the weight of the phone is around 200 gramm.
The battery weight is 66 gramm, the razer phone alone without battery and without backcover weight is 129 gramm.
So the battery has 1/3 of total weight.
If i can get rid of the battery INSIDE the phone and place it anywhere else, closer to the head. If we reduce weight of headset of the parts that are located more/most distance to the heasd, the leverage effect will result in smaller effect.

so iam looking for an smartphone FPC expansion cable with connectors.. male and femal ends.

the battery is connected with a FPC pin motherboad connector, 6 pins.
P+ ID P-
P+ P-
P+ NTC P-

ID – is a battery Identifier… this can be a showstopper
NTC – is thermistor, a resistor with a special ohm value curve alonge a certain temperatur range

the FPC connector at battery side looks that:
https://www.amazon.in/HUKATO-Battery-Motherboard-Connector-Nokia/dp/B085QN1P88
or here
https://de.aliexpress.com/item/32881726668.html
„2 STÜCKE Inneren FPC Steckverbinder Batterie Halter Clip Kontaktieren für Xiaomi 5 6 mi5 mi4C mi4i mi4 mi6 mi5S mi5S plus mi hinweis 2“

I could not find the part for razer phone BUT i found images for other phones that look visualy like razers one.

So, first try. i bought a 10,-€ battery for Nokia 5.1 ( HE342 / HE361).

When i plugged in the nokia battery, that has same voltage rating, but not same capacity, and not the razor like ID, the phone starts as normal, display shows animated logo..
BUT as soon as you can enter the PIN.. the phone shows message „Shutting down“.
So it does not work, i guess the software is not accepting the different battery ID.
If the startup would check that battery ID first on bios start.. the phone could shut down earlier.

Next Steps:
So i need the correct battery ID.. so i have to get a old razer phone battery from ebay and look if there is a chipset anywhere.. i wont break up my working razer battery for that.
OR
order the female and male connectors from Aliexpress.

other sources/suppier in Germany/Europe
https://de.aswo.com/index.php?id=1921&L=2

battery may be so expensive that it could be better to buy a used razer phone.

desolder from board parts…

https://www.ng-mobile.de/ platine service repair B2B

crazy equipment
https://www.mobileparts.shop/tools/by-category/battery_tools.htm?filter6=767&lid=3