Hello oldmobil,
battery was nearly dead, at least now it is... It accepted some charge when I wired it to a bench power supply (NiCd cells likes overvoltage), but charge was under 0,5 volts and dropping fast so I took out the cells. I saw your AA battery conversion earlier and I will try it too, thanks for that.
But I think the dock might be broken because I opened it, plugged it in, and multimeter saw ~12 volts coming to the dock but nothing on the contacts. Unless it senses batteries by impedance or something, it's broken.