The car starter is 18Ah. Divide maH by 1000 to get aH. It's 12 volts, so multiply volts x aH, and that starter pack is 216 Watt-Hour. I am not sure you could get 18Ah out of the starter battery. It drops in voltage as it runs down and the ad says it will not work at 25% charge, So figure you get 75% of that 216 WH. Call it 150 WH.
Your battery charger is 2A, so multiply by 48V and that's 100W. Divide 160WH by 100W and you get 1.5 hours. Figure electrical losses of 10-15% in the inverter/charger and that's like 1.3 hours? Just an estimate.
The number could be a little lower. The inverter might also shut off before the car starter pack runs down to 25%.
Edit/Update: See my later post. I don't believe this unit is really 18000mah,
Last edited by Doc_Wui; 05-05-19 at 12:11 PM.