I have been let down by my lithium jump-starter too often. I go to use it and the battery pack doesn't have enough charge, it is around four years old now and the battery pack swelled a bit, meaning the seams of the two halves of the rubberised plastic shell have separated. It's all held together with PVC tape now.
About nine hours ago I placed an order for a supercapacitor version instead.
There's no need to have this thing charged at all. As long as the recalcitrant vehicle battery has more than 3 volts, there's enough energy contained within to charge the. capacitor pack. The one I've chosen has 750 amps capacity, capable of cranking an 8-litre petrol engine of a 5-litre diesel.