LED and battery swap question.

[justanuthercap]

Hey all, I have what is probaly a dumb question. But can't find the answer so here i am with my hat in my hands.

I have a flashing 5 LED set up that runs on 2 AA's. The project I'm using it on will not allow the AA's to fit. Can I replace the two AA's with a stack of "button" batteries? They actually will need to fit inside of a standard 12 gram CO2 cartridge.

Thanks for any help.

 

[greg883]

Can you fit 2 AAA cells? They will provide the same voltage (~1.2), only less AH capacity than the AA cells. But if you are willing the accept the very short life of button cells, that shouldn't be a problem.

 

[jasonw2112]

Well the 5 AA batteries come out to 7.5 volts. Depending on what the button cells you want to use, come up to this voltage, it should work. But you may sacrifice battery life.
The biggest problem that lies in these kind of battery swaps ( that I've had to learn! ) is not necessarly the voltage, but the mAh.


Edit! I'm sorry, I don't know why I was thinking you had 5 AA batteries! So 2 AA batteries equal 3 volts ( 1.5 volts each ) so 2 button cell batteries, at 1.5 volts each should do it! You will just have to sacrifice time, for the space of a smaller battey.
You will also need to wire the batteries in series, positive, negative, positive, negative.

 

[DrewSmith007]

Some 9V batteries have 6 AAAA batteries. They're quite small, and you could rewire them in parallel to get more mAh out of your setup. The AAAA's should be 1.2 - 1.5V each. I've torn several apart though, only to find out they're not full of AAAA's.

 

[thatdecade]

Answer is yes. If your current setup runs on 2 AAs, that is 3 volts.

Like Jason suggested. All you have to do is walk into radioshack and find some button cells that fit. Look at the packaging to see what voltage it is, probably 1.5V or 3V depending on the type.

If it is a 3V, your job is done. Wire one button cell into your LEDs.
If it is a 1.5V, then you'll need two to get the 3V you want.


The reason these guys don't want you using button cells is that they don't last very long. Maybe 6 hours at the very most, more typical is about 30 minutes of battery life.

To guesstimate the battery life, you can check the battery packaging for the mAh rating and divide by 30 (LED forward current). Example: A rating of 15mAh will give you a battery life of 0.5 hours (15/30=0.5).

 

[greg883]

That's why I was asking if 2 AAA cells could fit in the case he is using.

I have some NiMH AAA cells boasting 1000maH (probably a "perfect lab conditions" rating) in my LED flashlight right now.

Even if you only get 50%, that's still 15 hours (@30mA).

If the case is difficult to open, put a small jack (2-3mm) on the case and use it to recharge them without having to open it up.

 

[exoray]

To guesstimate the battery life, you can check the battery packaging for the mAh rating and divide by 30 (LED forward current). Example: A rating of 15mAh will give you a battery life of 0.5 hours (15/30=0.5).

As always this is a good way to take a wild guess, but in reality it's never right... Battery companies test their mAh under perfect conditions and optimal current draw that will optimize the life for the paperwork...

In your above example I wouldn't be surprised if real life you only get about 5-10 minutes, not 30... That is if the internal resistance of the battery allows it to work at all...

To increase run time when swapping to buttons cells the more cells the more power you have, using a serial/parallel setup you can optimize within the space you have...

You know you need 3 volts, most button cells are 1.5 volts, so you need them paired up in sets of two wired in series to get thee volts... You can then parallel wire the twin series bundles to increase your amp hours...

See the very crude picture bellow...

Another option is too look at the newer "TrustFire" or other knock off rechargeable Li-Ion cells that are becoming popular with mini flash lights... They are nearly the same size as AA or AAA batteries but put out about 3.7 volts on their own, so all you need is a single cell... 3.7 volts should (most cases) be fine in a circuit designed to run off two standard alkaline cells as there is a real life swing in alkaline voltage, most name brand batteries push closer to 1.7 when when fresh, so in the end the difference is a small fraction of a volt that should easily be tolerated by most circuits... If you are concerned put a diode on one lead of the battery (mind the polarity) it will drop approx .5 volts...