I have four 3.7V ion-lition batteries each connected in series (14.8V). The voltage measured in the batteries before starting the test was 13.5V. When connecting a load (DC motor) with consumption of 220mA, the voltage in the battery started to drop and I disconnected the circuit when the voltage measured in the batteries was 7v. Then without the load, I noticed that the voltage in the battery slowly increased again until it stabilized around 12V. Why did the tension increase without a load?