Asus laptop battery slowly dying

Not sure if this is the correct tag or not, but I bought an Asus gaming tuff laptop that was refurbished and it was working fantastic but as of lately the battery has been sitting in the 40s (its currently at 42%) I have smart charging on so it will charge up to 80% but as of lately its dropped down to the 40s, I have done diagnostic checks on it and it says the battery life is good so I've got absolutely no idea what could be wrong with it and why its refusing to charge properly