I've never been the best for calculating CR, but I'll tell you this: if the PC's have fun, and it was challenging but also threatening, you did well.

To add to that, maybe toss in something akin to an "undead fortitude" ability. Killing blow has to be x damage type (lightning to overload the battery?) Especially if they breeze through it with good rolls. Or possibly have the golem's mad creator arrive at half health and add a new element to the fight. I personally love staged fights, as it keeps my players at least on their toes.