I have a nokia n78.I often use the gps onboard to find my way through the urban jungle of mumbai.The gps is surely a very useful tool for navigation but it tends to suck the battery juice pretty quickly. My phone can give only about 3-4 hrs of continous gps use on a full recharge.I’m a urban explorer kind of a guy so i like getting lost and found in the concrete jungle but that sometimes takes a lot of time.so much time that my battery runs out before my adventure is over!!. and so i decided to make myself a portable mobile charger.
Lets get to the basics of how the Li-ion/Li-po battery inside the mobile phone/gadget is charged. The wall adapter charger supplied with you mobile/gadget is basically a 5-7 volt regulated voltage supply.It may also be an smps providing a constant volatage.this volatge is not given directly to the battery but fed through a charge controller. the charge controller basically is another regulator that charges the battery at the precise voltage it requires.
http://www.batteryuniversity.com/partone-12.htm for more on Li-ion batteries.
This charge regulator is present in the phone itself. The charger we are going to design is going to supply regulated 5 volts to this charge regulator which will then charge your battery.
The simplest way would be to use a 7805 with a 9v battery. As shown in the schematic below.
charger using 7805
This is obviously a simple circuit and also the components are cheap. Well i found out that the the 7805 is a very inefficient device .It dissipates a lot of heat.I recon it might have an efficiency of about 60% or so and also the amount of charge that gets into your mobile is also very less.Also the capacity of an average 9 v battery is around 600mAh.To charge my mobile i connected the 2.5 mm charging connector i salvaged from an broken ca44 cable.The smaller end is the 2.5 mm male plug connector.The outer terminal of the barrel jack is the negative. while the inner terminal is the positive. this charger really works but the amount of charge that this circuit will provide is relatively small as compared to a boost regulator based charging circuit which will be my next post.