|Sounds to me like the A/C power adapter is faulty and your laptop is only getting power from the battery. When the battery runs flat you won't be able to use the laptop at all, so you'll need to buy a new power adapter.|
Not the cheapest option, but definitley the safest one, is to buy a new adapter from the laptop manufacturer via their website. Most laptop manufacturer's have a link on their website to order a new battery or a new power-adapter. And you can be sure if you buy it from them it will definitley be the right one and built to the correct safety standards.