lawrosa,
Thanks for taking the time to produce the video.
Voltage drop may be greater in six volt jars because of the relatively thicker plates.
12 volt jars have the advantage of having twice the number of cells. That means 50% less load per cell. The plates are of necessity thinner, too, so there is inherently lower voltage drop.
The biggest issue you have is a not very nice MSW inverter. There are some MSW that do a little better than what you have. I had a nice Cobra 2500 watt that would power up the 13500 btu air conditioner. It showed 118 volts on a Kill-a-watt meter.
