Mechanical Generation of Power is basically how we first got electricity. It's a fairly common application, and there are over a billion of them running around outside. Automobiles have been using rotating shafts to power themselves since the very start.
The schematic for it is fairly simple. An Alternator going into a rectification stage which is then tapped into a battery for power storage, or directly to the rest of the circuit if the shaft can generate a fairly steady speed and voltage.
Automotive alternators have been design in a way to couple the alternator to the shaft without directly mounting it around it, through the use of a belt or pully. As long as the shaft is fixed in place, and the alternator can be mounted along side, this works great (Safety first of course). There is a minimum RPM for stable generation, and output voltage will vary based on the RPM, so you have to work around that (Use a regulator circuit). The alternator's ideal rpm should match the Shaft's range of speeds.
A not typical setup for illustrative purposes:
First of all, how long must the computer be on? You mention that it must be activated every 108 minutes, and I assume this means it must be turned on every 108 minutes. Then for how long does it get turned on for? At a rate of 650 mA, if it is only on for say 1/2 hour every 108 minutes the you would only use 325 mA every 108 minutes.
Assuming you follow the 108 minutes 24 hours a day, then you would turn on the tablet 13 times for 1/2 hour in a 24 hour period. You are camping for 6 days so a total of 13 x 6 = 78 - 1/2 hour sessions (or however long you are turning it on for).
At 650 mA, 78 - 1/2 hour sessions (650 mA x 78 x 1/2) = 25.35 amps
Your Nimh are at 1.2 volts, so put 4 in series to get 4.8 volts (closer to 5.6 volts fully charged so be careful). When in series, the capacity is NOT added, so have only 2300 mAh capacity with the 4 in series.
25.35/2.300 = 11.02
So you would need 11 sets of batteries, or recharge them 11 times if you use the tablet for 1/2 hour every 108 minutes 24 hours a day for 6 days straight. I have a feeling that you won't be using the tablet this much, so you will have to adjust your calculations accordingly.
I know that you mentioned 18650 batteries, but given you budget and your options, I think that a good quality Nimh is your best choice. The main reason is because you can get a reasonably priced Nimh AA battery that can be charged in less than 2 hours! Many lithium ion batteries will take longer to safely charge. Plus the voltage of 4 Nimh batteries matches what you need. Get a fast charger that will charge 4 at once. You can get chargers that will charge Nimh in as little as 15 minutes, but that is hard on the batteries. I would go for a 1 hour charger with the generator.
Good luck!
Best Answer
Any generator may be driven by any mechanism - electric motor, gasoline or diesel engine, steam engine, water wheel, or hand crank.
The voltage produces by a generator will depend on its construction, and RPM.