I do some high altitude (4-4.5k metres) trekking. At night this can lead to very low temperatures (down to -20oC). My choice of battery type for my camera is lithium ion, or rechargeable lithium ion. My flash gun uses AA batteries, so I have open a very wide range of battery types - alkaline, lithium, metal hydride,.... I know that to have batteries survive these conditions, I need to have multiple batteries which are kept warm (usually next to the skin) and swapped over regularly. What type of battery copes best with low temperature? While my camera manufacturer (Nikon) provides some information it is very minimal and does not really answer this question. Other searches on the web have provided very little information comparing battery performance at low temperatures.
I keep hearing that trekkers have problems with their equipment as a result of altitude alone - usually at or about 3-3.2k m. On investigation, the equipment always seems to be Apple. Is there a general problem based solely on altitude? If so, what?