When Exposure introduced the first powerful bike lights which had the batteries in the same body they were revolutionary, but now most bike lights up to 2000 lumens are self-contained. Separate battery systems still have their place if you need lots of run time - or the option to switch to a second battery if the first one runs out. Separate head units are also more compact and lighter for helmet mounting or cleaning up crowded bars and modular systems let you mix and match to create your ideal set up, or just replace/upgrade bits as they wear out.