I have a batch of apple cider that started at 1.050, and has fermented to 1.000. Would adding sugar to increase ABV ruin my batch?
I want to add roughly 1Kg/ 10L of cider. Adding half now and the other half when it slows.
Your success will be affected mostly by the health of the yeast in the cider. If the yeast are healthy and vibrant, the added sugar should ferment quickly. If the yeast are less healthy, there may be a long delay before fermentation starts, it may proceed slowly, or it may even not start.
The primary contributor to yeast health is age. How long has it been since active fermentation ceased? If the yeast has been sitting at the bottom of the fermentor, you might have some problems.
If you're at all concerned, you're safest route is to pitch new yeast with the added sugar. Rehydrate as per the instructions on the packet, make sure the cider is within the yeast's optimum temperature range, and then pitch after adding the sugar.
I don't think you will have any issues with it. If you are concerned about sanitation you could always make a syrup by boiling the sugar with water, letting it cool then throwing it the cider. You could always just throw in without. I'm a pretty big sanitation Nazi, but when I make cider, i just throw in simple white sugar straight from the bag, and when i use honey in my beers I simply squeeze the bottle into my primary. At the very least, stir it in with a sanitized spoon. It would also probably be a good idea to throw in a 1/2 tsp or so of yeast nutrient, to help kick off and ensure fermentation starts. The container will tell you specifically how much to add.
1 pound of sugar is roughly 1 point ABV for a 5 gallon batch of cider.