Dear Luis Carvalho, the extraction of the batteries it´s explained at your shaver manual. See this PDF online at page 11 (english) or 19 (spanish).
http://www.service.braun.com/line/SH/S5493/S5493_13_MN.pdf
The problem is that this kind of shaver don´t offers replacement of the batteries. Once the batteries ends the shaver will be recycled or only works connected to 120V-220V power plug.
But you can try desolding the old batteries. I think will be a AA or AAA standard batteries of 1.2 Volts, but it don´t have +/- poles such as you see in any alkaline batterie. It will have 2 soldering blades for +/-.
If you want to do this, extract the old batteries and search a similar NiMH battery in any local electronic parts store, or try to solder any standard NiMH battery (batteries for digital cameras, MP3...)
It´s all I can do for you.
Adios, buenos dias.
Alberto from Spain, January 2008