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!