I agree it is better to use a Li-Po battery since K3 will try to charge it and it might be tricky if it apply an algorithm for Li-Po to Ni-Mh battery (or non rechargeable battery) since they have different characteristics, internal impedance etc.
I have some Ni-Mh and it would be the simplest way to do the experiment. I suppose I should use 3x1,2=3,6V, not 2 pieces? If you persuade me that this will not burn my USB port on laptop I could try
Thank you all for comments and advices