Dear sir i want to make a power bank using 3.7v rechargeable cells. Can u
help me to make a circuit for power bank. I want the output 5v 2amp...
thanks in advance.
What is a Power Bank
Power bank modules have gained significant popularity today due to their portability and ability to charge any cell phone while traveling and during emergency requirements.
A power bank is basically a battery bank box which is initially fully charged by the user at home, and then carried by the user during the course of a journey. And when the user finds his cellphone battery reaching low, he connects the power bank to his cellphone for a quick emergency top-up charging of the attached cellphone.
How Does a Power Bank Works
I have already discussed one such emergency charger pack circuit in this blog, which used chargeable Ni-Cd cells for the intended function. Since we had 1.2V Ni-Cd cells employed in the design we could configure it to the exactly required 4.8V by incorporating 4 of these cells in series, making the design extremely compact and suitable for optimally charging all types of conventional cell phones.
However in the present request the power bank needs to be built using 3.7V Li-ion cells whose voltage parameter becomes quite unsuitable for charging a cellphone which also uses an identical battery parameter.
The problem lies in the fact that when two identical batteries or cells are connected across each other, these devices begin exchanging their power such that finally an equilibrium condition is achieved wherein both the cells or the batteries are able to attain equal amounts of charge or the power levels.
Therefore, in our case suppose if the power bank utilizing a 3.7V cell is charged fully to about 4.2V and applied to a cellphone with a drained cell level at say 3.3V, then both the counterparts would try to exchange power and reach a level equal to (3.3 + 4.2) / 2 = 3.75V.
But 3.75V cannot be considered the full charge level for the cell phone which is actually required to be charged at 4.2V for an optimal response.
Making a 3.7V Power Bank Circuit
The following image shows the basic structure of a power bank design:
As can be seen in the above design, a charger circuit charges a 3.7V cell, once the charging is completed, the 3.7V cell box is carried by the user while traveling, and whenever the user's cellphone battery goes down, he simply connects this 3.7V cell pack with his cellphone for topping it up quickly.
As discussed in the previous paragraph, in order to enable the 3.7V power bank to be able to provide the required 4.2V at a consistent rate until the cellphone is completely charged at this level, a step up circuit becomes imperative.
Using IC 555
A simple boost charger circuit could be used for this purpose as explained in the following paragraph:
The inductor L is made by using 5 turns of 22SWG super enameled copper wire over any suitable toroidal ferrite core.
We can see two variable resistors (presets) included in the design, these are required to be optimally set for acquiring the most effective and efficient performance outcome from the boost charger circuit.
The voltage across "C" is the output which is used for charging the external device, and in our case the voltage here must be fixed at around 5V.
The entire setting procedure for the above boost power bank circuit can be learned from the following article: Calculating Inductor Value in SMPS
The circuit also includes a built-in 3.7V Li-ion charger circuit made up of a TIP122 emitter follower stage, a 5V zener diode and a small 6V/100mA incandescent bulb.
As long as the 3.7v cell charges through the transistor emitter lead, the series bulb remains illuminated and as the cell nears the full charge level, the illumination on the bulb becomes dimmer until finally it shuts off indicating a full charged 3.7V cell.
The 5V zener diode ensures that the emitter voltage of the TIP122 never exceeds the 5V range, which is further pulled down to around 4.3V by the associated series 1N4007 diode.
Another 1N4007 diodes can be seen connected with the base of the BC557 transistor, this configuration makes sure that the BC557 remains shut off disabling the 555 IC stage and boost converter stage during the charging phase of the cell.
The charging input is preferably obtained from any standard SMPS cell phone charger unit.
So the above takes care of a power bank using a 3.7V cell, the AH level depends on the load specs, or the cellphone required to be charged, for smart phones the AH level should be preferably above 5000mAH.
Using a Joule Thief Circuit
If you think that the above IC 555 based power bank charger circuit looks cumbersome and an overkill, you could probably try a Joule thief concept for achieving quite the same results, as shown below:
Here, you can try 220 ohm, 1 watt resistor for R1, and 2N2222 transistor for T1.
The coil is made over a T18 Torroidal ferrite core, with 20:20 turns for the primary and secondary, using multistarnd (7/36) flexible PVC insulated wire. This may be implemented if the input is from a pack of 5nos of 1.5V AAA cells in parallel.
If you select Li-Ion cell at the input source, the ratio might need to be changed to 20:10 turns, 20 being at the base side of the coil.
The transistor might need a suitable heatsink in order to dissipate optimally.
How to Charge the Joule Thief based Power Bank
The following image shows the complete design of a power bank with charger using Joule thief circuit:
Here the TIP122 along with its base zener becomes a voltage regulator stage and is used as stabilized battery charger for the attached battery. The Zx value determines the charging voltage, and its value must be selected such that it's always a shade lower than the actual full charge value of the battery.
For example if a Li-Ion battery is used, you may select Zx as 5.8V in order prevent the battery from overcharging. From this 5.8V, the LED will drop around 1.2V, and the TIP122 will drop around 0.6V, which will ultimately allow the 3.7V cell to get around 4V, which is just around sufficient for the purpose.
For 1.5V AAA (5 in parallel), the zener could be replaced with a single 1N4007 diode with its cathode towards ground.
The LED is included for roughly indicating the full charge condition of the connected cell. When the LED lights up brightly, you may assume the cell to be fully charged.
The DC input for the above charger circuit could be acquired from your normal cellphone AC/DC charger unit.
Although the above design is efficient and recommended for an optimal response, the idea may not be easy for a newcomer to build and optimize. Therefore for users who might be OK with a slightly low tech design but much easier DIY alternative than the boost converter concept might be interested in the following configurations:
The three simple power bank circuit designs shown below utilizes minimum number of components and can be built by any new hobbyist within seconds
Although the designs look very straightforward, it demands the use of two 3.7V cells in series for the proposed power bank operations.
The first circuit above makes use of a emitter follower transistor configuration for charging the intended cellphone device, the 1K perset is initially adjusted to enable a precise 4.3V across the emitter of the transistor.
The second design above uses a 7805 voltage regulator circuit for implementing the power bank charging function
The last diagram here depicts a charger design using an LM317 current limiter. This idea looks much impressive than the above two since it takes care of the voltage control and the current control together ensuring a prefect charging of the cellphone.
In all the three above power bank circuits, the charging of the two 3.7V cells can be done with the same TIP122 network which is discussed for the first boost charger design. The 5V zener should be changed to a 9V zener diode and the charging input obtained from any standard 12V/1amp SMPS adapter.