Li-ion can be a generic term for a battery with a number of different chemistries, some will run quite happily at temperatures below zero C, but most should NOT be charged below zero C. First question, what type of Li-ion battery chemistry are you using.Second question, are they both being used and charged below zero C, or just used at that temperatureGenerically, they will operate with reduced capacity below zero C, down to around -40C in special cases.There are manufacturers who supply low temperature cells and specify temperature performance for their products, so I suggest that you buy your batteries from a known manufacturer, and get the data directly from them. They do tend to be expensive, and are often configured for specific applications, however.The Battery University is a good place to start to get general information.Have a look at this paper re charging and discharging for a specific cell type.Incidentally, how is your battery pack configured, since maintaining parallel/series battery packs at extreme temperatures will be critical, the worst cell in the pack could easily deteriorate more rapidly than normal, bringing the whole pack down with it. Charging to maintain the cells is equally as critical, but that is another whole topic, and without knowing your cell configuration, no further comment is possiblecheers,Richard
