Discussed at some length here.
Temperature in 787 battery cells spikes in cold conditions: NTSB - 12/1/2014 - Flight Global
The NTSB was concerned that cold weather exposure could be a significant risk factor for the 787 main and APU [auxiliary power unit] lithium-ion battery," says the report.
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The NTSB notes that Boeing has since “revised the low temperature charging limit for the main and APU battery and the [battery charger unit] to reduce the likelihood of an internal short circuit.”