its normal, thats why companies recommend you getting a good battery during the cold winter months.
batteries are rated in cranking amps (CA) and cold cranking amps (CCA). cranking amps is how much power the battery can exert above freezing, CCAs are amps that can be output through the battery below freezing. (it may be 0 degrees, i'm not sure) more amps = more power exerted, and since CCAs are lower that CAs always, it will put out less amps/power at lower tempurature. this is also explained by electrons are excited when warm, and slowed down when cold. hope this answers your question.