Driving 12 - 15 LEDs using 4 CR2450 batteries.

[darkside501st]

Driving 10 - 15 LEDs, using 4 CR2450s or a cell phone battery

I was drawn to this site from the following post:

Originally Posted by barry82226 View Post
I'm new to this area. Ive been working on a circuit to turn on 6 leds and blink to conserve power. It will run in 10 to 15 second intervals running off an exising timiming circuit. I'd like it to run off of 2 cr2450 coin batteries or a 6v camera battery. Through my testing LED"s, i found that brightness is a function of current not always mcd's. Looks like 100mA drives most of the small LED"s I tested the best. I'm looking at led driver booster chips (linear LT3591) but not sure how much current it can handle. Also can i get that much current from coin batteries? Need long battery life too. I'm open fro suggestions on LED's as well.

How about giving us the LED's that you plan on using specifications. LED's do not always need the the full current for the intensity you want. As a side note I have built circuits that run over 20 LED's on 1 CR2450 battery.

This post was made on another thread back in 2011. I have searched for related articles but didn't find any. I was wondering if anyone could elaborate on how you can drive 20 LEDs on a CR2450 battery.

I will be using 12 to 15 ultra bright 5mm flat top LEDs. I am trying to use 4 CR2450s to drive the LEDs. I am not that technical when it comes to the intricacies of power supplies, required currents, etc. I would really appreciate your assistance if you have any help or advice to offer.

I have looked up a couple different listings for LEDs on ebay and here are the specs:

Listing 1:
color - White
Forward Voltage(V) at 20mA - 3.2 to 3.4
Dominant wavelength(mm)K at 20mA - 5000 to 6500
MCD at 20mA - 16000 20000
Reverse current(uA) Vr=5V - 10
Angle (deg) - 120-140

Listing 2:
Reverse Current (uA) : <=30
Viewing Angle : 180 Degrees
Absolute Maximum Ratings (Ta=25°C)
Max Power Dissipation : 80mw
Max Continuous Forward Current : 24mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V
Lead Soldering Temperature : 240°C (<5Sec)
Operating Temperature Range : -25°C ~ +85°C
Preservative Temperature Range : -30°C ~ +100°C

I have read a couple of articles saying that you can't get the required current from a CR2450 or any other coin cell to drive that many LEDs. Although, I have seen it done and the post I quoted said it is possible so I am trying to find out what is the best way. It looks like the forward voltage for the LEDs I want to use is between 3 - 3.2 volts and the recommended mA is between 20 - 25 mA.

I am not sure what would be the best way to arrange the batteries either. If I just hook them up in parallel to quadruple the capacity it would give the power source a voltage of 3 - 3.4 volts. Would that be enough voltage? I have read that if the power source matches the forward voltage you don't even have to use resistors. Although, when the batteries start to drain and the voltage drops below 3 volts the LED won't even light any more, right? That would leave a lot of unused capacity in the batteries. If that is the case maybe it would be better to hook up the batteries with two in series and two in parallel. This would make the power source 6v and it would double the capacity. I would have to use resistors for sure then. However, resistors unproductively consumes some of the power in the batteries though. Any assistance or recommendations you can provide would be a big help.

PS - this needs to be low profile due to the compact nature of the design. It is going to be used on an LED Arc Reactor. Like the one that Tony Stark/Iron Man wears on his chest.

 

[BioniC187]

How long do you want the led's to run for?
Yes, 20mA is roughly what is required for most led's to run bright
What are the current ratings of the battery you want to use?
Yes, 3 - 3.4V would be enough voltage. Current drain is what kills batteries, so having 4 of them set up in parallel, i am guessing, will hold up, how long though is not certain, because it is dependant on the current rating of each battery.
Yes you do not have to add a resistor to the circuit.
I would recommend trying to use a cell phone battery for this setup, its flat, so you can conceal it easilly and have good current ratings

 

[darkside501st]

How long do you want the led's to run for?
Yes, 20mA is roughly what is required for most led's to run bright
What are the current ratings of the battery you want to use?
Yes, 3 - 3.4V would be enough voltage. Current drain is what kills batteries, so having 4 of them set up in parallel, i am guessing, will hold up, how long though is not certain, because it is dependant on the current rating of each battery.
Yes you do not have to add a resistor to the circuit.
I would recommend trying to use a cell phone battery for this setup, its flat, so you can conceal it easilly and have good current ratings

Thanks for the reply. I think if I could get 4 or 5 hours of run time on four CR2450 batteries then I would be happy. Of course, the longer... the better. I might even be able to use fewer LEDs. Maybe 9 to 10 LEDs... it really depends on how well I can diffuse the light with such a small amount of space.

