A battery circuit must have source and return so current goes out one side and come back the other side. Ground is only serve as a reference and you can tie to either side of the battery. Ground should not be part of the circuit in carrying current. So a more direct question might be, what is the mechanism that actually 'completes' the circuit.
Don't let the - tag confuse you, batteries do not have negative voltage; the 0 V receptacle is almost always considered the ground and is connected as ground in DC circuits too. The voltage difference between the + and the - receptacles is what is called the DC Voltage of the battery.
Current from a battery leaves the positive terminal and it has to return to the negative terminal before any currrent can flow. So connecting it to ground has no effect although it also won't do any harm if you do connect it to ground. The circuit will operate just the same if you connect one side of the battery to ground.
Water does not have to complete a circle like electricity does. Current from a battery leaves the positive terminal and it has to return to the negative terminal before any currrent can flow. So connecting it to ground has no effect although it also won't do any harm if you do connect it to ground.
Like any DC source, batteries have a contact which is marked with + and is the receptacle for positive voltage and a - contact where 0 V is applied. Don't let the - tag confuse you, batteries do not have negative voltage; the 0 V receptacle is almost always considered the ground and is connected as ground in DC circuits too.
Measure the operating voltage of the battery pack V b. Measure the voltage (V 1) between the negative pole of the tested-device and the ground connection. Measure the voltage (V 2) between the positive pole of the tested-device and the ground connection.