I want to build a battery for an electric bicycle using 18650 batteries. But I'm not into spot welding so much. Would it be possible to build a large case for the 18650 batteries with these types of black plastic cases serial connected together and then just put the batteries in place when I want to use them instead?
From a strictly electronic point of view, one can construct a larger battery using the cases you've posted. In your example, an electric assist bicycle, you will have relatively high current draw requirements. The wiring and spring loaded contacts will fare poorly under such high currents.
It's possible that the wires will melt the insulation due to the heat generated, or the spring contacts will get hot enough to cause the case around them to melt. Once the insulation is melted, short circuits can occur.
It's common for batteries for e-bikes to be rated to 40 amperes current draw and those wires and contacts will not tolerate the level.
With soldering of this type of battery, the heat from the tool will likely damage the battery and might cause explosion. Multiple batteries composed of this type of cell use spot welders specifically made for this purpose. You can find a DIY resource by searching "DIY battery spot welder" for options to complete your project.
Any type of welding is not a good idea. But more to the point - if you don't include load-balancing electronics both for charging and discharging, you will be VerySorry(TM) .
To start out, you should read up on topics such as "Charge in parallel, discharge in series" depending on the anticipated charging voltage and output/drive voltage intended. each cell can only put out its own rated current (without voltage sag), and can only charge up to certain voltages and current rates; those current rates vary dramatically with the charge level of the cell.
There's a reason that Tesla Corp's vehicles have gigantic amounts of hardware and software control of the battery packs.
