If you shoot with a P+ back out in the cold or for long exposures I can highly recommend the CH-910 external battery pack/charger. You can attach two batteries, including the much larger video camera ones, to the unit and put it in a small bag along with a couple of hand warmers and shoot for ages. Last night I was shooting for two hours and the unit basically was less than 50% used even in freezing conditions.
The other good thing is that you can use the same unit as your charger and so I tend just to take this on the road with me in the bag when I travel. (The cautious side of me also has me stash one of the dual chargers in my checked luggage just in case).
They only seem to be available used these days and so you have to be sure to get the battery pack cable. Most of the ones I see listed don't seem to have this and so it's just a dual charger without it. The battery plate attachment for the back is the Canon DC920 which you can find on Amazon.