I calculated that that my cellphone battery (3.7V, 1.23Ah) can store 16.4kJ of energy and a battery for an RC car (7.2V, 3Ah) can store 77.7kJ, which could recharge the cellphone a few times.
So I was wondering if I could make a mobile USB charger of a Racing Pack and a 5V voltage regulator.
My questions are:
Does this work at all?
Can I damage something (for example with a too high voltage)?
What happens if the racing pack is discharged? Can I damage the phone / usb device with a too low voltage?
How can I determine when racing pack is discharged?
Hi. If you use a standard voltage regulator, you will waste a LOT of energy this way, and regulator will get hot and require heatsinking.
I would recommend a module like this http://goo.gl/nLeVz - they are very good at this job.
Please be sure not to over-discharge your battery pack, or you will hurt your batteries. You could make an auto-switch off circuit with some resistors and a transistor that turns off the load when the voltage gets too low.
Looks to me like a problem would be the dropout voltage of 2V. I read this to mean the 7805 will stop outputting when the input voltage dips below 7 volts. That discharging RC battery wouldn't last long like that, would it?
Would 7.2V really be a heat issue at that low of a power draw?