Oppinion/Feedback for a battery please.

I have no clue as to what you are trying to do. However, whenever you charge a battery, you need a regulator to limit the voltage (and/or current if the source is powerful enough). As to prevent discharge of the battery after your source disconnects, a simple blocking diode will prevent the battery from discharging back into the charging source. As an example, blocking diodes are used in solar panels to prevent the battery from discharging back through the panel when there is no sunlight hitting the panel. As to your problem of having insufficient voltage from your battery (that you are charging) to power a larger battery there are a couple of solutions. It is obvious that you need a lower terminal voltage on the battery you are going to charge than the battery you are charging it with. You could charge multiple batteries in parallel and then re-connect them in series to get the required terminal voltage. A more common way of doing this is to use a DC-DC converter. These devices work by converting DC to high frequency AC, running it though a transformer to get the required output voltage and then convert back to DC. Using one of these might just eliminate part of your process by charging the larger "battery" from the induction source directly. BTW--you cannot "charge" a typical fuel cell. A fuel cell generates electricity by a chemical reaction with a hydrocarbon fuel and oxygen to create an electrical output. Because fuel cells are slow to ramp up or down with changing loads, batteries are usually added to the system to stabilize the output. For example in an accelerating and decelerating vehicle, the battery output changes a lot while the fuel cell input remains fairly constant. Good luck with your ideas and projects--throw some of this out to your friends a maybe they can give you some ideas.

Your best shot would to be go to a hobby shop and by a Li-po battery that is used in model cars/airplanes etc. They usually have a nominal voltage of 3.6 volts per cell and have very high discharge rates. Capacity are measured in MAH (milliamp hours) and are available in various common sizes up to about 8000 MAH per cell. Some of the better cells can discharge at up to 40 times their C1 rate. That would be the cheapest thing you could do for your project. If you need even a higher discharge rate, you could use a capacitor in place of the battery. They can discharge all of their energy almost instantly---far quicker then your lead acid battery could absorb it.