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Old 07-18-2008, 12:58 AM #1
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Default Calculating battery length

I am going to make a laser much like drlava's posted here http://www.laserpointerforums.com/fo...num=1209532949.
I'm also using his driver, which from what I've heard was 88% efficiency.
I also heard mAh is MilliampHour or how much current a battery will discharge over a period of one hour.

I'm planning on using batteries similar to http://www.amazon.com/Energizer-Rech...917&sr=8-4, or 850mAh.
I dont think my local walmart has eneloops, the arguabley best battery.
So... 850x2 (using two aaas) = 1700mAh
The driver is 88% efficient and I'm driving my laser at 300mA. I'd multiply 300mA by 1.12 to get 112% (because the driver is 12% inefficient) to get 336mA

I divide 1700/336 and I get 5.09523... or 5 hours.
And since batteries sag, would it be reasonable I could expect anywhere from 3-4 hours?

Thanks


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Old 07-19-2008, 07:32 PM #2
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Default Re: Calculating battery length

No

The inverse of 0.88 is 1/0.88=1.136, not 2-0.88@!

Then, you have to place the batteries in series, not parallell, because lavadrives need more than 1.5V. So it's still 850mAh, not 1700.

Then, when your diode needs 300mA @ (for example) 4V, then the driver supplies 0.3*4=1.2W, so it consumes 1.2*1.136=1.364W, which boils down to 1.364/3=454.5mA

This gives you 850/454.5=1.87Hr time at most.

[edit] And... batteries don't 'sag' with a driver, and especially a boost driver, like they normally sag in a flashlight. The driver keeps slurping power from the battery until at last the voltage drops below 2.5V (which is when rechargeable batteries are below 10% capacity. And My guess is that you have rechargeables because they normally don't give mAh for alkaline, and if they did it would be a terribly weak one)
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Old 07-20-2008, 06:40 AM #3
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Default Re: Calculating battery length

I forgot that when batteries in series do not double, however they do in parallel.
I also didn't even think about that I was using a driver! :-[
Thanks for taking time to properly solve the problem.

I apologize for my misunderstanding.
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Old 07-20-2008, 06:44 AM #4
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Default Re: Calculating battery length

Battery length is usually calculated by a ruler.

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