they are all in one place - spread them out between the LEDs?

Will need at least 3 to get 4.5V. Sounds like a balancing challenge.
Take a look at your battery options here, such as batteries with solder tabs so you don’t need holders.
http://www.batterymart.com/p-powerizer-c-nimh-rechargeable-battery-solder-tabs.html

Then consider a charge control chip, maybe like this
http://www.maxim-ic.com/datasheet/index.mvp/id/4534

Oooh, this one looks good, simpler to implement with fewer parts.
http://www.maxim-ic.com/datasheet/index.mvp/id/3809

MarkT: Only if they are all in one place - spread them out between the LEDs?

Yeah, all circuitry would definitely have to distributed evenly around hoop, hopefully the least challenge in the project, as compared to charging the thing :)

CrossRoads: Will need at least 3 to get 4.5V. Sounds like a balancing challenge. Take a look at your battery options here, such as batteries with solder tabs so you don't need holders. http://www.batterymart.com/p-powerizer-c-nimh-rechargeable-battery-solder-tabs.html

Then consider a charge control chip, maybe like this http://www.maxim-ic.com/datasheet/index.mvp/id/4534

I like the capacity, but C size might be too large to fit inside the tubing. Looks like they've got AAs with solder tabs (http://www.batterymart.com/p-aa-2100-aa-flat-top-nimh-rechargeable-battery.html), that should work. Thanks for the tip about control chip!

MarkT:

Should you drive all 25 leds continuously at full power, they'll draw a current of 25 x 60 = 1500 milliAmperes, or 1.5 amperes an hour.

Quite a lot of NIMH-AA batteries can deliver up to 2700-3000 milliamperes. The arduino itself does take a few mA as well, but that's probably not worth counting.

I think this is confusion between current and capacity.

I was indeed calculating a time by just dividing the numbers. A burning hula hoop might be nice, but only when the dancer knows hows to handle it, so thank you for responding.

One thing I don't understand, while photographing (certainly when using flash) I have no problems emptying my AA batteries in less then an hour. It still show "replace batteries", so they aren't empty completely, but could I there for conclude a camera (well... mine) is more or less battery unfriendly ?

I don't think so - my relatively new Canon Powershot SX120 seems to go thru batteries a lot quicker than I'd like too. They are after all mini computers, between the display screen, auto focus, lens moving, writing all the megapixels to memory cards ...