Dell L322X battery issue

Hi all,

I bought a Dell L322X laptop off eBay around 8 months ago. Its worked fine till now, with no issues or problems. I recently had the "Plugged in, not charging" issue with the battery; it was fixed with a charger off eBay that wasn't Dell made. I've since ordered a new original Dell charger off Amazon, it worked fine for around 2-3 weeks.

However, the battery has since reduced its capacity on what seems like a weekly basis. First it stopped charging at 78%, and kept on going down by around 2-3% a week. It would charge up to 78%, and then stop and say it would take 4-5 hours to fully charge the battery. Even when left overnight on charge it wouldn't go any higher then 78%. It now stops at 60% but when I hover over the battery icon it displays the "Plugged in, charging" message. I've used the Dell detection program and it says there's no fault with the battery so I'm not sure what the issue is here. Do you think its a battery issue and I need a new one? That's what the local computer shop suspected was wrong but I've read that it may be a software issue with Windows 10, and was wondering whether anyone knows anything that could help me here? I'm a student so need the full capacity without having to charge it every few hours.

Thanks.