Today I took a quick trip to Radio Shack for some PP3 connectors. Returning to the bus I attached the connectors to the fan via a terminal block. A PP3 battery was connected to the connector and attached to the bus wall with a magnet.
I didn't use a new PP3 battery and a PP3 battery is not the ideal battery to use anyway. PP3 batteries are good for about 500mah while my fan probably uses around 115mah. Thus, a new PP3 battery would run the fan for around 4 hours maximum. Not only that but as a 12v fan, it won't be running at its optimum anyway.
I took a few notes while testing the fan. I wasn't surprised that it didn't make much difference. With 4 inch tubing I really should use a 4 inch fan. 90mm is not optimal. It's all good for testing though. After setting the fan going, I cupped my hands around the exit vent. There was a gentle rush of air so clearly my fan is expelling air. Putting 12v through should expell more. It's even possible to put a T connector where I currently have the inlet vent and to install two 4 inch fans.
There is room for encouragement. There is plenty latitude in my design for improvement and I am heartened by the outside temperature rising a degree with no internal temperature rise.
When I was in Radio Shack I saw lithium batteries on sale, complete with lithium chargers. That was very interesting. It gave me thoughts about running lithium batteries. They're high capacity though odd voltages! Having said that, my CPU fans can take anything from 5 to 25 volts with no ill effect. Indeed, a single motorbike battery sized lithium cell could provide all the power I'm likely to need for fans, lighting, electronics charging elf. Charging the battery is the big issue however. This is largely why I'm going for disposable save for powering the ventilation which I'll try to run off solar.