I was just going to order some of them off eBay so they probably would not compare to an Energizer brand. This is a generic brand datasheet for this type of battery:
https://www.electro-tech-online.com/custompdfs/2012/05/CR2450.pdf

A cellphone battery could work. That is what they used on the real prop. But I am not sure what to look for. Check out this video:
https://www.youtube.com/watch?v=o5LHIR8O3G8

The arc will fit in a cylindrical container that has an inside diameter of 3 1/4 inches. Does anyone make flat disc-shaped Lithium batteries that would be about 3 inches in diameter?

I have given some more thought to using a cell phone battery and I am now thinking that this is a much better option.

1. The battery would be rechargeable
2. I could set it up to be able to be charged via a USB cable
3. The battery would be easy to switch out with a backup battery to extend the amount of time the arc will light.

I read on another forum that you could use a **broken link removed** in order to be able to recharge the battery with a variety of input sources including being able to recharge it by hooking it up to a USB port.

Unfortunately, I am really out of my element with this. I mean I have a basic knowledge of batteries and LEDs but I don't really understand how this recharge controller works. Anyways, I am sure I can get some technical assistance from the company on that.

Now I need to find a cell phone battery that I can order a small bulk order on that will not be too expensive, that will be about 5mm or less in height, have a high mAh rating, and fit inside the 3.25 inch diameter casing. Has anyone set up a cellphone battery for something like this before? I did a duration under load web calculator and in order to get about 5 hours of run time on 12 LEDs it said I would need a battery with at least 1500mAh. Do you have any suggestions on the model of cellphone battery that will work for my application? Can you offer any tips or advice?

Again any advice on this would be a great help and is very appreciated.

Thanks

 

[BioniC187]

Spare phone, get a battery for that, so then you can just remove the battery from your arc reactor model and charge it in the phone, almost all phone batteries are +3.4V and have a current rating of+700mAH.
the batteris you have selected are 550mAH and 3.0 V.
Firstly the LED's will not be at their brightest, Coz of the battery voltage being at 3. You won't really notice it though.
So at 25mAH and with 2 of the cr2450 batteries in series, You get 2(550)/12(25) = 3hrs 40min meaning you should get that time use out of those batteries.
So now feasibility :
How often will project be used? If it will be like A LOT, then rechargable will be the best option, but if not, use the button cells, cheaper option possibly

 

[darkside501st]

Spare phone, get a battery for that, so then you can just remove the battery from your arc reactor model and charge it in the phone, almost all phone batteries are +3.4V and have a current rating of+700mAH.
the batteris you have selected are 550mAH and 3.0 V.
Firstly the LED's will not be at their brightest, Coz of the battery voltage being at 3. You won't really notice it though.
So at 25mAH and with 2 of the cr2450 batteries in series, You get 2(550)/12(25) = 3hrs 40min meaning you should get that time use out of those batteries.
So now feasibility :
How often will project be used? If it will be like A LOT, then rechargable will be the best option, but if not, use the button cells, cheaper option possibly

Well, a couple of my friends want me to make them one as well so getting a spare battery for my phone and recharging via the phone would not be the best choice cause then they wouldn't be able to recharge their batteries. We go to a lot of conventions and costume parties, etc so it won't be used everyday but often enough that the cost of the coin cells would add up. Not to mention that one of my friends would love to have this thing as a display prop so being able to plug it in is great. Also, rechargeable batteries are better for the environment as opposed to disposing of all those coin cells.

I have found several cellphone batteries for smart phones that have had a 1500 - 1750 mAh ratting. The trick is to find a site that gives you all the information (dimensions, mAh rating, etc). Most sites selling batteries ask for the make and model of your phone. You can't just browse cellphone batteries by size and capacity.

Another thing I have been wondering is if they make generic cellphone battery adapters/sockets/holders. Just like they make coin battery holders that you can attach to your project. It would make switching batteries much easier (like changing a battery on your phone).

Your calculation of 3 hrs 40min sound about right for 1100 mAh. But that would also mean that the voltage would be at 6v and not 3 so the LEDs would be pretty darn bright as long as the coin cells could provide the 300mAh rate of discharge. This is why I want to go with a battery that has a little more capacity. I am going to do some more research on this today so I will post back about what I find. Thanks again for your input.

