The 2 batteries inside are connected in paralel to the smal PCB that are inside the
battery pack. So the capacity is doubled, not the voltage. So I searched for some similar capacity battery on ebay and buyed this
https://www.ebay.com/itm/3-7V-5500mAh-4795118-Li-po-Polymer-Battery-For-Tablet-PC-Power-Bank-PAD-laptop/121961352525?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
I placed them at the outside of the back of maxisys with some double adhesive tape. And they are connected with wires to the smal PCB board inside, I glued the PCB to the case.
Next time the batteries must be changed then it will be a 5min job :-)
This solution works great for me, and its very cheap !