When I dropped my engine in last time, I had slow cranking which was funny since I had a good 6V battery and it always cranked OK. Eventually I saw the throttle pushrod smoking. I had forgotten to hook up the engine ground and it was grounding through the throttle rod!
Another time I somehow twisted something inside the starter while putting it under the car and it would hardly turn over. I had apparently internally shorted the starter which was easily corrected.
Both of these are cases where trying 18V would have been Very Bad.