I've got 4 motors, 1.25W power each, (6V, 0.25mA in top)

but these motors can run on higher V lvl, so I need to calculate maximum Voltage i can pull from my powerbank that has 5V & 2A on out.

PS: efficiency can be ingored so far

How are you going to get more than 5V out of a powerbank that you say outputs 5V ?

using dc-dc step up transformer

So you are not aiming to get more than 5V out of the powerbank after all, are you ?

As to the maximum voltage that you can convert 5V to, then any voltage is possible, even up to thousands of volts, albeit at low current

You are thinking about the the wrong way. What is the maximum voltage that the motors can run at and how much current is required ? Once you know that then you can look for a boost converter that can output the required voltage and current

I have a moscito killing device that creates plenty of kilo volts. Out of one single AA cell.

No upper voltage limit. Check the current such a converter can deliver.

Nothing satisfies quite as much as listening to pesky mosquitos ride the lightning.

not any

each motor eat 0.25A

if I raise voltage lets say to one hundred volts, than motors just won't get enough current, thus they won't work

so I need to find balance between how much I get from powerbank, and to what lvl I can speed up motors

The current does not remain constant it will probably draw several Amps at 100V, it will vary with the applied voltage.

There will be an optimum voltage for the Motor where it can deliver it maximum power.

It's probably whatever the voltage is when it consumes 1.25W as you stated.

yeah, that what I looking for

I can't find relation between how much I can raise voltage, and will it be enough current left for feeding motors

What is the maximum power output of your power brick? 5V at what current?

2A i already written

If the data sheet does not give a power curve then the max power is usually delivered at the rated motor voltage. If the data sheet states 5V then there is no reason to assume otherwise.

So that's 10 watts. That's the power available.

Took down a fist sized wasp nest built under a balcony tabel. Smashing the fallen nest zapped every wasp exiting the crashed nest. Zapp and flashes. Now they're gone.

that's true, but motors won't work if i use 10W

less current will be delivered

i already tried

10 W at what voltage? If you run the motor at 10 V for example, 1 A would be available for the motor. It would probably burn up as well.

10 Watt is what the phone bank can deliver. Suppose You step up the output to 100 volt from the converter. Then the max current availble will be 100 mA minus the losses in the converter.

if i run at 10V, it can damage power supply due to lack of current,

because if i raise up to 6V, i already have near 1.67A left, but each motor will pull 0.25A on this voltage according to datasheet

so I don't understand how much current will left to motors if i will continue raising power?

Sounds fun. For long range weaponry vs wasps, I recommend brake cleaner

A power bank will limit the current to its rated maximum so there is little danger of damaging it. With 4 motors you will have 0.5A available for each motor. At 10V that would be 5W per motor, probably far more than the motor can tolerate.