Using external HD & Time Machine

Obviously cannot leave the external HD plugged in all the time on portable laptop so Time Machine can do its hourly backups so will Time Machine still work if I do twice weekly manual backups?

I have read Time Machine can do snapshots when on battery power is that only on batt power or does it do them anyway and then when the external HD is plugged in again then transfer the snapshots to the disc?

First time Mac owner so all is new, wonderful & confusing!!

Thanks