[amy2203]

I think for what you need you need 4 different sales, as you want to specify the free product for each option, or you could create a new product called Free Battery, set it to hidden but available for sale, and select that as your free product. Then you can just pick the right one when you pick and ship the order.

I've found onsale very good, it can take a bit of getting your head around, and depends on your products and what kind of offers you want to do I suppose.