Yeah, that is the only issue I ever have when I do it, you can practically see the Nexus S battery start dying.The only concern is that the hotspot app will drink your smartphone's battery.
Something like this monster? Extreme Battery Kit DC AC Inverter Power iPad Laptop Portable Converter Battery ChargerMy sister bought this thing that she can carry around with her. It is pretty large, about the size of a car battery. Anyway she can charge it up and then use it as a plug. At any given time she can just plug her phone and laptop in to it and get surfing the net. I can't remember what that thing is called though.
Thank you captain obviousUnless you need to be actually moving (in which case I recommend a power inverter for your car), you might be better off just getting free wifi from a hotel or restaurant (some businesses have it broadcasting where you can get access from their parking lot).
Here in the US the quality of WiFi (free or pay to use) provided in hotels varies enormously. IMHO, the pricier the hotel and the more it charges for the service, the worse the connection!There are times I've stayed in a hotel and have preferred to tether my cellphone than pay their crazy wifi charges.