Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I want to build a mobile charger that is battery operated and found this diagram. At first glance, this diagram looks kinda simple and I thought that this won't work. I'm a newbie on making this kind of projects and its look like a simple one so I want to know how and why this diagram works. If this diagram really works I really need help in converting it into a schematic diagram.enter image description here

and also what is the diffference between the two in charging power?...diagram


share|improve this question
... Some people will watermark anything, won't they... – Ignacio Vazquez-Abrams Aug 31 '14 at 22:14
But on topic, they're both pretty terrible, and nothing I'd use with a battery. – Ignacio Vazquez-Abrams Aug 31 '14 at 22:16
are you suggesting me that this both diagram is complicated for a newbie or both only work on ideal conditions? – Kevin Aug 31 '14 at 22:26
They will both drain your battery, and waste most of it as heat. – Ignacio Vazquez-Abrams Aug 31 '14 at 22:37
Well I really want to build a portable mobile charger that is powered by DC so i could charge on emergency instances (power breakages, middle of the journey, etc.) – Kevin Aug 31 '14 at 22:40
up vote 0 down vote accepted

The simple, first circuit will deliver 5V at 0A, and be limited to 1A but voltage will drop off rapidly with increasing load; at 1A output, it'll be delivering only 2.5V.

The more (needlessly, I might add) complicated drawing will deliver 5.6V at 0A and voltage will drop to about 4.91V at 180mA.

Neither will regulate very well, and I don't think either will do what you really want it to do.

Let's back up and find out exactly what you want to charge. Battery voltage, chemistry type, and current rating. Different batteries require different chargers, and an inappropriate one may damage batteries or start a fire.

share|improve this answer
My mobile's battery is a Li-Ion Battery, 1200mAh (4.44Wh), Nominal Voltage is 3.7 and Chargeable Voltage is 4.7V. – Kevin Aug 31 '14 at 23:53
@Kevin, neither of the designs show above will do. You have a single Li-Ion Polymer cell, which at least doesn't require balancing but does require a three-stage charger. It first needs to charge at constant current while monitoring the cell voltage until the voltage reaches 3.7V, at which point the charger needs to switch to constant-voltage, maintained until charging current drops near to 0, at which point the charger must shut itself off automatically. Improper charging can make the cell explode. Frankly, you can't build a good charger as cheaply as you can buy one... – TDHofstetter Sep 1 '14 at 0:23
Could you show me an example of a 3-stage charger schematics, not that I would make it this time but It could be a good project to make when time comes – Kevin Sep 1 '14 at 0:51
@TDHofstetter, can you expand on your critique of the first circuit? I'm not an EE, but it looks like a pretty straightforward use of a linear regulator, with caps on both the input and output. Shouldn't it be able to keep the output at 5V plus or minus a small margin for it's entire current range? – Duncan C Sep 1 '14 at 2:26
@DuncanC, the first circuit wouldn't hold a stable output because of that pesky series resistance. With that resistor in place, the 5V would be divided between that resistor and the load, whatever load that might be. If the load is 5 ohms (which would have drawn 1A from the regulator @5V), then the voltage at the far end of that series resistor would be only 2.5V. If that series resistor was removed from the circuit, then the regulator would provide a smooth even 5V... but that's too much for a Li-Ion Polymer cell. – TDHofstetter Sep 1 '14 at 2:41

To pull together the various responses you've gotten, in comments and in answers:

To charge a LiIon battery directly you need a 3-stage smart charger, as described by others. Failure to charge a LiIon battery correctly can cause it to explode, possibly causing a fire or personal injury. This is not something you want to get wrong.

If the phone will do the actual charging then remove the resistor and your first circuit will work fine as a 5V regulated supply for your phone (although it will waste a lot of power.) If you want to charge a LiIon battery directly, you've got a lot more work to do, and should have an experienced EE check your work before you actually use it on a battery so you don't burn your house down.

share|improve this answer

Are you really interested in building it, or is getting an off the shelf product acceptable? What you're looking for is a step-down (buck) switchmode DC/DC converter. Unlike the circuits you presented, a switchmode circuit is able to convert a DC voltage to another DC voltage at high efficiency (80-95% depending on a variety of factors). As a first project in electronics, there's no hope that you'll build one correctly on the first try (no offense, it's just that hard). What's feasible would be building one with a linear regulator, such as the circuits you've shown on your question, which by their very nature can't be more than 41.7% efficient. What this means is that you'll drain your battery twice as quickly, if not more, using a linear regulator instead of a switchmode converter. Plus you'll have to figure out a way to dispose of all the waste heat that a linear regulator generates (inefficiency = heat).

For instance, here's a DC/DC converter that accepts a 12 VDC input and outputs 5VDC @ 3A max, which should be plenty enough for charging any cell phone. It also claims to be 96% efficient. It's certainly cheaper than anything you could build.

share|improve this answer
I really want to build something like this since I am an engineering student I may build this kind of project on my higher year. – Kevin Sep 1 '14 at 0:52
Kevin, swineone speaks truth. I'm an electronics hobbyist, not an EE. I've done enough reading that I would feel comfortable building a regulated power supply using a linear regulator. It's not that hard, and well documented. (I'd rely heavily on research and build to somebody else's specs.) Linear regulators waste a lot of power however. They basically use a transistor in linear mode to control voltage, which makes it into a variable resistor. That burns all the excess voltage as heat. A buck converter is much more efficient, but a lot trickier. – Duncan C Sep 1 '14 at 2:33

The first diagram is based on the good old 7805, an integrated linear regulator that's pretty much an all-in-one package (except for the small capacitors that it needs on the input and output). It takes 7 to 25 volts in and gives 5 volts out. The 5-ohm resistor shouldn't be there - without it, you could certainly charge a mobile phone with that. The problem is, as a linear regulator the 7805 drops voltage by turning it into heat, so it's very inefficient for your application.

As others have commented, the second schematic is just overly complicated and wrong for this application, don't even waste time on it :-)

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.