Li-ion batteries have battery management systems per cell . The functions of the management circuits are:
To prevent overcharging the cell (4.1 V) <== 4.2V
To prevent under discharging the cell (3.5 V) <== 3.0V
To prevent an instantaneous overload current (This depends on the cells capacity)
The connections to the battery are done with stainless-steel strips and the normally the connection is with a spot weld. If you use soldering for the connection do not overheat the battery