Smartphone Battery pack - Charging & Voltage Regulation

[Auzeras]

Hi all,

Recently I embarked on a project to make myself a nice extended battery pack for my smartphone. Some of you may know "Ingress" if you do you'll know why a battery pack is important  Anyway, I looked at various name brand designs, and all the high capacity ones (+5000 mAh) were rather expensive, getting up into the $80+ NZD range. I bought an advertised 12000 mAh pack off ebay for $22. Didn't expect that capacity but oh well. It performs "ok" but I don't think the 18650 cells used are that great or if the circuit is that efficient. It's hard to tell because my phone is broken (part of the charging circuit) and I refuse to pay the measly $20 to buy a part of fix it

I decided to build my own high performance pack and settled on using 3000 mAh (or thereabouts in capacity) 18650's from panasonic and put them into a cheap and crappy chinese built battery pack/charger, something like this http://www.ebay.com/itm/5V-2A-DUAL-USB-Power-Bank-Battery-Charger-18650-DIY-Box-Case-for-Mobile-Phone-/370921122281?pt=Battery_Chargers&hash=item565c9f29e9 but I still didn't trust the efficiency of the circuit design.

My current china pack looks like it's wired in series parallel with 2 cells parallel (for the capacity) and then in series to get about 6v, I plan to do the same for mine. Right now I have a few questions that my amateur brain needs to get around. Finding schematics for each function individually is no problem at all, Lithium chargers and 5v regulators are all over the internet and discussed in various magazines and books, but for the most part they use inefficient L7805's for the 5v and only address single cell charging.

A more efficient voltage regulator isn't hard to design/come across (although if someone has a nice one I'd like to see it) it's the charging that has me a little confused. As far as I know, charging Lithium in series is the same as charging a single cell, it just takes longer and you can take your ordinary charging circuit and use it (I may be wrong though!) and charging in parallel requires balancing. Am I wrong in thinking that if two parallel cells are constantly connected, charged and discharged at the same rate etc they shouldn't need balancing? And balancing is only required to charge cells in different states of charge?

If that assumption is correct, can I take a very simple Lithium charge circuit and use it to charge my 4x cells in series parallel? Or do I need to introduce some complexities such a balancing? If so is it worth making my own circuit, or are these cheapie china ebay options kinda okay for what they offer? The main reason I don't trust them is most of them show the 18650's in parallel, which means you'll get plenty of capacity, but the voltage will have to be stepped up to 5v, which seems inefficient. It also runs into the issue of balancing, since the cells are all connected on a single rail, no extra wires for the balance.

Sorry if this is an overly complex long read 

tl;dr 

I want to make a 5v external phone battery using 4x 18650 cells, help stupid me design charging/voltage regulator circuit.

Also it might pay to note, this isn't a one off, I'm likely to make about 10 of these units for other people. I don't mind hand soldering or making the PCB's.

Also, if someone had a brilliant alternative to my haphazard home made frakenstein such as a premade board or two that does this, please let me know, I often overlook the gloriously simple things.

Many thanks,

Az

[amyk]

First make sure you know what is series and parallel. You seem very confused about that. Parallel lions can be charged like a single cell, series is what needs balancing.

[MinK]

When you have your batteries in parallel you can charge them as one single cell. Just before parallel connecting cells make sure each cell voltage match to other cells voltages 
If you design your own circuit, make sure it has under-voltage cut-off function. Deep lipo cell discharge may damage cell. Don't discharge bellow 3 V or 3.3 V to keep it safe.