Saturday, September 20, 2014

DIY Powerbank 2014

This post is about an easy circuit diagram that can be used as a power bank. It is not ideal for high current operation though as linear regulator will be used in this case. For storage purpose either Sealed Lead Acid or Lithium battery can be used.

[UNDER CONSTRUCTION]
With the development of mobile devices like mobile phone or tablet or anything similar their power requirement is also increasing rapidly. Let's look back a few years, that time mobile phones had batteries like 850mAHr to 1200mAHr and with that it could run for days after days and now we have 2000mAHr to 6000mAHr rated batteries on our mobile devices but we can hardly go a day with that. Power consumption is not the only thing either to be honest, now a days huge amount of things can be done on a mobile device hence we use them more.

 So what will happen if you are on the way to someplace or in someplace where it is hard to find a AC wall outlet and your phone's battery is dying? For that purpose many portable charger are available in the market ranging from low capacity to very high capacity. Making such a device is a very easy and fun not to mention very useful too. So why not making one for yourself?

Now before we jump into the diagrams I will highly recommend you to read this post where I tired to explain how these portable chargers work, if you can grasp the idea of that it will be far easier for you to make one.

How does portable chargers work.

We can make this device using many different diagrams and techniques with output targeted for many different devices so there will be multiple diagrams and I will be dividing the whole diagram into blocks so that it can be easily understood. Let's look at the block diagram first.

Block Diagram:

Let's take a right to left approach. The battery that we will be using, which I will talk more on later, might not have the proper voltage that we need to charge our phone or tablet. Most modern phones takes in 5V and it will be really handy to attach an USB port so that we can plug in any cable and charge any device that supports charging from USB. Sadly to keep stuffs simple I won't be going into the fast charging capability in this post. Now let's take a look at 3 different diagrams that can be used as converter circuit.

Converter Circuit:

First one is plain and simple linear type using the good old 7805, that can provide 1A at 5V. Enough for most devices but might waste a lot of power. Also the battery voltage must be 6.5V or higher. Before looking into in let's just take a look at the USB port configuration as we will be wiring the output of it to the USB female port.


USB port Configuration

So now we know where to connect the Ground wire and where to connect the +V wire. So let's check out the first diagram.


This probably is the simplest circuit that will work just fine but the problem is the current output is limited to 1A and the 7805 chip will get a bit hot which means it will waste a good amount of power. Yet it will work, if you give the proper input voltage it will convert it to a steady 5V, even if the input voltage decreases over time and if it is supplied with a battery it will decrease over time but the output will be constant 5V. The indicator will lit up showing that the circuit is in operation.

If you want to learn more about such regulators read this post.
Linear voltage regulation with 7000 series integrated circuits.


Time for the second diagram. This one is similar to the first one in a sense that it will take in a voltage that is slightly larger than 5V and lower it down to a constant 5V. There are some benefits of using this one, which are higher efficiency and higher output current. This small circuit can provide about 3A and can achieve a very respectable 80%+ efficiency. So this one is better suited for using it with battery.


This one is a bit complicated than the previous one. The output 5V will be connected to the USB A just like the previous one with proper polarity.

To better understand this circuit read the post linked below.
5V Multi-purpose SMPS.

Now lets take a look at the third diagram. Probably you have already noticed that in the previous two diagrams you will need an input voltage higher than 5V to operate but what if you are using single cell lithium ion which is a very common practice and we know single cell lithium ion will have a voltage of 4.2V maximum. So the diagrams above won't work with that. So we will be needing this one.



Diagram:

 


Explanation: 

As you can can see very simple diagram with only a few components, I will explain the whole circuit step by step.

Let's start with the input section, a standard female jack is used as input and putting a source of around 11.5V to 26V will be fine for this circuit but keeping the voltage as low as possible will generate less heat during voltage regulation. 

The input is directly fed into a positive voltage regulator, 7809. It will give a constant 9V output. A 470uF electrolytic capacitor is used for smoothing and decreasing voltage ripple. A LED is connected with it in order to indicate charging. Next the 8.4V Lithium Ion batteries are connected via a 4007 diode. This diode performs two very important role, firstly it blocks the DC voltage to flow from the battery to the regulator and secondly it drops around 0.7V thus making the output around 8.3V. Charging a Lithium ion battery is a very complicated procedure but lithium ions need to charged like through that procedure and the voltage needs to be very stable. So here we are charging a 8.4V pack with 8.3V. It's okay actually, we might loose its capacity slightly but we will get more charging cycles out of it.

The next thing is the battery is connected with another regulator via a switch. This regulator is a 5V one, 7805. This chip can provide continuous 1A at 5V. Which is enough for charging most modern mobile phones and tablets.

A LED is connected to the output of the regulator to indicate that the regulator is on. Another 220uF electrolytic capacitor is added to smooth the output of the 7805 regulator. Then the output is connected with the a USB female port.

I have added a few more LEDs through a 0.5W resistor and a switch. This will work as an emergency lamp too. 


Notes on a few topic:


When selecting an input voltage you have to remember that this is a linear voltage regulator so it will waste energy by producing heat. The generation of heat will be Output Current * ( Input Voltage - Output Voltage) so it is clear that the higher the input voltage is more energy will be burnt and more heat will be generated. So, I fitted the both regulators in a heat sink. The Tabs are actually connected to the 2nd pin of the regulator. If you want to learn more about this you can read this post.

7805 on the left and 7809 on the right


Next thing is the battery. Lithium ion is a very sensitive type of battery. Misusing that may cause severe damage to the battery and may cause damage to the user. Lithium ion needs to be charged with a constant current at the first stage then a constant voltage is needed. To keep stuffs simple I'm going to use low current constant voltage. Another thing is lithium ion shouldn't be charged with more than 50% of its current capacity. That means if you have a 2000mAHr pack you should not cross the charging current over 1000mA, higher current will generate more heat and damage valuable battery life. To learn more about how to charge lithium ion battery you can read this post. For my purpose I'm using two 4.2V Cell each rated at about 3300mAHr-3500mAHr, and I'm charging that with a maximum of 1000mA at 4.15V per cell so it should be fine.

Two 4.2V 3.3AHr lithium Ion Cells

 
Now you need to be very cautious with the polarity of the USB port. A picture of the port diagram is shown here. Make sure you connect the proper pin to the proper polarity. Check the polarity before connecting any device. As this will be a small circuit I used a small piece of board for assembling a few components. 

On the left a USB female jack






 

As I was saying lithium ion shouldn't be charged with more than 50% of its capacity, mobile 
devices have lithium ion batteries so they shouldn't be charged with more than that. As this device provides about 1A at 5V that means its safe not to charge any cellphone battery that has a capacity of less than 2000mAHr.


Possible Upgrades:


1. Like I said before this diagram incorporate constant voltage charger for the Lithium ion battery, you can surely swap it with a more accurate proper type of charger that will provide constant current at the starting and changes to constant voltage as the current requirement for the battery to charge drops.

2. Next thing that you can do is swap the linear 5V regulator with a switched mode regulator. Switched mode regulator burns less energy so less heat and more efficiency.

3. You can also add a output current controller and maybe a selector for different output currents.

4. I have used bulky lithium ion batteries, you can use the 18650 type battery.

5. Adding a battery voltage meter might come in handy, you can do that too.

6. Multiple USB ports can be added.


Warning:

1. Lithium Ion batteries are very sensitive and misusing them might cause catastrophe. Don't do something unless you are 100% sure about it. Fire Hazard.



Final Product:


Top View of The final Product.



Resources:

1. Datasheets.
2. Visit my full blog.

That's pretty much everything, good luck everyone.



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