Measure the battery voltage with a multimeter. The voltages of the two batteries we have are 3.66V and 3.76V respectively.

As you can see, both batteries have the anode on the left and the cathode on the right.

Anode (Positive terminal)

Cathode (Negative terminal)

We are combining a 2900mAh iPhone 7+ battery with a 2675mAh iPhone 8+ battery and turning it into a 5575mAh power bank.