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 BankPower 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
In order to correct the above situation and make sure that the 3.7V power bank is 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.
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.