Portable DC charger help

I need to create some sort of power source for my Raspberry Pi, it requires a standard DC cable, and what I have got is 4 AA rechargeable batteries that all add up to 24000mah. The device takes 1 watt and in continuous use that would last around 16 hours with the batteries. I need some sort of way to massively extend that time period. I'm new to this sort of thing, so I thought that I could go with some sort of thermal power that is initially powered by the batteries, and then creates more power that would in turn go back into the batteries and power the raspberry pi, this will be incorporated into a plastic casing that will attach to a hard surface, so it can't be very large at all. And solar power isn't much of an option due to lack of sunlight in the area it will be placed and it unreliability to run continuously.

Portable DC charger help