Use one big table or multiple small tables for described scenario
You definitely should not have one table per device. That means you'll have to create dynamic SQL to merge the device name into the query, and create a new table every time you add a device. So version 2 is much more appropriate than version 1.

But you don't necessarily need separate tables for voltage and current. You could have a single table device_log, with columns:


id - primary key
sub_device_id - foreign key to sub_devices table
timestamp
type - 'voltage' or 'current'
value 


Or if you don't need so much generality, and always read current and voltage at the same times:


id - primary key
sub_device_id - foreign key to sub_devices table
timestamp
voltage
current