It is quite common to have significant sparking when attempting to connect an alternator
outut or an inverter/charger to a battery
terminal. The main reason is that the equivalent circuit is a series or parallel resonant RCL (resistance, capacitance, and inductance) circuit that exists with such sources/loads IN CONJUNCTION with the fact that it is extremely difficult to make the connection without "switch contact bounce". It happens very fast as the connection is made with varying values of resistance until finally the mechanical connection is steady.
Switch designers are aware of this difficulty and attempt to slam contacts together with mechanisms designed to do so. By doing that well contact wear and erosion will be minimized increasing the life and rating of the switch. You just can't do it by hand quickly enough with sufficient pressure to come close to zero Ohm contact in a smooth fashion.
The RLC circuit resonates and can create thousands of volts as the connection is made via varying resistance over short periods of time and sparks occur.