 

[ronv]

I would think about a couple of AA batteries and a Joule theif circuit to drive the LEDs. AA chargers are pretty cheap and use can always use the batteries in the TV remote. I think Collin55 has lots of Joule thief circuits. Perhaps he can give you one for 10 or so LEDs.

Here is an IC for driving 8 from 3 volts. Really small so it may be hard to hook up.

https://www.linear.com/product/LT1932

 

[darkside501st]

I would think about a couple of AA batteries and a Joule theif circuit to drive the LEDs. AA chargers are pretty cheap and use can always use the batteries in the TV remote. I think Collin55 has lots of Joule thief circuits. Perhaps he can give you one for 10 or so LEDs.

Here is an IC for driving 8 from 3 volts. Really small so it may be hard to hook up.

https://www.linear.com/product/LT1932

Oh, I wish AAs would work... it would make things easy. Unfortunately, If I want this thing to have internal power it is gonna have to be by a battery that is about 5mm thick. AAs are a great deal thicker than that.

I have found several mobile type batteries that have anywhere from 1500 mAh to 2600 mAh but finding the dimensions of them is proving to be a bit more difficult. I may have to go to an electronics supply store and check out their mobile batteries in person. Once I get the model number of a battery that will work then I can hunt for prices and generic alternatives. I can't believe that there isn't a bigger market out there for this type of battery as a generic power supply and a site where you would be able to get what you need by looking at the dimensions and capacity of the batteries.

 

[ronv]

Can you make it with 7 mm? One of these would run 10 of your LEDs for 3 hours or so at 15 ma. Should be able to find a cheap charger on Ebay.

**broken link removed**


There are some smaller ones but of course the capacity is less. Guess you could do 2 sets of 5 LEDs??

**broken link removed**

 

[BioniC187]

Yes, my calculation is correct, but batteries in series do not change the voltage rating, so they would stay at 3 volts.
As for the battery holder, i fashioned one myself, just get the battery in a fix position that you can remove and put back easily, then you make the power terminals to the battery. based on how the battery sits in fixed position.
Yes going to the store to look at batteries will be the best idea, just get a few within the current rating spscs you want, and then choose an appropriate one according to its size

 

[darkside501st]

Can you make it with 7 mm? One of these would run 10 of your LEDs for 3 hours or so at 15 ma. Should be able to find a cheap charger on Ebay.

**broken link removed**


There are some smaller ones but of course the capacity is less. Guess you could do 2 sets of 5 LEDs??

**broken link removed**

I have looked at that type of battery pack and they are easier to search for and select the right size and power rating but they would be harder to remove and then you have the bulky wires and clips to deal with to. They also make those batteries with much higher mAh ratings than the ones in the links you provided.

I was just thinking in my previous post that there should be someone out there that manufactures and sells the mobile phone type of battery (with the hard case and the flat terminals) in the same way as these battery packs where you can look at them and sort them by size and capacity.

Yes, my calculation is correct, but batteries in series do not change the voltage rating, so they would stay at 3 volts.
As for the battery holder, i fashioned one myself, just get the battery in a fix position that you can remove and put back easily, then you make the power terminals to the battery. based on how the battery sits in fixed position.
Yes going to the store to look at batteries will be the best idea, just get a few within the current rating spscs you want, and then choose an appropriate one according to its size

Oh I know that two batteries in series would double the capacity and not change the voltage. I was trying to say that if I used four batteries and connect two in series and the other two in parallel then it would double the capacity and voltage. I could easily use for of those coin cell batteries not just two.

Anyways, I am pretty sure that I am going to go with a rechargeable mobile type battery. I know how to mold and cast so I could make a battery holder to fit whatever battery I settle on. Also, they sell the same kind of leaf spring or pin spring terminals that are in cellphones so I could get those and incorporate it into the cast design. It was just wishful thinking that they might manufacture a battery holder for this type of purpose. I think I will stop by a Fry's Electronics tomorrow and take a look at the mobile phone batteries they have... another possibility is a 3DS battery... it is the same kind as a mobile phone.

