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### Topic: Making a USB charger of a Racing Pack (Read 4903 times)previous topic - next topic

#### marian42

##### Feb 06, 2013, 01:55 pm
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

#1
##### Feb 18, 2013, 06:56 pm
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

#2
##### Feb 28, 2013, 12:49 am
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?

#### Klaus_ww

#3
##### May 17, 2013, 09:26 pm
P=U*I
Calculate the power you waste with the 7805.
Freizeit-Programmierer mit moderatem Ehrgeiz besseren Code zu schreiben.

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