battery charging - How to make 5V 650 mA on a summer camp?

On our summer scouts camp, I need to power a tablet (it simulates the computer from The Lost, and the children must activate it every 108 minutes). It requires 4.3 - 5.3 V and about 650 mA. I need to power it for 6 days, but the problem is, there is no electric outlet or something nearby. So basically we must make the electricity or use batteries. And for it to be harder, the financial limit for this project is 18 EUR (20 USD), which isn't a lot obviously. I would also like to note that we have a 1000W electric generator, but it can run for only about 2-3 hours/day.

So I came up with two solutions how to power the tablet:

Solution 1: Use batteries

I can buy and use a bunch of batteries. Problems:

1. Capacity - I need 15,6 Ah/day, which is a lot for batteries and this financial limit.


2. Charging - I can use the electric generator, but can I charge a battery in 2 hours?

Solution 2: Generate electricity ad-hoc

I have a dynamo from old Czech car, which (according to the internet) can output 12V 55A (is it possible?) and maybe connect it to a stationary bike, and children will have to use the bike to charge 18650 lithium batteries, which will then power the tablet.

I like the second option more. What do you think? Do you have more solutions? Any advice appreciated. Or feel free to ask for more details. Is it even possible?

Notes:

• My level of knowledge of electronics is low-medium, but I don't have much practical experience.


• I made a practical test. 4 AA NiMH 2300 mAh batteries charged the tablet for 8 hours. Those numbers don't make sense, theoretically it should have been 15 hours!