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# How to use a rechargeable battery in a circuit? Answered

I want to use a rechargeable battery in a circuit, but I'm having trouble understanding it. Let's say I want two switches... One to use an alkaline to charge a rechargeable battery, and another to power an LED using that charged battery... can some draw the absolute SIMPLEST way to do that using a AA battery and this battery:
http://www.coolight.com/LIR2032-Rechargeable-Coin-Cell-Battery-3-6v-p/1xlir2032.htm

(The LED works fine with 3.6V)

Remember, I'm very dumb, don't assume I know ANYTHING!

Thanks!

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Ok, well... right now I'm working on capacitors... I have a 25V 120uF capacitor that I'm trying to charge with two AA batteries... no matter how long I let it charge though, it never gets any much voltage, it won't even blow out a 3V LED which leads me to believe that it's only putting out as much as I put in... but I thought the whole purpose of capacitors was to build up lots of energy... what am I doing wrong? (I just have 2 AA attached directly to it...)

Have you looked at my instructable?

Capacitor LED circuit

Try the equations I give using your capacitor values to see how much energy you are storing.

Depending on what type of AA's you are using (alkaline, NiMH, Li-ion etc) you may not have enough voltage to meet the forward voltage of the LED. Does the LED light if you connect it directly to the batteries?

Yes the LED lights, but that's not the point... the capactior should blow the LED away... it's 330V... you use a 5(.5)V capacitor with 5V USB, so that's not quite what I'm after... I need to know how to recharge my disposable camera capacitor... How do I do that?

If you'd have asked this question at the start, you would have got the right info sraight away! The circuitry between the battery and the capacitor steps up the voltage. That's why the capacitor on the camera flash is rated at 350V or higher.

Uhh... what? I have a 330V capacitor, but it never chargers to that much, I can only get exactly what I put in... Let's put it this way, I have a 330V capacitor, two AA batteries, and some wire, and nothing else and let's pretend I want a massive shock. How do I do it with only what I have?

You don't. You need a circuit to raise the voltage. If you didn't need anything else, why would the camera have the extra circuit?

To do all the other... camera stuff? Ok, I'm going to Radioshack tomorrow. NOW what do I need? Transistors? How many and what kind? Resistors? What? Thanks

Charging Li-ion batteries is somewhat complex, and even dangerous, since they are rather prone to exploding if charged improperly. (those batteries are much larger than the LIR2032, though.)
More than you ever wanted to know about lithium batteries From the RC Forums Group

If you're "very dumb", your best bet is to buy the LIR2032 charger from coolight as well, and get IT to run off batteries (which is probably a simple matter of replacing the wall wart with the equivalent number of batteries...

I DON'T WANT TO BUY A CHARGER! I'm sorry for getting upset, but I WANT TO KNOW HOW IT WORKS! I WANT TO USE IT IN MY OWN CIRCUITS! I don't understand how to do it, and before I start using it, I want to understand it! It's been done many times here with the tic-tac light and the usb light (well that was a capacitor, but similar) I just want the very BASICS of how to do it!

To charge a capacitor, you simply connect it to a voltage source less than the maximum rated working voltage of the cap and let it charge. Trivial. The cap will stop charging all by itself (current goes to zero.) You can tell exactly how much a cap is charged by measuring the voltage.

A good battery has a relatively flat charge/discharge curve, which means that it's hard to directly relate cell voltage to amount of charge.

For Lead-acid batteries, you use a constant voltage source slightly higher than the rated voltage of the battery. Current goes down as the battery charges, but never goes to zero, just to a smallish "float charging" level.

NiCd and NiMH batteries are charged with a constant current source, and you have to use relatively complicated scheme for detecting "fully charged", especially if you want to do a full charge in less than 12 hours (lots of cheap chargers just keep charging after the battery is full, but that's only "allowed" at the C/10 charge rate...)

Li-ion and Li-polymer batteries MUST be charged using a three phase scheme: First (if necessary) you trickle charge till the voltage reaches about 3V, then you do a current limited charge till the cell reaches 4.1 or 4.2V (depends on exact type of cell), and then you hold the voltage at 4.1/4.2V until the charging current goes to near zero or a timeout expires. A typical CCCV lab power supply does a pretty good job if you watch it carefully. The simplest circuit I've seen that comes close while using "traditional" components is a circuit by SH Designs for the RC crowd. There are MANY special purpose chips from various vendors that do most or all of the difficult parts. Li-ion/Li-poly cells should (ideally) be charged one cell at a time, and battery packs typically contain fancy circuits that cause this to happen (along with limiting other error conditions that would otherwise damage the battery or the people around the battery.)

Except for the capacitor, it's NOT easy stuff to understand.

Thanks, that works great, but one more thing... I don't really want to know HOW much charge the battery has, I just want it to have a charge... Does that work just like a capacitor?

I don't understand any of that... I just want to know the SIMPLEST way to charge and use a battery or capacitor in a circuit!

Why do you want to charge a rechargable cell/battery with an alkaline cell/battery? This doesn't make sense. L

I don't... I want to know how it works. I see no point in wasting a USB cable if I can't even get it to work.

wouldn't it make more sense to charge the rechargeable battery off from AC power or even USB? that way you can plug in the light to charge up the battery and then unplug it to go mobile thus not needing to replace the battery for a very long time (if you are using a NiMh Battery) that would be my suggestion.