Question about power inverters

I figured this was as good a place to ask.

In my work van I’m just running a 140w little power inverter out of the 12v outlet on the dashboard. I have an 8’ extension cord running to the back of the van with the only thing plugged in is my dewalt battery charger. It’s fairly thin, I’m assuming 16ga but the label doesn’t say. It’s from Harbor Freight. Anyway the light on the inverter flashes after 10 minutes or so and the power cuts out to the charger.

This doesn’t happen without the extension cord.

Do I need a thicker gauge extension cord or a higher wattage inverter?

All I’m running is one battery charger at a time.

Thanks