Inverter Generator to charge in remote locations?

Barrio Barranco

Esteemed Pedelecer
Nov 24, 2018
Can anyone brainy here tell me what Wattage output I'd need to power my 4A Bosch charger by using a petrol inverter generator? It's a 36V 500wh battery (13.4Ah).
I've got the second battery but plan to get away stealth camping at weekends also in my VX Combo "Micro Camper" and to have the ability to charge one/both up -fully or even just 4 bars/80% in the evening would be good...I don't want to get a 6A charger
I saw this but don't know if it is enough- I read somewhere that Li-ion batteries are better charged with inverter generators as the output is more stable and better for batteries?
Or I could run a 12v>230v car inverter -but then I'd need to sit with the engine on rather than away from the vehicle protected from the elements??
A petrol generator seems more versatile to me and I'd rather not mess with the van battery in the middle of nowhere...