!!PLEASE HELP!! Portable iPhone charger project

Discussion in 'The Projects Forum' started by n00by, Feb 25, 2014.

  1. n00by

    Thread Starter New Member

    Feb 25, 2014
    Hi, I have a basic understanding of electronics and data logic.
    It's been a long time since I've studied.

    I'm building a portable iPhone charger.
    It will be powered by 4 3.7v lithium cells,
    2 cells in series(7.4v) in parallel with another 2 in series.
    This 7.4v bank will be connected to an L7805CV regulator to give an output voltage of 5v.
    This battery module is what my iPhone will connect to via its normal USB charging cable.
    Now, inorder to recharge this battery bank, I will be connecting a 5v mini USB charging module so that I can for example recharge it with my ps3.
    Now this is where I'm lost.... I need to wire the 5v mini USB charger to all the individual cells in parallel so that each 3.7v cell is being charged with 5v.
    This confuses me though because 2 cells of each bank will already be connected in series.
    I'm pretty sure the answer is a couple of diodes or transistors, but I can't work out how to do it. Could somebody please help and possibly draw me a quick circuit diagram. I would be very appreciative.
    Dylan. (77dylan77@gmail.com)

    Also, I know these types of things are already on the market, but I wish to do it myself as I'm beginning a bit of a hobby.
  2. wayneh


    Sep 9, 2010
    It might be a good time for you to learn about integrated circuits designed specifically for battery charging. A small chip will provide all the smarts you need to tend your batteries properly. I believe there are ICs that can even handle two cells in series and yet charge them both properly and independently.

    You might also want to learn about DC-DC converters. These are very cheap on e-bay these days and can be used to buck or boost a voltage to USB 5V. They are far more efficient than the linear regulators like 7805, which simply burn off as heat any voltage over the target.
  3. n00by

    Thread Starter New Member

    Feb 25, 2014
    Thank you for that I will start looking into some ICs. Do you have any specific ICs in mind that I could use for my project?

    FYI - the cells I will be using:

    Full charge 4.2v
    Empty charge 2.75

    4 cells. (2 in series) paralleled with(2 in series)
    Last edited: Feb 25, 2014
  4. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    Is this a charger to just plug into the phone? Or a charger to charge batteries separate from the phone?

    If you are plugging to the phone then battery management is not needed. All you need is a 5V regulator. The phone will take care of the rest. Consider a buck regulator. Buy one of those cheap car chargers and see how they do it. Many use the 34063. Not the best regulator on the block but it has been around for years and you will find tons of support on it.

    If it is a charger just to charge the batteries then look for battery management chips on a place like Mouser. But you need to be really careful charging Lithium batteries, Just ask Boeing. ;) You just might want to consider another project.
  5. burger2227


    Feb 3, 2014
    Some lithium chargers can also supply 5 volts, but not do both at the same time.

    One lithium battery is all they need to boost it to 5 so you really don't need more than one battery or you will need special circuits to monitor each cell as they charge. That will require a charger and a charger manager.

    Lithium batteries are not something to mess around with. They must be charged correctly or you could have a fire!
  6. n00by

    Thread Starter New Member

    Feb 25, 2014
    The battery pack itself is to charge my phone and yes I will be using a 5v regulator.
    But I need to figure out how to recharge the battery pack.
    The cells will be configured in 2S2P. To give an output voltage of 7.4v.
    I have a module that is mini USB input and I need to use that 5v output apply 5v across each individual cell, without interfereing with the series configuration.

    I will look on Mouser, but if you could put me in the right direction I would really appreciate it.
    Thank you for taking the time to help.
  7. burger2227


    Feb 3, 2014
    The battery pack with 2 in series will need a wire to monitor the middle connection as illistrated here: http://www.youtube.com/watch?v=h5Z9GiavZX4

    Actually I am trying to make up a 3 battery pack in series also. That requires 4 wires with 2 in the middle going to a managed charger. I happen to have a charger I got from Rockwell when they sent me a bad oscillating tool. They sent me another whole package with 2 bad batteries. I took one of them apart to figure out the connections.

    They do sell charging managers or protectors here:


    BM is middle wire. The charging voltage comes into the board on the same line as the load. If the protector is in the device with the batteries it also protects from over discharge and over current.
    Last edited: Feb 25, 2014