Thanks for the input guys. So after I find out about the battery and chose a model then the only other question I have is how to make it so that I can connect it to a USB port to recharge it. The only good option I have found is that MAX1811 chip. Another less viable option I came across was someone that just used a resistor to bring the voltage down from 5v to 4.2v. Although, I think that there is more to charging a Li-Ion battery than that though. I think the internal protection on these Li-Ion batteries just provides feedback as to what voltage the battery is charged to and then the charger cuts off the current when it reaches full voltage. If you just use a resistor then you can overcharge the battery and cause it to explode. Admittedly, I have not done a whole lot of research on the subject yet but if anyone can confirm or deny my conclusions then please do. Also, if you guys have any other alternatives to the MAX1811 then I would be happy to consider them. I like choices.

 

[audioguru]

I think you are making so many mistakes that your lithium cell phone battery will catch on fire and all your LEDs will burn out.
You need a lithium battery charger IC set for the absolute max voltage of 4.2V, not a resistor that allows the battery voltage to go too high and cause a fire.
Since the lithium battery is 4.2V when fully charged then you need a current-limiting circuit or resistors to limit the LED current and limit the battery discharging current to prevent a fire.

Some but not all cell phone batteries have a protection circuit inside. Batteries for RC helicopters and airplanes do not have a protection circuit in them.

Two batteries in series provide DOUBLE the voltage. Then you will have a lot of heat to throw away in a current-limiting circuit for the LEDs.

 

[darkside501st]

I think you are making so many mistakes that your lithium cell phone battery will catch on fire and all your LEDs will burn out.
You need a lithium battery charger IC set for the absolute max voltage of 4.2V, not a resistor that allows the battery voltage to go too high and cause a fire.
Since the lithium battery is 4.2V when fully charged then you need a current-limiting circuit or resistors to limit the LED current and limit the battery discharging current to prevent a fire.

Some but not all cell phone batteries have a protection circuit inside. Batteries for RC helicopters and airplanes do not have a protection circuit in them.

Two batteries in series provide DOUBLE the voltage. Then you will have a lot of heat to throw away in a current-limiting circuit for the LEDs.

I know we are covering a few different options here but I am not sure how people are getting mixed up on what I am saying. I was not suggesting that anyone or myself charge a li-ion battery with just a resistor to drop the voltage from 5 to 4.2... I even said in my post that I think that is dangerous and could cause the battery to explode. The MAX1811 is a current-limiting circuit of some kind... I don't know that much about it yet. But I provided a link to it in one of my earlier posts.

I did get mixed up in my last post on series vs parallel, I was going by what BioniC187 had posted. I just checked and you are right. Batteries connected in series increases voltage and batteries connected in parallel increases capacity. I did have it right in my first post though... I just get the terminology mixed up sometimes. Since the coin cells are only 3v and the LEDs forward voltage is between 3 and 3.2 volts the LEDs would not be at their brightest with all four batteries connected in parallel. I realize that using resistors to drop the voltage from 6v to 3.2v would have burnt up some power as heat. I wasn't even sure if I would have done it that way or not. It depends on how much difference it would have made in the brightness of the LEDs.

However, I am not even going to use the coin cell batteries anymore. As I stated earlier, I have decided against them. I think the mobile battery will work much better for the reasons I pointed out throughout this thread and also because the voltage on load is 3.7 volts and I could use small resistors to drop the voltage to 3.2 volts. So with my current plan I won't be burning up any LEDs and I wont be turning too much power into heat.

Believe me the last thing I want to do is catch something or myself on fire so I always do as much research as I have to in order to understand how things work before I jump in and start working. That is one of the reasons I came on this forum to ask advise from people that already know more than I do about this stuff. I am always open for constructive criticism and fair warnings. I just want to assure you that before I start this project I will understand it and any potential risks.

If you know of any other current-limiting circuits besides the MAX1811 or if you know of a reason why I should not use the MAX1811 then please offer this helpful information.

 

[audioguru]

A resistor in series with a rechargeable lithium battery does not limit the voltage. It limits the current. The voltage will go higher than the absolute maximum allowed voltage of 4.20V then the battery will become chemically unstable and catch on fire or blow up!

A resistor in series with an LED does not limit the voltage. The chemicals an LED is made from limits the voltage and the resistor limits the current.

An ordinary rerchargeable lithium battery is 4.2V when fully charged and 3.2V when its load MUST be disconnected then recharged. Its AVERAGE voltage is 3.7V.
Your battery will have too much discharge current (causing it to catch on fire) and the LEDs will burn out when it is 4.2V, unless the LEDs have something to limit their current.

