 # efahrenholz

## Achievements

100+ Comments Earned a bronze medal
• Unless someone has done a real time test, mathematically solving this only works when the LED is on full till the battery dies. These devices use a trick to dim the light over time to make them last longer. However, lets do the math.The replacement batteries are 150 mAh, at 1.2v. Now, typically the way these devices work is they boost the voltage from the battery up to meet the forward voltage of the LED. This would be roughly 2.8 - 3.3v. The conversion process isn't perfect, so some power is lost. The cheap board on here is probably in the lower end of efficiency, so lets just assume it's 80%. If we take (100x1.2)/3, we get 40. In this case, that's 40% (1.2v is 40% of 3v). Lets take 150*0.4, which equals 60 (in this case, 60 mAh). Why we reduced the miliamp hours is due to converting to …

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Unless someone has done a real time test, mathematically solving this only works when the LED is on full till the battery dies. These devices use a trick to dim the light over time to make them last longer. However, lets do the math.The replacement batteries are 150 mAh, at 1.2v. Now, typically the way these devices work is they boost the voltage from the battery up to meet the forward voltage of the LED. This would be roughly 2.8 - 3.3v. The conversion process isn't perfect, so some power is lost. The cheap board on here is probably in the lower end of efficiency, so lets just assume it's 80%. If we take (100x1.2)/3, we get 40. In this case, that's 40% (1.2v is 40% of 3v). Lets take 150*0.4, which equals 60 (in this case, 60 mAh). Why we reduced the miliamp hours is due to converting to a higher voltage. We can't get power for free, we exchanged higher voltage for a lower amperage. The power is the same till we add in the drop in efficiency of the conversion. Normally, here we could say we have 60 mAh to work with at 3v, however we have to add in the drop in efficiency. So take 60*0.8, which gives us our final 48 mAh.If you are still with me, we have 48 mAh (roughly, really). Lets calculate the power to time. A typical LED at maximum current uses around 25 mA. From here we just take our 48 mAh and divide by 25, which gives us 1.9, or about 2 hours. This doesn't seem right though, does it? That's because these devices actually dim the LED using a Pulse-Width Modulation based on the amount of voltage reaching the converter. Towards the end of the night, the lights will appear very dim and will have a flicker to it. So sadly, there is no real way to calculate the longevity. It is far to complicated and requires a real time test.

• You did an excellent job. It was very smart to stick that LED matrix inside the glass case. Is that a display case for a model? You were very lucky it fit so perfectly. Congratulations.

• efahrenholz commented on RegisHsu's instructable Handy Power Supply

Unfortunately that won't work with this. This is a plastic chassis.

• Your Mac won't need a file cleanup before long because you won't be able to use it with that cracked screen.