Peel, quarter, core and chop apples. Put as many apples in the slow cooker as possible and put the remaining in a bowl. Drizzle with a half of the lemon juice so they won't turn brown.
Add sugar and cinnamon, the rest of the lemon juice and orange zest. Cover the slow cooker and set it to maximum. Set the time for 8 hours. Stir from time to time to make sure nothing sticks to the bottom.
Add the remaining apples plus the accumulated liquid. Mix to combine, then cover the slow cooker. Let cook at maximum setting for a further 6 to 8 hours. Stir from time to time to make sure nothing sticks to the bottom.
Taste and add more sugar if desired. Fill hot apple butter in sterilised jars and fill close with lids. Turn the jars upside down till cooled down.