If you ride in cold weather, lead acid or AGM are your only choices. The other chemistries lose their CCA capacity dramatically as you get near freezing. Porsche makes light weight batteries for their cars, but does not recommend them below 50F as an example.
Lighter batteries are nice because they take weight off the top, making bikes more flickable and, for those of us who like to clean up the mid sections of our bikes, they are also easier to squeeze into tight spots like under the cowl since they are smaller.
They can also be made in complex shapes to fit available nooks and crannies. But, ask Boeing how they like the new lithium ion formulations ;-)
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