I purchased a 20,000mAh “jump pack” for $120CAD two years ago to use with my boat. I’ve jump started it with no problems, and this was a Mercruiser 350. I’ve also boosted a car and a truck in -25C weather. They work great, are super portable and only weigh 2-3lbs.

Caveat - unlike a standard battery booster pack (lead acid), lithium battery does not take kindly to freezing temps, so it is useless to keep it in the car over winter. I wouldn’t expect to be able to start anything in the middle of winter up here unless it was kept indoors at room temp.