Hi Genster, I use any hearing aid battery marked 675. Zink Air. Its the closest to the voltage required.
They don't last as long as mercury but they don't deteriorate until opened. Give em a hour to power up.
Comes in packs of 4 or 6. I just checked it in a 500G
Hearing aid batteries are available from pharmacy's & some supermarkets.
hello genster ..... i have checked and the battery in my 500 G is AG13 (357) 1.5v. I have a couple spare if you have any difficulty though i think very easy to find .