So finally i came across my cheap accumulator drill set that contains 2 powerful rechargeable batteries with 18V with 1.300mAh capacity and a charging device for around 39€.
Combined with a really cheap L78xx voltage regulator we can choose the demanded voltage for any chosen consumer device.
In this case i had to power a Lego NXT brick, several Lego servos and additionally an Android smartphone device. As the Lego NXT brick operates on 7.2V i took a L7809CV voltage regulator to get 9V out of the drill accumulator (i already checked that the NXT brick worked with a 9V power adapter). You can see the PIN layout of L7809CV (78xx_datasheet) in following image:
You can find the full project and many more at : www.smartlab.at