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Flashligh Battery Selection
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Flashligh Battery Selection

by Lord Cupcake on Mon Apr 12, 2010 11:06 pm

I have recently been designing a flashlight, and it requires 5V to run. However, I have very limited space, so I decided that I needed to run it off of a small 3V coin cell, such as the CR2032. In order to get the necessary 5V, I was going to modify Adafruit's mintyboost design, removing all but the 3V to 5V conversion circuit. This all sounded good until I found out that the CR2032 has a very low mAh rating.

Since this is a flashlight, I need the maximum lifetime possible from the batteries, but without sacrificing valuable space. I have 1.25X1.25X.29 inches of space to work with, maybe down to .2 inches in the height dimension. I need to be able to supply between 75mA and 300mA to up to 15 LED's each drawing between 15mA and 20mA, arranged in parallel, for extended periods of time. I do not want to modify my design, or have to make more space available. Are there any suggestions?

Any and all help is greatly appreciated.
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Re: Flashligh Battery Selection

by zener on Tue Apr 13, 2010 1:03 am

Do some mAh calculations and report back. You can't power it with wishful thinking.

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Re: Flashligh Battery Selection

by Entropy on Tue Apr 13, 2010 9:35 am

Lord Cupcake wrote:I have recently been designing a flashlight, and it requires 5V to run. However, I have very limited space, so I decided that I needed to run it off of a small 3V coin cell, such as the CR2032. In order to get the necessary 5V, I was going to modify Adafruit's mintyboost design, removing all but the 3V to 5V conversion circuit. This all sounded good until I found out that the CR2032 has a very low mAh rating.

Since this is a flashlight, I need the maximum lifetime possible from the batteries, but without sacrificing valuable space. I have 1.25X1.25X.29 inches of space to work with, maybe down to .2 inches in the height dimension. I need to be able to supply between 75mA and 300mA to up to 15 LED's each drawing between 15mA and 20mA, arranged in parallel, for extended periods of time. I do not want to modify my design, or have to make more space available. Are there any suggestions?

Any and all help is greatly appreciated.

In addition to a very low capacity, coin cells like that have very high internal resistance.

This is why you are able to safely run single LEDs off of CR2032 cells without burning the LEDs out.

It is also why you are not going to be able to run more than 1-2 LEDs off of a single CR2032 cell, no matter what you do.
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Re: Flashligh Battery Selection

by franklin97355 on Tue Apr 13, 2010 5:18 pm

I do not want to modify my design, or have to make more space available. Are there any suggestions?

Find another project?
Run wires to an external battery?

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Re: Flashligh Battery Selection

by johngineer on Wed Apr 14, 2010 12:09 am

Lord Cupcake wrote:Any and all help is greatly appreciated.


Have you tried a burnt offering, accompanied by prayer?

Seriously though, engineering is all about compromise. You may not want to make dimensional changes, but you might just have to.
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Re: Flashligh Battery Selection

by Lord Cupcake on Wed Apr 14, 2010 5:15 pm

johngineer wrote:Seriously though, engineering is all about compromise. You may not want to make dimensional changes, but you might just have to.

Sounded like words of wisdom, so I went back over my dimension calculations to try to find extra space. It turns out that instead of mounting my battery in the middle of my project, I could mount it on the side and use it as a handle. I could also use two AA's for my 3V, but that still leaves some lifetime calculations. I'm completely new to these calculations, so correct me if I’m wrong.

If AA's have 3000mAh of charge capacity, and Adafruit's mintyboost design has ~85% efficiency, and my load current goes up to 300mA, then 3000mAh * .85 = 2550mAh of current after the boost, /300mA = 8.5 hours of battery life on the highest setting. That would be way better than a CR2032, as it only has a charge capacity of about 220mAh, meaning that the lifetime would be more than 10 times less.
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Re: Flashligh Battery Selection

by johngineer on Wed Apr 14, 2010 9:50 pm

Lord Cupcake wrote:
johngineer wrote:Seriously though, engineering is all about compromise. You may not want to make dimensional changes, but you might just have to.

Sounded like words of wisdom, so I went back over my dimension calculations to try to find extra space. It turns out that instead of mounting my battery in the middle of my project, I could mount it on the side and use it as a handle. I could also use two AA's for my 3V, but that still leaves some lifetime calculations. I'm completely new to these calculations, so correct me if I’m wrong.


Good thinking about moving the battery.

Lord Cupcake wrote:If AA's have 3000mAh of charge capacity, and Adafruit's mintyboost design has ~85% efficiency, and my load current goes up to 300mA, then 3000mAh * .85 = 2550mAh of current after the boost, /300mA = 8.5 hours of battery life on the highest setting. That would be way better than a CR2032, as it only has a charge capacity of about 220mAh, meaning that the lifetime would be more than 10 times less.


Bear in mind that AA batteries do not put out a stable 1.5v throughout their discharge cycle. And you have to know if that 3000mAh is based on an average over the usable lifetime or over some other period, and the load that was used when it was calculated.

Calculations are quite valuable, but you're really going to want to know real-world performance before you fab this thing. So I would recommend breadboarding it and using a data collector to record the voltage drop of the battery. With a little work, you can rig up an arduino to record the data periodically over time, or send it to the serial port on your computer where you can view it in a terminal window.
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Re: Flashligh Battery Selection

by Lord Cupcake on Wed Apr 14, 2010 11:51 pm

johngineer wrote:Bear in mind that AA batteries do not put out a stable 1.5v throughout their discharge cycle. And you have to know if that 3000mAh is based on an average over the usable lifetime or over some other period, and the load that was used when it was calculated.

