Bluetooth Headset Charger Siemens Gigaset S810

This is a small housing for the electronic needed to charge a Bluetooth Headset using the power supply of the Siemens Gigaset S810.
The Gigaset S810 uses a power supply which is used to charge the mobile telephone. On the power supply a voltage of 4 V is given but the output is 7.2 V which is strange for a original part. So it is better to measure it. I uses this step down converter
http://www.ebay.de/itm/DC-6V-40V-To-5V-3A-Double-USB-Charge-DC-DC-Step-down-Converter-Module-DP-/111856162513?h