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I'm very glad I never tried them! My old-school Li-ion batteries perform well in temps down to -20 F. Maybe even colder than that, but fortunately I haven't experienced any temps lower than that yet!

You can still charge them technically but at a highly reduced c rate. But for the health of the battery it's best to heat them.

Luckily my pytes setup has built in heaters that kick in around 45f and if they cannot keep up charging is reduced by the build it BMS.

That would never do for me, winters up here just get too cold to be using electric heaters on the batteries.