If i need 12Vs from 18650 power bank then i will connect four 3.7V cells in series to get 14.8V and to increase capacity i will have 3 or series arrange in for example 4s3p bank

But what if i connect all 18650 in parallel which will increase the capacity of the power bank but voltage will be 3.7 and to get 12 volts i will use step up circuit to make it 12v

the question is what are the disadvantages of this approach. in parallel the voltage is increased but capacity will be equal to whatever a single 18650 assuming all 18650 are of same brand with same capacity.

Why people use parallel batteries to increase voltage but keeping the capacity the same while if they connect in parallel it will increase the capacity of the charge and to get higher voltage use step up circuit

You still haven't answered the questions :

READ THIS !

What's the load and the the minimum operating

time between battery changes ?

In other words , what is the application mAh spec ?

Let me repeat it for clarity:

A- WHAT IS THE APPLICATION CURRENT REQUIREMENT ?

B- HOW LONG DOES YOUR APPLICATION NEED TO RUN AT ONE SESSION WITHOUT STOPPING TO

CHANGE THE BATTERY ?

Do you know how to calculate that ?

ie:

Exampleapplication contains the following devices:

1. Device A (current draw 50mA)

2. Device B (current draw 170mA)

3. Device C (current draw 200mA)

total current draw= 870mA

Application needs to run 12 hours without powering down for a battery change.

870mA * 12 = 10440mAh

Run requirement = 10440mAh

10440/3400= 3.07 batteries =>either round up to 4 or reduce time to 11 hrs, 43min.

Minimum current capability at any given time = 870mA (0.870A)

18650 current capacity = 3400mAh

Does it matter which you calculate first ?

Will there be any difference ?

Typically you calculate the number of batteries to get the voltage you need.

Then you calculate how many of those SERIES battery packs you need to get the operation time

required by the application.

Conclusion:

Application requires FOUR batteries IN SERIES to get 14.8V (3 batteries is only 11.1V)

Application requires FOUR SERIES 18650 battery packs (of FOUR batteries) in PARALLEL) .

This is a 4S4P battery pack.

How many batteries do you need in SERIES ?

Your post starts with "If I need 12V ...."

12V/3.7 = 3.24 batteries, (3*3.7=11.1V)

If 11.1v is unacceptable then you have to round up to FOUR 3.7V batteries to get 14.8V

Even if you use 4 batteries IN SERIES, you STILL need to tell us HOW LONG THIS APPLICATION

NEEDS TO RUN CONTINUOUSLY WITHOUT POWERING DOWN.

Once you tell us that , we can tell you (or you can calculate) how many 4-series battery packs

you need to put IN PARALLEL.

If you say FOUR, then the resulting battery pack is a 4S4P. (because 3 in series will not give you 12V)

If you say THREE, then the resulting battery pack is a 4S3P. (reducing run time to 11 hrs 43 min))

But what if i connect all 18650 in parallel which will increase the capacity of the power bank but voltage will be 3.7 and to get 12 volts i will use step up circuit to make it 12v

You lose current capability by using the step up because it is unlikely to be able to source the

current the batteries can source without it, in addition you waste energy through the silicon devices

in the converter.