Hello. I have a laptop Battery for dell with 4 cells inside 14.8 volts. Because the cells was 5 years old my battery lasts like 10 minutes. So i tried to open it up and replace the cells with new ones . After i finished and closed the battery package with the multimeter i checked the voltage and it was 0. I searched a little bit and i found that you maybe have to reset the bms of the battery by simply connecting the last possitive battery 15 volts to the possitive of the bms but nothing happened. When i checked with the multimeter again the voltage was not zero but started from 6 volts and ended up zero. But to be honest i wasn't so careful. Is there something i could try ro fix the problem or i just ruined my bms?

Thank you in advance

 

Most likely you blew it.
With that much info that is all I can say

 

most lappy bms mcu's die if they detect one cell groupe below a certain voltage(so called saftey feature),you need to keep power to all the bms pcb battery inputs when renewing the cells,no way to reset them as far as i know.