These older winbook laptops were made by Twinhead. Winbook does list a
5021 model and I am not familiar with it either, so you could contact Twinhead if all else fails. Are you sure it is a Winbook 5021, typically the legacy Winbooks are XL, Si, XP, LM modeled units. I had a Winbook XL2 that I went through the same struggle. I found that without having to charge a battery (mine was dead) the 19V (65watt) power requirements were rather flexible. My notebook ran just fine anyware from 12V - 19V with a universal power supply, which means you could use just about any laptop power supply as long as the polarity is correct and the plug fits. All the legacy Winbooks I used to play with had the same polarity in which the center was positive. A battery will cost you anywhere from 80 - 130 dollars along with a power supply in the range of 20 - 50 bucks. Upgrading these old machines is not neccesarily cost effective anymore.