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Topic: Making a USB charger of a Racing Pack (Read 3362 times) previous topic - next topic

marian42

Hi,

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?

Zapro

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.

Good luck with the project.

// Per.

meanpc

I've been reading the datasheet for the LM7805 -

http://www.fairchildsemi.com/ds/LM/LM7805.pdf

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?
http://www.meanpc.com

Klaus_ww

P=U*I
Calculate the power you waste with the 7805.

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