camelback

I've just come into posession of a '47 Buick Super 8 and I've been driving it about once a week to maintain it. Last week, I went to drive it and it wouldn't start. I'm located in Phoenix where we've been having average temps over 100 degrees and my predecessor said that the high temps cause the battery to die. I used the electric charger that was left to me and charged the battery for about 15 hours and still the only thing that happens is what sounds like the starter - the engine just doesn't catch. I've got the clutch totally disengaged and the accelerator floored just like it says in the owners manual. I also ensured that it has gasoline. I'd really appreciate any help. I'm definitely new to the game of caring for a car, especially a classic such as this one, so layman's instructions would be appreciated.