Why does my laptop only work when plugged in?

Hi,

i have a Lenovo ideapad 320S laptop which only works when the power adaptor is plugged in. When I remove the plug, the laptop will remain on for about another 5 minutes before shutting down quickly (despite the fact it shows 100% battery). I recently took it to currys pc world in which they claimed to have fixed it yet when I took it home the problem arose again (I’m guessing they plugged the power adapter in straight away to investigate the issue rather than seeing how the laptop worked on battery alone)

wondering if if there’s anything I can do myself or whether the laptop just needs a new battery ?

many thanks