The test ended up showing a higher efficiency than I expected, it seems nearly too high.  I conducted the test with as robust of a technique as I thought I could do with the equipment that I have.


Affiliate links:
Battery Load

https://s.click.aliexpress.com/e/_AfM7OW



This is an updated version that I haven't bought yet, but will.  It has a 4 wire connection and improved connectors so will be way more accurate.


https://s.click.aliexpress.com/e/_AUNvbA


Watt meter
https://s.click.aliexpress.com/e/_9QCPWq



Power supply:  This is a decent grunty supply (fairly expensive) It's not really suitable as a lab supply since it has large capacitance on the output - this means that if you are current regulating a sensitive device that has changing loads (driving an LED or something) it could temporarily overcurrent the device.  Great for charging batteries or just a good constant current constant voltage supply for general purpose. 32 Volt 32Amp.  I've tried the smaller form factor supplies similar to this, however they do not seem to be as good or robust.
https://s.click.aliexpress.com/e/_AV63mI