At the moment it's not set up to try and stand or hold its own weight when it's switched on. It lays flat with the legs pointed straight out, and the weight resting on the base in the middle. I have tried to make it stand, but at 2.5V it fails at that pretty quickly.
The legs move fine as long as they're just waving about and not pushing down in any way, but even if I can just about get it to stand (by positioning the feet directly under the "knee" joint so the outer servos aren't holding an angle), it still sits at 2.5V.
I'm not sure how much control I have over attaching servos since I'm using the Adafruit PWM boards that drive up to 16 servos each (or 9 each in this case). I guess I could try switching on one board after the other, rather than both at the same time. If that is the issue though, would the voltage stay consistently low?
I will try and weigh the robot, but again I'm not sure it applies if it's basically laying flat, and at 2.5V it doesn't have much torque anyway.
The buck converter is 6-40V input and 1.2-36V output. The batteries are in series so are over 8V when fully charged, and it does give the right voltage when nothing is connected to the output. It claims to be 20A/300W max, it was from Amazon so I'm not sure where to find a datasheet:
https://www.amazon.co.uk/gp/product/B06XYH1KKJ
It's a lot bulkier than any of the lower rated converters I have. If it does really have a lower limit than stated though, would joining two of them have raised the output voltage on each one slightly?