Do it Yourself Power-Bank Part-1

Almost everyone nowadays owns a smart-phone. Obviously the merits of having a small computer(smart-phone) in your pocket is not necessary to explain.

But as we know every coin has two sides, this heavy computation power, packed with awe features like stunning graphics, high-speed data connectivity and great multimedia abilities require a great sacrifice of battery power. 

So many smart-phone users spend much of their device usage time connected to the wall-outlet charger.

But while on the run when a wall-outlet was not available came to our save our Great Hero :D the Power-Bank.

So nowadays power-banks are a usual gadget for most users and you can usually get a good bargain for quite a decent capacity power-bank in market.

But i wanted to build one on my own, just for fun and also because that technically i had most of the stuff required to build it lying around my workbench.

And this is what mine looks like :

So lets begin....

I will start by listing out first what parts are required and explaining what role do they play in our project on a need to know basis.

The list:

• A couple of 18650 batteries. 
• LM2577 Step-Up or Boost converter.
• A USB-A Type Socket/receptacle.
• A control Circuit.(We need to build this one).
• A piece of Dot-PCB or veroboard for Prototyping.
• Needless to say a soldering iron, and accessories like solder flux and solder wire.
• Some general DIY tools.
• And finally the housing, a box in which we will fit our parts and circuits.

The explanation:-

The working principle of our Power-bank is very simple and is shown in the diagram below.

Now the 5v DC supply available via the USB socket can be used to charge your mobile phone using your data cable. Almost all mobile phones require 5v DC input to charge-up but the current required varies from device to device. 

However most mobile phones chargers provide somewhere around 1A and thats our target too.

And when you are not using your power-bank you can charge its internal battery with your usual mobile charger only.

The Battery:

Now lets focus on the voltage part. It obvious that we are going to take power from those batteries i listed above. 18650 as battery size standard. It is the type of battery that most power-banks in the market contain. It is a lithium-ion rechargeable battery and even most laptop battery-packs contain these 18650 cells internally.

!!Caution!!:- DO NOT EVER TRY TO SHORT-CIRCUIT THESE BATTERIES. IT  CAN LEAD TO EXPLOSIONS AND DON'T USE LEAKY OR SWELLED OR FUMING BATTERIES. 

IF BECAUSE OF ANY REASON THE BATTERIES HEAT-UP, REMOVE ALL CONNECTIONS IMMEDIATELY AND DISPOSE THEM OFF PROPERLY.

DON'T PLAY WITH STUFF YOU DON'T KNOW ABOUT. Read usage and safety information about these batteries in the datasheets if the manufacturer provides any or else the generic ones at here or here, before proceeding any further.

These lithium-ion batteries generally have a full charged voltage of 4.2 - 4.3v max at no load. But when you load them it may decrease to about 3.7v to 4v. So lets assume that the moderate working voltage would between 3.3v to 4v.

You can get a variety of these batteries on ebay or other sites of various capacities and brands at various prices. So choosing the right one would be difficult. 

Most batteries come at standard 2600mAH capacity.

Now the more batteries you connect in parallel more is the total capacity of your battery-pack. Suppose you get 4 x 2600mAH batteries and connect them in parallel, then their voltage will stay the same but their charge capacity will add up, so the total capacity will be 10400mAH.

Capacity of battery-pack directly translates to the number of times you can charge your mobile phone battery. Now some people might have a misconception that "my mobile battery is of 2000mAH and 10400/2000 = 5, so i will be able to charge my mobile battery 5 times" . 

NO thats wrong actually.

How many times you can charge your mobile battery depends on many other factors too.

First of all nearly 10-15 % of the battery energy is wasted as heat during conversion from 3.8v to 5v.

Now we provide 5v to the phone but the phone's internal charging circuitry also wastes some 5 to 10% power.

When we say 2600mAH, we mean the total charge of the battery when used from full charge to full depletion. But if you deplete a rechargeable battery completely then you can't charge it any more, because a irreversible chemical reaction will occur in it . So we can use it until the battery voltage stays above 3v.

So that translates to roughly 70-80% of the total battery energy that is directly going into your mobile battery.

You would be surprised to know that many of us could get these batteries from used up old laptop battery-packs, and actually i built mine from used laptop battery only, but there are some quirks related to using cells from a depleted laptop battery-pack.  So i will explain it in the last post of this series.

OKAY now i think we have enough info regarding batteries, lets proceed to the next part in the list. 

Step-Up Converter:

The LM2577 Step-up(boost) converter. It is a DC-DC conversion circuit which takes any DC input voltage between 2.5 to 27 volts and can provide a constant regulated output anywhere between 3v to 30v, provided that the input voltage doesn't exceed the set output voltage.

If the input voltage exceeds adjusted output voltage then output will start directly following input and no conversion or regulation will take place.

To set the output properly first connect the battery at the input pins keeping in mind the correct polarity and then connect a multimeter across the output and keep turning the trim-pot until you get 5.0v on your multimeter.

Just take look at the video below for further clarification..



Once the output is adjusted, the step-up converter tuning part is complete.

Now just take a piece of dot-pcb (prototyping PCB) and mount the USB socket/receptacle on it and solder its legs firmly, then solder the output pads(O+, O-) of the LM2577 step-up circuits to two wires and solder the other ends to USB socket mounted on the PCB.

So now you have setup the output portion of our project successfully.

We will look into building the Control Circuit in the next part of this blog.

Click here to proceed to the next part in this series.