Ipad battery goes DOWN as I am charging while Uong safari and iBooks

I am outside in bright sunlight and my ipad is about to run out of battery. With my ipad charger and an extension cord I charge my ipad but the battery actually goes DOWN as I am using it. I do have max brightness because i am outside, but is this supposed to happen? Is there a way to fix this? All I am doing is using iBooks and safari.