2006 E320 CDI , no issues , new fuel filter , all maintenance done recently.

Problem : I live in Canada and this november it's my first time owning a diesel in winter , our temps have dropped to around -8 Degress celcius. Now everytime I start my car it only starts on the 2nd try , the first time it cranks for 3 seconds and stops. 2nd time it cranks and fires up. It also says my battery is low which could be due to an after market alarm. Either way you guys have any ideas to why that could be happening? (NOTE : It never did that during summer time)