The following is not scientifically based, it's just my latest experience...
About 2 years ago, my wife parked her 2010 Focus on a city street while she ran an errand. When she came back, the lights would turn on but the car wouldn't start. Up to that point, the car had been slow to crank a couple of times, but had exhibited no other symptoms of a failing battery. She did not leave on any lights or other power drains. Plus, I'm pretty careful about keeping both ends of battery terminals clean, so the problem wasn't corrosion.
I tried to jump the car from my truck (decent cables - don't know the rating) but could get no results. After about half-an-hour of trying, I called the owner of the dealership where we bought it. (Thank goodness he's an old and close friend!) He just laughed and said that they see this fairly regularly. It seems that the cause is that many newer ignition systems draw so much power that only super-heavy-duty jumper cables have even a chance of getting the car started.
He lives just a few blocks from where this happened and drove over with a better set of cables - the car still wouldn't start. He finally called the dealership and confirmed that they had the correct battery in stock. He then called his son-in-law (who works for him) and had him drive the battery to where we were. We installed the new battery and the car started perfectly on the first twist of the key. My friend (the dealer) then told me that when a customer calls with this problem, sending a tech with a new battery is now pretty much their standard response instead of having the car towed.
Just my two cents...
Dave B.