2 order low pass filter

[Harry (Gast)]

hello guys,

I would like to contruct a 2 order active low pass filter to satabalize 
the input voltage for my lithium ion battery since it is occilating. I 
tried with Opto coupler and digital isolator but it does not work 
perfectly.
Was thinking to use an OP amp. The "Grenzfrequenz" could be between (0,2 
- 5 HZ).
Have no idea which component i have to choose that i will get a stable 
signal at the end with an Amplification gain of (+-5).

[Jacko (Gast)]

Yeah, it's always the same problem with that damned line-voltage.
In Europe it's oscillating at 50 c/s and the worst: it's much too
high for those lithium-cells.

A really OLD friend told me something about transformers and bridge
rectifiere... UNBELIEVABLE!