Portable USB phone Charger Design Help

[A_Dude]

Being new to soldering I have searched the web for interesting projects to try out. One that has caught my eye was a portable USB phone charger. I have seen multiple designs, that can be split into two types, one involved using a 9V battery, while the other opted to use 3V from two AAs. The one that impressed me the most is the Minty Boost, but unfortunately it is not compatible with my phone, the LG nexus 4.

So I intend to attempt to design my own circuit that will be able to charge my phone. Although I have a rough idea of how the system should work I could use some help by being pointed in the right directions for this circuit design. From what I understand the main principle is to convert the 3, 9 or whatever input voltage used to a 5V for the phone using a buck or boost circuit of some kind, after that I am a little fuzzy and could use a helping hand. The main problem I face is trying to understand why my phone was incompatible with the Minty boost and how I could alter that circuit design to suit my needs.

If anyone can help me, I would appreciate it greatly. Hopefully if successful I will be able to improve upon this to perhaps include a rechargeable element of the portable charger, but that will be a problem for another day.

 

[dorke]

My 2 cents:
Whatever you build don't hook it up to your LG phone.
It will be a shame to destroy it...
furthermore ,chargers done the wrong way ,can cause the battery to explode.

So for your own good try some other project to learn.

 

[Martaine2005]

So I intend to attempt to design my own circuit that will be able to charge my phone. Although I have a rough idea of how the system should work

A_Dude,
Give a clue to your own circuit. And what is your rough idea of how it works?

Martin

 

[BobK]

My 2 cents:
Whatever you build don't hook it up to your LG phone.
It will be a shame to destroy it...
furthermore ,chargers done the wrong way ,can cause the battery to explode.

So for your own good try some other project to learn.

Really? As long as you have a 5V regulator you are not going to destroy a phone or explode any batteries. The charging algorithm is built into the phone, all the phone needs is a 5V constant voltage supply.

The only other consideration is how much current it will draw. Different phones will look for different voltages on the data pins to determine the current capability of the charger. If it does not find this signal, it will default to the 500mA that is always legal on a USB port, and probably charge 1/2 to 1/4 as fast as it would with a compatible charger.


Bob

 

[BobK]

And to the OP.

A charger using a 9V or 2AA will charge the phone less than 1 time for a set of batteries.

There are inexpensive "power banks" available on Ebay that will charge it 4 times or so, and are rechargeable themselves. Something you build will cost you as much and be far inferior.

Bob

 

[Gryd3]

My 2 cents:
Whatever you build don't hook it up to your LG phone.
It will be a shame to destroy it...
furthermore ,chargers done the wrong way ,can cause the battery to explode.

So for your own good try some other project to learn.

@A_Dude, he has a point, but there are some missing details.
Unless you know exactly what you are doing, never try to directly charge a battery!

However... cell-phones, video game controllers, etc... anything that typically has a built-in / included rechargeable battery typically has it's own built-in charging circuit.
All you need to do is provide this charging circuit with the appropriate Voltage / Current. You do not need to worry about charge profiles or anything fancy.
Things to consider though...
-Capacity of the battery you wish to charge.
-Capacity of the batteries you are using in the portable charger.

9V battery here... anywhere from 120mAh to 1200mAh per battery
https://en.wikipedia.org/wiki/Nine-volt_battery#Technical_specifications

AAA battery here... anywhere from 300mAh to 1250mAh per battery
https://en.wikipedia.org/wiki/AAA_battery#Use

Cell phone batteries can be much larger. 2400mAh is common.

You also need to consider the type of charger you use, and the losses associated with it.
Using a 9V battery a linear type converter (LM7805), you will waste a lot of power stepping down the 9V to the 5V required by the phones built-in charger.
Using a 9V battery and a switch-mode converter is much more efficient, and will actually allow you to squeeze more mAh out of the 9V battery at a lower voltage!
However... using a pair of AAA is not quite as nice to work with...
Using batteries in parallel lets you add their mAh together, but using two AAA in series adds their Voltage together instead to let you get 3V. In order to get 5V, you need a switch-mode 'boost' converter which is still efficient, but if you are converting 3V to 5V with this kind of thing, the mAh available will drop.

Here is how conversions work:
9V battery w/ 1200mAh <> Linear Regulator = 5V w/ 1200mAh
9V battery w/ 1200mAh <> Switch-mode Buck Regulator = 5V w/ almost 2000mAh
3V pair of AAA w/ 1200mAh <> Switch-mode Boost Regulator = 5V w/ less than 700mAh

The outcome is directly proportional to how much you change the voltage by... if you need to double voltage with a converter, the mAh will halve.
This also comes into play with the current requirement though as well!
Phones like to charge with at least 500mA. This is a measure of how much current is being pulled at any given moment... mAh is a measure of a combination of how much current you can draw over time.
So... your poor AAA pair of batteries will be sweating trying to put out over 830mA in order for the regulator to convert that to a higher voltage.
The 9V battery will be putting out about 300mA to feed the switch-mode regulator.

*There is a catch!
Batteries do not last as long under a heavy load!
The capacity rating of a battery is under an ideal low current load. So a battery rated at 1200mAh may only deliver 800mAh if you try to pull more current than it's meant for... both AAA and 9V cells are not meant for heavy current draw. They are typically used in low power operations.
This is why bigger batteries or different battery chemistries are used for things RC toys, and retail portable cell phone chargers.
So... as a project to learn, it's worth while, but as a solution to a problem... it's not.
You would need at least 8 AAA batteries to fully charge a cell phone, or a pair of 9V batteries AND a very efficient regulator... you would also need the device's built-in charger to be very efficient as well... and don't forget that if you are using the phone at the same time, the charger may not be able to charge the device faster than the device is using it's own battery.

Best of luck, and let us know if you have any questions.

 

[dorke]

Really? As long as you have a 5V regulator you are not going to destroy a phone or explode any batteries. The charging algorithm is built into the phone, all the phone needs is a 5V constant voltage supply.

The only other consideration is how much current it will draw. Different phones will look for different voltages on the data pins to determine the current capability of the charger. If it does not find this signal, it will default to the 500mA that is always legal on a USB port, and probably charge 1/2 to 1/4 as fast as it would with a compatible charger.


Bob

The 5V USB adapters for phone charging are dirt cheap-that is the way to go ,
not build your own.

A_Dude has very little experience if at all,
experimenting on a his phone isn't the way to learn... unless he seeks a budget hole.

a word of caution is a must here...
Can you be sure what he builds will produce 5V ?maybe more than a bit higher....

 

[BobK]

Look at my post again. "As long as you have a 5V regulator" is that condition for everything that follows. Even a beginner can make a 5V regulator work.

Bob

 

[AnalogKid]

The one that impressed me the most is the Minty Boost, but unfortunately it is not compatible with my phone, the LG nexus 4.

There is nothing magical about the Minty circuit, meaning nothing that makes it compatible or incompatible by design with various devices. It is a generic boost circuit making a generic USB power output. If your phone won't work with it, it almost certainly will not work with anything else you build. Note that this has nothing to do with you, and almost nothing to do with the circuit you choose.

And "compatible" is a subjective term. Did you test it, or just read about it? When connected to a generic USB source, my LG phone puts up a message complaining about the cable (???). For anyone selling a product, this qualifies as incompatible. After all, I would not want to field hundreds or thousands of phone calls explaining to non-technical users that they should ignore their device's warning and use my product anyway. And, if the phone should fail for some other reason, guess who their lawyers are going to call?

However, my phone charges just fine no matter what I hook it to.

ak