Observations about the universe, life, Lausanne and me

Friday, November 7, 2008

iBrick: external battery for the iPod

My ancient iPod mini only gets about 2 hours of battery life. A problem, since I'll be in the TGV for four hours today. The only solution: Quickly cobble together an external battery.

Thankfully I already head everything I would need: A 12 V, 1.2 Ah lead-acid cell for the power. An USB car-charger to convert the 12 volts down to an iPod-friendly 5 volts.
A switch, for convenience.

I connected them all up, and then used my friendly purple electrician's tape to hold everything together.

Success! How long will it last? Well if we assume a current draw of about 150 mA for the iPod (a number I just made up - but it shouldn't be too unreasonable), it eats 150mA*5V=0.75 Watts. My iBrick has 1.2 Ah at 12V, which is 14.4 Watthours. The car-charger is probably using an LM7805 chip or something similar, so we can assume a 90% efficiency for the voltage conversion. (14.4*0.9)/0.75=17.28 - about 17 hours. Good enough for me!

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