If you are using a 6volt battery then for most of the time you need a boost switching regulator except for those periods when the generator might be producing voltages at about 6volts. The problem here is that the generator is about the same voltage as the battery and this effectively means using a boost switching regulator generating maybe 8volts followed by a buck regulator that takes the 8volts and charges the battery thereon.
The alternative is to use a 12volt battery and avoid the buck regulator. In effect you just have a boost regulator and it will be about 90% efficient.
Ensuring the battery does not lose energy back into the charger when the generator is stopped is trivial so you have to decide on your battery voltage and your battery technology. This, might warrant a fresh question should you struggle to decide.
Added at top as updated question modifies best response:
I am building a generator designed to output 3-phase power for industrial applications. ... The idea (best-case scenario) is to have multiple people riding bikes, and then convert the power into 3-phase electricity. ...
I am not sure whether to create AC (and use a variable frequency drive to convert it to 3-phase power) or use DC, and then convert it to 3-phase power. The idea is [to use] stationary bike[s] (probably just a bike on a stand) to turn the generator shaft.
My prior general comments below still apply but my specific answer is:
There are a number of ways to do this and none is 'best' as all are compromises, and the final configuration depends on what assumptions are made.
However, if you wanted the industrial norm of constant voltage constant frequency AC then you almost certainly need to store energy from the bikes and produce the AC from the energy store. As advised below, the most likely bike power producer would be a permanent magnet alternator producing multiple phase AC (usually 3 phase). Voltage and frequency and power level are immensely user dependant and the best method is likely to be to convert this output to DC, store it in a battery and then produce fixed voltage fixed frequency AC using a DC to AC converter - an off the shelf product.
A good way to handle the bike AC is to arrange for the alternator AC Voltage to be higher than the battery DC voltage at all useful power output and speed ranges, convert the AC to DC and then "buck convert"(= voltage down convert) the DC to battery level voltage. A charger-controller would handle the input from all bikes and manage battery charging. Depending on design requirements users may be requested to pedal at constant power or constant voltage (both of which can be enforced by a controller with feedback to the user) or be free to provide input as desired.
It would be possible to transfer energy directly from bike rectified DC via down converters to the DC to AC converter input directly without battery storage - and this is essentially what happens to most of the energy when bike user input is <= load, but completely batteryless operation would be difficult as the battery provides a stabilising influence and, in a properly designed system, an energy source that has no drop puts to below load requirements.
In a past lifetime I designed controllers for alternators used as loads for exercise machines so have a good feel for what is required to achieve this task. Realistic load levels for typical fit but non athlete users are.
50 Watts for say one hour with reasonable ease.
100 Watts for one hour for a very solid work out.
200 Watts - getting extremely strenuous.
500 Watts - I could do about 10 seconds :-).
I can answer specific questions if you have any.
Is this a real-world idea or an investigation of a concept or ...?
All considered, schemes like this would not prove economic relative to grid powered electricity at current grid prices.
"Generators" output DC directly by converting the alternating voltages within the machine to DC. This is typically done using a commutator and brushes - effectively a manual "synchronous rectifier". This arrangement has some drag, complex mechanical requirements, lower lifetimes and losses in the carbon to metal contact of the commutator.
"Alternators" output AC = "alternating current" (and voltage) which is converted or "rectified" to DC outside the machine proper. Electronic conversion methods and components allow this conversion to be highly efficient.
Alternators come in two main "flavours" -
Those which create the AC in the rotor and transfer it to the non rotating frame of reference (the one you are standing on) with slip rings, while the fixed stator is used to create the field that the rotor turns in to produce the AC voltages.
Those where the AC is made in the stationary stator windings with the rotating part (rotor) providing a rotating field that interacts with the stationary output windings to provide the AC.
There are two main subsets of these stationary output winding machines.
Wound rotor - the rotating magnetic field is produced by rotating windings which are fed DC field power via slip rings. Automotive alternators usually work like this. Advantages are that magnetics provided by wound copper coils are relatively cheap and the field magnitude can be controlled by varying the DC power which is fed to the winding. Disadvantages are mechanical complexity from slip ring feed and wound rotors.
Permanent magnet rotor. Permanent magnets are sound to produce an alternating output voltage in the stator windings. Advantages are no need for DC feed to the rotor, relative ease of rotor construction, modern high strength rare earth magnets allow very energy dense alternators to be produced. Disadvantages are the inability to control the field strength.
There are variants such as AC induction motors used as generators but these are usually best used for specialist applications and can be difficult to control.
For your application where you require efficient energy conversion and probably low cost, low complexity and ease of "doing it" the best solutions are either a dedicated alternator OR a brushless DC motor (BLDCM) - sized to be of the wattage range desired in each case. Electrically these are essentially the same but one was produced with alternator roles in mind whereas the other (the BLDCM) was designed for motor use but will work very well as an alternator. Small dedicated alternators are rare but BLDCMs of the size range of interest are used 'everywhere'. These are typically found in computer printers, powered toys (especially flying ones), disk & DVD drives and much other equipment that uses small motors.
BLDCMs can be converted for alternator use or it may be practical to build your own alternator based on the same principles.
As above, when used as alternators, BLDCM's have permanent magnet rotors and generate AC in the stator with no mechanical connections (such as brushes or slip rings) from rotor to stator. The generated AC is converted to DC - usually with diodes. This is the overwhelmingly most common and sensible method to use in a very wide range of power levels and applications. There are exceptions but this is usually the best approach.
To decide how to proceed from here you need to know
What order of power you require.
Where and how you would like to mechanically power your device and why.
eg on a bicycle you may wish to use wheel rim , hub, pedal crank or chain drive. Or ...
A concise but complete description of the application will help.
Ask more questions ...
Tell us about power levels,application, more ,... .
Best Answer
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!