12 volt battery dead

I live in Maine, which has very long and cold winters. I just recently replaced the original 12 volt battery in my 2007 Prius with 144,000 miles on it. That's nine years, so it looks as though cold weather is more beneficial to the battery than living in a hot climate. You might call it the refrigerator effect - it helps to preserve things longer. I never gave the battery a thought, as I never had a problem until I recently changed the ATF fluid and then the car wouldn't start. At first, I figured that a low ATF sensor must have triggered the no start condition as a safety preventive measure. After being told that there is no such sensor, and reading that a low battery would cause the symptoms I was seeing on the dash panel (red triangle of death, shift indicator flashing, etc.) I knew it was time to replace the battery. Bought a Exide blue top AGM battery locally, which is 1.25 inches longer than the original, and of course it wouldn't fit the battery bracket, so modified the bracket by removing the "L" shaped end piece. This battery has guaranteed replacement for 4 years, and has considerably more power, so seemed like a good choice. The price was $135, and other area parts stores were asking as much as $196 for the same battery, so it pays to shop around. There are some good deals online, too, but the shipping charges tend to eat up the savings.