A method performed by an apparatus, the apparatus comprising: a second rechargeable energy source, a power connector configured to be connected to a removable first rechargeable energy source and configured to supply power from the first rechargeable energy source to the apparatus via a power supply line, a balancing element placed between the power supply line and the second rechargeable energy source, and a second power supply line and at least one electrical entity connected to the second power supply line; wherein the second rechargeable energy source is connected to the second power supply line, wherein one of the at least one electrical entity connected to the second power supply line represents a power amplifier, wherein the balancing element comprises a regulator and a switch, wherein the regulator is connected to the power supply line and to the second rechargeable energy source and the regulator is configured to prevent that power is supplied from the second rechargeable energy source to the power supply line, and wherein the switch is placed in parallel to the regulator, the method comprising operating the apparatus in a first state, the method comprising in said first state: - providing power from the power supply line to the second rechargeable energy source via the balancing element, - decoupling the second rechargeable energy source from the power supply line such that power supply from the second rechargeable energy source to the power supply line is prevented, - drawing power from the power supply line by the regulator and charging the second rechargeable energy source via the regulator in the first state, - changing the status of the apparatus from the first state into a second state; the method comprising in the second state providing power from the second rechargeable energy source to the power supply line via the balancing element in the second state, wherein in the first state the switch is configured to disconnect the second rechargeable energy source from the power supply line and in the second state the switch is configured to connect the second rechargeable energy source to the power supply line. a second rechargeable energy source, a power connector configured to be connected to a removable first rechargeable energy source and configured to supply power from the first rechargeable energy source to the apparatus via a power supply line, a balancing element placed between the power supply line and the second rechargeable energy source, and a second power supply line and at least one electrical entity connected to the second power supply line; wherein the second rechargeable energy source is connected to the second power supply line, wherein one of the at least one electrical entity connected to the second power supply line represents a power amplifier, wherein the balancing element comprises a regulator and a switch, wherein the regulator is connected to the power supply line and to the second rechargeable energy source and the regulator is configured to prevent that power is supplied from the second rechargeable energy source to the power supply line, and wherein the switch is placed in parallel to the regulator, the method comprising operating the apparatus in a first state, the method comprising in said first state: - providing power from the power supply line to the second rechargeable energy source via the balancing element, - decoupling the second rechargeable energy source from the power supply line such that power supply from the second rechargeable energy source to the power supply line is prevented, - drawing power from the power supply line by the regulator and charging the second rechargeable energy source via the regulator in the first state, - changing the status of the apparatus from the first state into a second state; - providing power from the power supply line to the second rechargeable energy source via the balancing element, - decoupling the second rechargeable energy source from the power supply line such that power supply from the second rechargeable energy source to the power supply line is prevented, - drawing power from the power supply line by the regulator and charging the second rechargeable energy source via the regulator in the first state, - changing the status of the apparatus from the first state into a second state; the method comprising in the second state providing power from the second rechargeable energy source to the power supply line via the balancing element in the second state, wherein in the first state the switch is configured to disconnect the second rechargeable energy source from the power supply line and in the second state the switch is configured to connect the second rechargeable energy source to the power supply line.