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# What battery should i use? Answered

So I started a little home project where I am using 28 LED lights and I need to know what kind of battery I should use.
The lights I have are 5mm, 3-3.2 volts, 20mA-30mA. I want to wire the lights in a parallel circuit for each one. I want the LEDs to be bright! They are rated at 2,500 mcd @ 20mA.
-I need a battery that is a very small battery less than 1"x1"x2" or very close to that.
-Also is it possible to use a battery that is a 3.0 volt battery that is something like 2000 mAh
- I did the math and it seems that my mAh is 560 because (20mA)x(28 LEDs)= 560 mAh. Is that right? if so could I use a battery that is 3 volts and 2000 mAh and have a battery life of 3.5 hours? because 2000/560. is that right?

So in a summary, I need a small battery powerful battery that can EQUALLY light up 28 LEDs and if so what kind of resistors would i need if I use a battery that is 3/ 6/ 9/ 12 volts.

Also do i need to have a resistor for each LED light or can i have one at the end of all the parallel circuit?

Thanks!!!!

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## 11 Replies

jduffy54 (author)2012-05-31

get a few 123 li-io batteryies. They are 3V, and will light up your leds. Most LEDS will also run at 3.6V, so three rechargable AAs will work well too. for size constrains, I'd suggest a rechargable 3.6V cell phone, mp3 player, or rc helicopter/plane/mini car battery.

snotty (author)2012-05-22

EZ!!! (well, except the battery size requirements)

There are a couple good ways to do this. I think you should start off with easy parts just to get the project working and then refine it with a smaller battery etc.

There are two parts to this project:

Part 1: Power supplly
Get enough battery power to make it go. Your math looks ok except "20mA)x(28 LEDs)= 560 mAh" has an extra h at the end. "mAh" indicates how long a battery will function at a given constant draw. Anyway for your project I'd advise starting off with 4 rechargeable (and easy to use and cheaper) NiMH AAs in a flat case. They're 1.2 volts (more like 1.35 when freshly charged) which gives you around 5 volts.

This is too much to run straight into your LEDs but you need a little headroom with the voltage because of...

Part 2: Regulate the current.
I started off using a couple of resistors. Find an LED resistor calculator that uses one resistor for multiple LEDs. This is a bit dangerous since it can kill all of your LEDs once a few of them blow or get disconnected. But its easier than soldering one resister per LED and with that many LEDs a difference of one (or two or three) LEDs won't cause a major overload in my experience.

Alternatively. Use the cheap driver that Dan designed: https://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/ I use this a LOT. There are three of them on my bike helmet.

Finally once you've got a sense of how it works, then refine with fancy, expensive lithium batteries and a buck/boost driver or whatever.

Your battery size constraint may be a tall order. You can experiment with cellphone batteries, standard 9v batteries, A123 cells and all that. But lithium batteries are a deep rabbit hole and there's lots to know about charge and discharge protection and exploding polymer batteries.

steveastrouk (author)2012-05-22

Its not feasible, with what you have. You need a LOT of current to run them in parallel - 28 x 20mA , 560mA - and from a very small battery. Your battery has to be more than 3V, because you need to overcome the forward voltage of the LED to make it light up properly, and once its lit, you HAVE to use current limiting resistors.

mh76dk (author)2012-05-17

As for the battery size, you can just connect a few coin-batteries in series to get the desired voltage (that is assuming they give you enough umph) even 5 or 6 of them should be within your stated size limits

jnajmy (author)2012-05-17

do you think if i put them in series until i got 3 volts and then did parallel circuits to the battery so i could raise the mAh it would have equal brightness to a 6 volt battery?

Do you know what makes an LED bright? is it the voltage or the mA going to it?

Thanks for the help!

steveastrouk (author)2012-05-22

The current. The voltage is pretty constant. DON'T put them in parallel across the battery, you will kill them

mh76dk (author)2012-05-17

I am sorry, I cannot answer that. Perhaps someone else will

nutsandbolts_64 (author)2012-05-20

I shall pass my 3rd-hand knowledge about LED's to you lol.

Anyway, LED = Light Emitting Diode. A diode has a forward voltage drop (whatever that means) which I take as a "minimum required voltage". For your LED's, its forward V drop should be around 1.7 volts.

Now, your math all seems well, but I haven't heard of a battery that's small which goes up until 2000mAh. That's about 2 amp hours you're talking here. A lantern battery would be your most efficient choice (although somewhat expensive, at least for me) since it gives 6V at 4Ah.

This means your circuit should involve 2 parallel circuits connected in series. W/ the 2 parallel circuits connected in series, each parallel circuit would be supplied with a good 3V. Now, each parallel circuit's LED would receive 142mA each. Way over the limit, so therefore you must add a resistor to each parallel circuit.

According to the calculator, use 4.7 Ohm resistors w/ a good power rating of 0.607 watts for safety, but calculated is 0.364W. So yeah, a 1/2 watt resistor power rating would be nice.

To be safe, wait for more answers lol. I can't guarantee your LED's' safety.

mh76dk (author)2012-05-17

I was lazy and just searched the web for "led resistor calculator" and found quite a few, here is one example: http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

I hope that helps (if the random one i picked from the list of hits arent exactly what you want, try your own search for similiar keywords)

jnajmy (author)2012-05-17

thank you for that link, but that is not really what I am after.

I think i am going to use two 1.5 volt batteries then wire multiple batteries to those in a parallel circuit. would i need a resistor if i do that because it would be 3 volts (with a lot of mAh) and the LEDs are 3 volts

Thanks for the help!

mh76dk (author)2012-05-17

Well, my "help" consisted of searching for your answer. I do not know enough about the subject to give any personal advice, so id rather not start speculating.