According to Adafruits math at http://www.ladyada.net/make/mintyboost/process.html, it shouldn't matter that much if the battery voltage drops with time. Since AA's are considered dead at ~1V, the lowest I should ever go is a total battery voltage of 2V. The boost regulator should still be able to boost voltages that low, and still deliver acceptable levels of current. It's probably just a matter of finding the right chip to optimize the output current.

johngineer wrote:Calculations are quite valuable, but you're really going to want to know real-world performance before you fab this thing. So I would recommend breadboarding it ...

I couldn't agree more. I am planning on trying a few different boost chips, but as I mentioned earlier, it would need to handle voltages as low as 1.5V (just in case I can squeeze out that last bit of juice) and be able to supply around 400mA (so I can have a little bit of overshoot room). I am planning to breadboard it first. Thanks for the advice.
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Re: Flashligh Battery Selection

by Entropy on Thu Apr 15, 2010 9:51 am

Stepping up to 5v and then using dropping resistors for each LED will result in a lot of lifetime being wasted by heating up the resistors.

Unfortunately driving lots of LEDs in parallel in an efficient manner is somewhat difficult. Is there a particular reason you are looking to use 15 or so 5mm LEDs as opposed to maybe a single high power LED such as a Luxeon Rebel?

In that case, there are plenty of constant current boost regulator circuits designed to drive LEDs such as this from a pair of AAs.
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Re: Flashligh Battery Selection

by Lord Cupcake on Thu Apr 15, 2010 4:18 pm

Entropy wrote:Unfortunately driving lots of LEDs in parallel in an efficient manner is somewhat difficult. Is there a particular reason you are looking to use 15 or so 5mm LEDs as opposed to maybe a single high power LED such as a Luxeon Rebel?

The main reason I am using 15 LED's is because I am basing this project off of a flashlight that I used to own, with a pentagon of green LED's in the middle, surrounded by a decagon of white LED's. This and a laser diode were powered by 3 AA's, with no boost regulator. I designed the same LED layout, minus the laser diode, with an adjustable 555 timer circuit, switching between 15mA and 20mA per LED, and the ability to independently turn each LED layer on and off. It also has the ability to switch between strobe mode and constant on.
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Re: Flashligh Battery Selection

by Entropy on Thu Apr 15, 2010 5:14 pm

Hmm, then why not continue to power it with 3xAA? Sounds like you're looking to do a head retrofit?
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Re: Flashligh Battery Selection

by Lord Cupcake on Thu Apr 15, 2010 5:39 pm

Not exactly. I am making a completely new flashlight, with the LED arrangement and only the LED arrangement being what I am basing off of the original flashlight. I was at first intending to do a head retrofit, but after first reverse engineering and replacing the inner 5 LED's of the original flashlight with a different color, I decided that a flashlight with more options would be better. The original had only 4 options. Inner 5 on, all except laser diode on, laser diode on, and all off. My new design, under no circumstances, could fit in the original case. Even if it could, I would have to drill holes for 2 pots and 6 switches, defeating the purpose of having the flashlight.

I don’t want to power it with 3 AA's for a few reasons. It is more expensive, It defeats the purpose of boosting to 5V, I don't want to have to make any more extra room than I need to, and there is already a design and parts list available from Adafruit that boosts 3V from two AA's to 5V, with an output current that is at the level that I need.
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Re: Flashligh Battery Selection

by zener on Thu Apr 15, 2010 6:18 pm

Here's what you need to do for your calculations:

First get the mAh rating of the battery. You say 3000 mAh which seems high for an AA, but there are many different kinds. Keep in mind this rating is based on a maximum discharge current of 1/10 or 1/20 of the rating. So for example, if the 3000 mAh rating is based on 1/10 of 3000 that means it will put out 300mA for 10Hr. However, if you draw more current then the mAh rating goes down. This is important to understand.

Now for your current calculations. When you step up a voltage, your input current to the boost regulator goes up proportionately. So for example, if you boost from 3V to 5V, that is a boost of 67%. So if your output current is 300mA, then your input current is 300 x 1.67 = 500mA (at 100% efficiency). Then you need to adjust for the (in)efficiency, so if it is 85%, then you divide your 500mA by .85 and you get 588mA. You now look up the mAH capacity of your battery at 588mA discharge current and you find it is 2000mA maybe (I am guessing). So that means you have 2000/588 = about 3 hours. HOWEVER, that assumes your batteries are 3V all the time which they are not. They will end up near 2V, so your input current goes up significantly as they die, at the worst possible time. The voltage will "fall off a cliff" toward the end. So that means you could maybe get 1 hr, likely less. You could do something with Excel to figure it out probably. Or just build it and see how long it lasts.

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Re: Flashligh Battery Selection

by Lord Cupcake on Thu Apr 15, 2010 7:22 pm

1 hour or less is unacceptable by my standards. This is going to be an "I told you so" moment for Entropy, but powering it with 3 AA's and no boost could be better. I can't use 2 AA's straight up because my LED's have a voltage drop of 3.2V, so using 4.5V from 3 AA's to power it could actually work. Looking at Duracell's Ultra AA data sheet, running my LED's at .39W and .3A, I could power my LED's for ~8 hours.

If I were to put the 3 AA's in series with each other, would they each individually take the full load of .39W and .3A, of would they each take a third of the load, with each battery handling .13W and .1A. If the latter is so, I could make my batteries last even longer, around 30 hours! If this is not the case, then would arranging the batteries in parallel do the trick?
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Re: Flashligh Battery Selection

by zener on Thu Apr 15, 2010 10:38 pm

If you put them in series the current goes through all of them (like water in a pipe). If you put them in parallel, then they split up the current, but the voltage doesn't add, so you just have 1.5 volts. Have you seen the price of Duracell Ultra? It might be cheaper to hire a lawyer to follow you around with a lantern. You might consider NiMH or even Li-ion. At least those you only have to buy once.

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