This is a nice concept, but please remember that you don't see the title on the shirt, and so not many people are going to understand the original concept. I usually look at the shirt before I look at the title or the designer's comments, to see if the shirt's concept can be easily understood. My first thoughts were that the big battery had taken the smaller one's place or something.
My suggestion is just to work on clarity, which seems to be the biggest problem designers on threadless have. If you can get clarity, you'll be a step ahead of the crowd, and you'll definitely have my vote.
Yeah I didn't really get what you were going for until I read the title. I like the idea though. Also, the fact that the car battery is labeled "car battery" makes it feel like you're forcing the joke. Could I suggest maybe putting a made-up brand name on it instead? Like "Sure Start" or something like that. I think it will feel more natural.
Personally, I don't think you need the title to *get it* I think it's the sort of design that could leave itself open to a number of interpretations, and that's fine. The little battery could be afraid of/mad at/in love with/dreaming of being the big battery. The ambiguity kinda adds something, I think. $5
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