Unfortunately, he wears 2 Ni-MH that are not standard (AA type). Each battery has a price of 10 euros.
Panasonic has recognized the problem and has launched the product EW-DJ40, which uses two standard AA batteries. The EW-DJ40 gives the same pressure (590 kPa, 1500 pulses) and has a capacity of 165 ml (against 140 ml) - see intructions webpage.
I'll retoruner product and buy the EW-DJ40.