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# what button cells can you recharge?/ super capacitor help Answered

what button cells can you recharge? For example, some are in calculators wtih solar panels, do they need a complex charging circuit? I'm building a bunch of tiny circuits and I don't want to keep on changing the battery. edit, I now need help with super capacitors

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thanks for the bump NEW CALCULATIONS I think I need a 3.1482 Farad Capacitor for my application, because doing some bench test, a 33000 uF capacitor charged to 5 volts takes 20 seconds to discharge down to 2.7 volts, the threshold point for my IC to work. The resistor I use for the test was 1k, so it leaked about 5 ma at 5 volts, sloping down to 2.7 mA... ahh crap, there goes my calculations, I forgot to slope the mA... I still need help now!!!

im missing some info to do the math. how long do you want to run the circuit for? if one minute, you need a 99 000 uf capacitor, allowing for ideal situations and maximal charging level.

Here's the whole thing. I'm trying to make a dream machine that allows you to lucid dream by pulsing and led for 1 second every 5 minutes after the first 5 hours. Lets say you sleep in, and say you get about 10 hours of sleep, so the capacitor need to last that long. the led will flash 60 times before the capacitor is discharged, or take up the same amount of energy as being on for a whole minute. and if I run it at 15mA it will consume 15 mam, or 0.25 maH. The uController consumes 0.24 ma, so if it's on for 10 hours thats 2.4 mah. The minimum voltage for the uController is 2.7 volts. So overall mah needed is 2.65 mah and the voltage can't go below 2.7 volts, preferably 3 volts so I odn't need fancy circuitry to drive the white led. It will charge off of usb, so the capacitor will get charged to b5 volts. I need to know how many farads I need to accomplish this.

oh, and the problem with my test with the 33000 uf capacitor was that using just a resistor, as the voltage slopes down, the current does to, and I didn't take that into my calculations, and to be honest I have to idea how to calculate it.

oops, one thing wrong with what I said when I said whole thing I actually have 2 leds runing, so it'd be 0.5 mah instead of 0.25mA so, a grand total is 2.9 Mah

why dont you just attach several supercaps in paralelle? start with 2 or three, and add more as needed.

With a supercap, you're also going to be affected by the rather non-ideal discharge curve; your cap will still contain 1/4 of the original energy when it's at half its original voltage...
You know that Cooper Bussmann, one of the supercap vendors, has a nice spreadsheet for this sort of thing?
At ~5mAh/day power requirements, a typical non-rechargable battery will last quite a long time...

I might just go with a watch battery, I mean an average cr 2032 battery (get a bunch from dealextreme, 3.57 for 20, probably 15 cause some will be dead, but that's a great deal. One battery could power my device for about 800 hours, or about 80 nights, which is a terrific deal! This equates to about 0.2 cents a night to keep it on, I'll definately go with a battery, thanks!

I'd just use a capacitor (designed for this sort of thing).

Now that I think of it, I remember seeing a LED light that you plugged into the USB port to charge -- it was a really simple circuit: a button, an LED, and a resistor or 2)

it was the super capacitor light instructable, right? That's actually a pretty good idea, I'll look into how many farads I need for my aplication. Super capacitors are a bit more expensive than a cell though...

to find out what size cap I need, does anyone know how many farads to a Mah? For example, how many Mah does a 1 farad cap have at 5 volts?

I just calculated it, and I need 2.65 mah, so how many farads is that?

can someone double check me on this, do I need a 7.3611111111111111111 Farads for this

capacitors give almost unlimited amps, but only for a short burst of time. they are fairly unlimited in that respect

As for batteries, there are places like this that sell these batteries, but that is only an example, I know nothing about the company.

bump Come onnnnn.... COME ONNNNNN

They make rechargeable watch batteries. My friend's dad was a jewler, and he used them