I have one of these with a compressor built in to it so that I can pump up my tires with the same convenience. Damned if I can figure out why I always have at least one leaky wheel whenever the tires are changed from summer to winter.
Another Canadian Tire special -- watch for the sales.
Since mine is being used weekly on one of my tires (spring is just 11 weeks away!), I charge it regularly using the handy cigarette lighter plug. With respect to keeping the thing charged, the answer is a cigarette plug on board -- thus when your engine is running you are topping up the booster pack too.
I had a season where the alternator did not work on board, nor did I have a reliable charger. I used a similar battery pack to charge the boat's battery by leaving it connected as if boosting the boat, and then plugging it in to the shore power circuit to charge. The unit's charger sensed that the voltage was low across the two batteries and applied current to bring everything up. Not the best thing for the batteries nor the unit, but it did work very well.
The guts of these booster packs are lead acid gel cells -- 7 to 12 amp hours. They charge using a low current trickle charger.
Happy winter from snowy Montreal!
Christopher