good, lightweight battery?

So about two months ago I went away for a while, came back, and my battery was dead. I charged it, and it stayed fine for a while. Then I noticed it wasn't cranking over as fast as normal. Friday it died at the autostore, so they charged it, and It fired right up. Then last night I used my bottle heater for my n2o and the battery died AGAIN. It shouldn't have drained the battery because the heater uses very little electricity. So I'm pretty sure this battery is shot. I just don't wanna buy a new battery if the battery isn't the problem (starter or alt). But my car was rapidly clicking. And the starter wasn't making any noises. It seems to be the stock motorcraft battery, and I have close to 70k miles!

So what would be a nice lightweight battery? Thanks for any comments/replies!

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