A calendar life study was conducted on lithium ion pouch cells which were stored under float charge condition at five temperatures. The half cell study showed that the anode experienced severe loss of the active material, especially at high temperatures. The capacity fade mechanisms were then proposed. The capacity fade at low temperatures could mostly be caused by the loss of lithium inventory to side reactions and impedance increase. The capacity fade at high temperatures demonstrated a two-regime pattern. The fading mechanisms in the first regime could be similar to those at low temperatures. The capacity fade in the second regime could be dominated by the severe loss of active carbon material. The impedance rise plays a minor role in the second capacity fade regime.