"high capacity" battery powered USB charger

[twobad]

I would like to build circuitry for a battery to charge cell phones via USB interface. I have seen designs for 9V and AA battery chargers. They are "elegant" in form factor and light weight.

However, I am looking for longer charge/run times and the ability to charge several phones simultaneously. I would like to use a sealed lead acid battery, something like one of these

http://www.portablepower.com/6_volt_und ... vctgjl7fm5

in either 6 or 12 volt DC (whichever would be better/easier), I'm looking for 5 volts DC to my USB output(s).

This would be used for camping, at the beach, 4 wheeler riding etc.- all those places where everybody starts with a fully charged cell phone but after a few hours everybody's is dead, almost at the same time. I like to use the GPS function of my phone to record my offroad tracks and after 5 or 6 hours the battery is "toast." As the phone is tucked away in storage anyway, having it connected to a medium size battery while running would let it use the external battery during traveling and still have a full charge for "normal use" and/or charging at the destination.

I have seen a design that utilizes a zenier diode and resistor and another that uses a 7805 voltage regulator (although the 7805 VR design is said to require more than 7V input to function so that makes the 6V battery less attractive. Also, I read that the 7805 simply looses through heat, all voltage not required by the device)


Advice is appreciated!

Thanks,
-=T=-

[I_Daniel]

The easiest is 12 Volts as most motorcycles and motorcars use 12 volt batteries.

A circuit using a LM7805 is all that is required. There are a number of circuit diagrams for ac to 5 volt using this regulator. You merely require the part of the circuit using the LM7805 with the two capacitors. You can then connect one of those PC frontpanel USB port(s) to the output. With a proper heatsink the 7805 will handle 3 Amps.

The input will be the 12 Volt battery instead of the output from the transformer and diodes which are not required for your purposes. So basically you require only four components. The LM7805, two capacitors and the USB connector(s). Plus of course the box you will house it in and crocodile clips on the input-side to connect to the battery.

All regulators dissipate heat whether they are discrete components or an IC so the IC must be mounted on a heatsink. if Not the LM7805 can only deliver about 100 milliamps without a heatsink.

[twobad]

Thank you. Since my original post I have disassembled a factory made cigarette lighter micro usb charger and am examining the pcb and componets. Your design seems to be less work and more what I'm looking for. Thanks again!
-=T=-

[newtonph]

That's really sounds like a brilliant idea Daniel. I would try to make that experiment for my students.

[I_Daniel]

@ Newtonph

Since you wish to make this a school project you could add the following -
Q. What can be done to prevent reverse-polarity from the power source (Battery)
A. As a safety measure and to prevent reverse-polarity which would blow up the circuit the bridge rectifier could be left in circuit or one 3-5 amp diode could be inserted in the positive line. (of course the bridge rectifier is the safest - because reverse-polarity would not even be noticed. Check the current flow when the Battery+ is connected to either of the ~ (AC) inputs- on the output side positive remains positive)
******************
Please note - Most 7805 IC's only handle 1.5 Amp. But higher Amp 7805's are available.
The 7805 is a positive regulator. I haven't seen a positive earth car for ages but if so a 7905, which is a negative regulator can be used) I say this because a cigarette lighter's centre pin could be positive or negative, depending on whether the car has a positive or negative earth system. (i.e. earth-, pin+ use 7805 ; or earth+, pin- use 7905)

[LynBat88]

Thank you, I appreciate your advice