Battery issue..... i think :(

hello to all!!!

so as you can tell from the title its about my battery

at startup without the charger it dies after the dong.... if the computer is already on (when the charger is plugged in) the computer works fine, but when i unplug it the computer will last me between 5 and 20% (only sometimes the 20%)

what can it be?? i already had the motherboard changed once because the computer died on me

appreciate the answers, TIA