How much current does the inverter take from the battery when running the fan?

I am assuming my calculations when I will connect another 10ah battery in parallel with existing battery and running the engine with giving a little gas.

 

If that is with the engine running then it seems you won't be able run the fan.

 

If that is with the engine running then it seems you won't be able run the fan.

But when my 5ah battery is fully charged and I connect the inverter it will give a 10min backup, like ups.

 

If that is with the engine running then it seems you won't be able run the fan.

I have a idea, what about using a DC fan and led lights, it would eliminate all the troubles and will increase efficiency, right ?
If yes please suggest me how I can build a DC table fan, I can figure out the led myself. Google doesn't seem to work with DC fan.

 

Search ebay for car fan.

 

Search ebay for car fan.

Car fan ?

 

Do the search.

 

Thanks a lot !