I own a 10545 as well. The gentleman is correct...anything less than 12 volts, and the starter won't engage. I also agree that they are so easy to start, that (for me) the power start feature rarely gets used...BUT, it does get used by my girls or the wife, if any one of them decides to cut the grass...which they do a fair bit of the time in light of my work schedule.
As far as buying a battery online...I don't. I go to my local "Batteries Plus+" and have them rebuild them. I get a lot better battery, very quick turn-around, and costs less...especially considering that you don't have to pay shipping & handling.