Finally I modified my Rohde&Schwarz FSH6 spectrum analyzer to run from a Li-Io battery instead NiMH. Dying batteries are an annoying problem of the old R&S FSH3/6 series by design. The device has no real power switch but goes in standby on power off. Drawing a standby current of about 1.2mA in combination with the memory and self-discharge effect in NiMH batteries let them die sooner or later.
To solve that I decided to install a Li-Io battery with protection board. Apart from a better Li-Io life cycle these batteries double the capacity with a lower cell count. Original 6S1P 3200mAh NiMH replaced by 2S2P 7000mAh Li-Io type 18650 cells. 2S2P means two cells in parallel with another two cells in series.