Just an assumption that you are questioning the Volvo hybrid in this thread ?
The voltage is 400. If you can locate the inverter that drives the motor, that will show the HV cables.
About the extra cells that you want to solder together, if this is what you mean, I would vote against soldering and get a resistance welder that spot welds nickle strips to the cells, quickly, without heating the cells, which will have very detrimental effects on the cells.
If you are not sure about the inverter location, respond here and I will do some research for that location. I will also try to find a printout of the cables exiting the battery for the Volvo hybrid ? Does this image help you ?
https://commons.wikimedia.org/wiki/F...nd_battery.jpg