Well im going to try to get a mutlimeter here pretty soon so I can measure the actual voltage that the batteries are outputting. But as far as the solar panel goes, the 6.7 volts is the max voltage output, it isnt always going to have enough sunlight to output a full 6.7 volts.
Keep in mind the following is a pretty "toy" description of some of these items - don't be too rough on me.
Roughly speaking, the voltage is set by the chemistry, not the amount of sunlight. Voltage (more accurately "electrical potential") is roughly analogous to pressure or height in a mechanical/gravity system. A 20m high water tower supplies the same water pressure as a 20 meter high dam across a huge lake. The pipe size is what determines the water flow rate, and the volume of the reservoir limits how long it will last. Thus the big fat "D" cells, the little "AAA" cells and the tiny little button cells all provide roughly 1.5V of "push". Each of them can provide surprisingly high current levels (set mostly by the specific chemicals used, and by how they are layered and things like that), while the total amount of energy roughly matches the amount of those chemicals that are in the battery - not too surprisingly the big fat "D" cells would last the longest.
Most circuits will work reasonably well for a small range input voltages, thus you can use 1.5V alkali batteries, or 1.2V NiCad batteries pretty much interchangeably.
For solar panels, the arrangement of chemicals (actually the electrical properties of the crystal lattices is probably a more accurate description) sets how much electrical potential (voltage) the electrons in the material gain when a photon strikes the material. If there is only a little light, only a few electrons will have this increased energy, so the maximum current will be smaller than in brighter light, but the "open circuit" voltage is pretty much the same in either case.
For a given voltage, the rest of the circuit limits the current depending on the resistance of the circuit (through voltage = current x resistance). Thus for any electrical system, if your power supply is of the correct voltage, you can use any power supply rated at or above the maximum current your circuit would draw would work fine, with no danger of oversupply. That is why one can use the power supply for the big 17" MacBookPro to supply the tiny 11" MacBookAir with no issue, but if you try the tiny MacBookAir power supply to power the larger laptop, the powersupply gets really hot and the laptop charges very slowly.
For your solar panel system, providing 6.7 volts of input could result in damage to the circuit that is only expecting at most 3V (two batteries) or 4.5V (three batteries) (different keyboards use different numbers of batteries). To get down to a lower voltage, I think there are various designs of "voltage divider" resistor circuits, or diodes, or transistors, or voltage regulator packages - see
http://www.electro-tech-online.com/...-ideas-reviews/86779-diodes-drop-voltage.html for examples. Alternatively most individual component solar cells provide about 0.5V, so rewiring the panel you currently have (which might be essentially impossible for a sealed panel) could set it up for the desired output voltage.
Once you have a system set up to deliver the correct voltage (or close to it), you can hook up multiple solar panels in parallel to provide the higher current needed. Basically, the max current output is set by the light intensity and the physical size of the panel capturing that light.
What you might consider is trying to wire up the solar cells to installed rechargeable batteries so that the batteries can provide the higher current needed for keyboard operation while the slow trickle of solar power would recharge the batteries for the times when the keyboard is not demanding the high current. I don't know about energy density, but perhaps a big capacitor might be sufficient.
It looks like Logitec uses a rechargeable button battery:
http://www.logitech.com/en-ca/keyboards/keyboard/devices/k750-keyboard
$60 at Amazon
http://www.amazon.com/exec/obidos/ASIN/B005L38VRU/jbenterprises/
