Actually Epson did not tell me tha tthe warranty was void, the camera was repaired in warranty, he only told me that in all the observed instances of that fault (shutter lock up) a third party battery was used, however they could not issue a formal warning since they have not managed to discover the exact technical reason.
That episode, together with the cost of the camera, and the fact that my RD-1 will be out of warranty in a few weeks time, are reason enough for me to use only Epson batteries.
Of course you may feel differenly, it's your camera after all and you are entitled to use any source of power you like with it. 😉