Is it time when your car doesn’t start on a -30 degree day or is that just because it’s insanely cold out??
Yes and no. The colder it gets the harder it is for the battery to produce the juice, but a good strong battery with a healthy charging system shouldn’t have a problem.
First, define “car doesn’t start” - I’m assuming that you’re saying it won’t turn over or turns weakly and then dies. If so, then you need to have your charging system and battery tested. You might, e.g., have an ok battery, but a dying alternator. Or you could just have bad or dirty connections.
Most big box auto parts stores (like advance or autozone) will test your battery & system for free and will install a new battery for you if that’s what it turns out to be.
A brand-new fully charged correct CCA rated battery should start your car in any temperature encountered on this planet. A borderline battery will sometimes work again once it gets warmer out, but it’s only delaying the inevitable.
Your battery may be perfectly good for another two or three years, but extremely cold weather can still defeat it. Car owners accustomed to extreme cold have a number of tricks for starting on such Arctic conditions. These include engine block heaters, a few minutes connected to a charger, or bringing the battery indoors overnight. Also consider one of the popular portable starters.
Stop by an auto parts store and ask them to check the battery. Pick a warmer day or remove the battery from the car and bring it to them.
If your battery is over 4 years old it might be time to change.
Have it checked with a load on it.