Regardless of the windchill, to a piece of machinery or a thermometer -35 is -35.
Copied from the Yuasa website: "What Are Cold Cranking Amps? Essentially, Cold Cranking Amps is an industry value to rate a batteries’ ability to start an engine in cold temperatures. The numerical value refers to the number of amps a battery can produce, and sustain for 30 seconds, at a temperature of 0 degrees Fahrenheit." - "Check voltage every 3 months (or more frequently if stored at higher temperatures as high temperatures cause higher self-discharge rates) Recharge battery if voltage is below 12.4V. (12.48V for conventional flooded lead acid batteries). Once activated, the battery is permanently sealed and must never be opened. There is no need to add water to AGM batteries. Adding water to AGM batteries will result in irreparable damage to the battery or the vehicle."
It is reported that a lead acid battery (conventional or AGM) will be 10% weaker for every 15 to 20 degree drop below 80 degrees F. At this rate a battery is only at 30% of it's capacity at -20 degrees F. Now @Mark Anderson
stated that he let his machine run for a couple hours to charge the battery. Again, from the Yuasa battery website; "Magnetic stators don’t charge at low RPMs." Letting it idle may have actually been discharging the battery unless the idle speed was increased to about 3000 rpm. If the battery was discharging while idling, shutting the machine off would allow the battery to self charge to a low state of charge, but possibly enough to start the engine.
From Lithium Pros website: 'ALL batteries will perform poorly in cold weather, regardless of whether they are lithium or lead-acid. But, a lithium battery will still outperform a comparably-sized lead-acid when the temperature drops. However, since most people try to run the smallest and lightest lithium battery possible, there are some things to be aware of: For a daily-driven vehicle, almost no extra effort is required. On a very cold morning, however, one might find that the cranking performance of the battery is not as strong as usual; if it is very cold and the vehicle has been sitting for a while, the engine might not crank fast enough to start. This does not mean that your battery is dead.
Simply turn on the headlights for a few minutes to drain some energy from the battery. As a result, the battery will warm up internally and gain enough “juice” to start the engine.'
It's one of the reasons I like lithium over lead acid batteries