It's always wise to carry a spare battery while shooting. Batteries seem to die at the most inopportune moments. Remember when you bought your Leica M camera and there were those little bags of extra goodies that were looked at once and then thrown in the drawer? I use one of those extras all of the time. The DC cable for the battery charger is so often overlooked. It is a pro trick to charge your batteries while traveling between locations during the day. Never again do I have to stop shooting because I don't have a charged battery.
Leica DC Charger