Your LEDs are not 3.2V. They have a range of forward voltage maybe 3.0V to 3.6V.

 

[darkside501st]

Ok, thanks for clearing that up on the differences between voltage and current and what does what. Yes, it makes sense now that I think about it... I knew that if you charge the battery too long it would catch fire/explode and when you charge a battery it is the voltage that is increasing. So the voltage would continue to rise without a circuit or chip to cut it off after the battery reached a full charge (4.2v). So, if the battery has a built in protection circuit then can you just hook it up to a USB cable to charge it? From what I have learned about them I do not think this is the case but something you mentioned made me think that maybe I am wrong.

In my last post I wrote that the forward voltage of the LEDs was between 3 - 3.2 volts but the specified range on the LEDs that I am looking at is actually 3.2-3.4v according to the datasheet. I do plan on putting a resistor on each LED to limit the current so that the LEDs don't get damaged. The 3.2 - 3.4 forward voltage is for 20mA. So if I go with a mobile phone battery then do I need to use a source voltage of 3.7v or 4.2v to calculate the appropriate LED resistor?

 

[audioguru]

Some lithium batteries have a protection circuit inside that prevents over-voltage, over-current and disconnects the load when the voltage becomes too low. But it is not a charger circuit.
The cell phone has the charger circuit.

You need to calculate the LED current when the LED voltage is high and low with a battery voltage that is high and low. Do you need the maximum current in each LED to be as high as 20mA?
When the LED voltage requirement is high and the battery voltage is low then you might see that the LEDs will be too dim most of the time.

 

[colin55]

If you read the article Joule Thief <snip>
You will find the following circuit:
**broken link removed**
By winding the transformer on a larger core and increasing the supply voltage, the output will be nearly 30v for 3.7v to 4.2v supply. This circuit does not need a dropper resistor and the pulse operation of the LEDs makes them very efficient.

 

[darkside501st]

Some lithium batteries have a protection circuit inside that prevents over-voltage, over-current and disconnects the load when the voltage becomes too low. But it is not a charger circuit.
The cell phone has the charger circuit.

You need to calculate the LED current when the LED voltage is high and low with a battery voltage that is high and low. Do you need the maximum current in each LED to be as high as 20mA?
When the LED voltage requirement is high and the battery voltage is low then you might see that the LEDs will be too dim most of the time.

well from my understanding of LEDs, in order for the LEDs to reach their full potential they have to have full voltage and current. If the voltage was below 3v and the current was like 10-15mA then the LEDs would not be at their full brightness. However, I do not yet have these LEDs in my possession so it could be that at full current they are too bright or rather that I may not need the full brightness to achieve the effect that I want. They are rated at between 16000-20000mcd so that is pretty bright.

So are you saying that all these LED resistor calculators out there are not entirely accurate because all they ask for is the source voltage, forward voltage (voltage drop), and the required current for the LED?

If you read the article Joule Thief: http://www.talkingelectronics.com/projects/LEDTorchCircuits/LEDTorchCircuits-P1.html
You will find the following circuit:

By winding the transformer on a larger core and increasing the supply voltage, the output will be nearly 30v for 3.7v to 4.2v supply. This circuit does not need a dropper resistor and the pulse operation of the LEDs makes them very efficient.

Why would I need/want to increase the voltage to 30v? I have heard that pulsing the LEDs can help the battery last longer and that the human eye cannot see the pulse but I did not want to get into that if it wasn't necessary.

 

[audioguru]

I have some name-brand Fairchild red LEDs. At 20mA their forward voltage is spec'd at anywhere from 1.5V to 2.4V. Any LED resistor calculator will be completely wrong.
I can't find a part number for a Name-Brand white LED to see its datasheet.

LEDs are DIMMED by pulsing because then their average current is reduced by pulse-width-modulation.

 

[colin55]

Why would I need/want to increase the voltage to 30v?

In your OP you said you wanted to drive 10 to 15 LEDS.

LEDs are DIMMED by pulsing

But your eyes see peak. and persistence-of-vision takes over.

 

[audioguru]

If your eyes see the peak brightness of pulses then a PWM dimmer (there are plenty of them) will not work.
Persistence of vision works for pulses with a low repetition rate so that their duration is 30ms or longer. That is a repetition rate of only 33Hz.

The peak current in a multiplexed LED display must be much higher due to the dimming caused by PWM.