Battery Low - Cards not working properly

I have an old mainboard (3-4 yo). I left my PC alone for about 2 weeks, shuting down it's energy supply. After that I built in a new CD-RW/DVD-drive and started the PC. It showed by start a warning about low battery and shows values between 1,4-1,6 V.
The Problem: The first time I started the PC it worked OK out of the msg in the beginning, I even checked the internet about this Warning MSG. Then I had to restart the PC cause of the new installed software of the CD-RW, and then:
1. My network addapter started to behave strange - it sends packets to the root but doesn't get any (I checked the root). The addapter is working.
2. My sound card misses to play the windows welcome sounds and there is a beep by ajusting the volume values.
The units using energy supply are: 2 HDD-s (I built 1 away), a monitor, mainboard and the new CD-RW/DVD-drive (I removed the old CD-R-drive). The energy supply is up to 250 W.
I am wondering if the problem could be found in the energy supply or it's deffinitelly a mainboard-problem. And if the values of the battery are changeing (not going down only), will I have to change the MB or I could have the chance to reload the battery (is the battery dead or just discharged). Any advices will be welcome. Thank you!

(Oo;?(In such a case, it is possible that all is bad or all is good but misconfigured.)

You cannot but calm down and do things step by step. Remove all devices and drivers also. Clear CMOS and Install them again one by one like a tortose. Consider Bios Update on this occasion. Don't jump at the goal like a busy rabbit.

Well, I bought a new MB. After i have seen the problem was still there ... I tried another network cable !!! It's awfull to have a cable which only sends to the server, but gets no visible packets. Anyway thank you for the help and for the community you have built here I'll watch some times if I can help also.