I want to charge a 7.2V-2000mAh NiMH battery. I have many solar panels and a voltage regulator but I do not know what voltage is required to charge it. I've heard people say that as long as it is any voltage higher than the battery peak voltage it is good. So I can use a 12V panel or even a 24V panel. I understand that low current is important for trickle charging, like 10%. I have an LM317T to limit current to 200mAh. I just don't understand the voltage requirement.
NiMH batteries should be charged by constant current until they are full. For your battery pack, since it is 2000 mAh, C = 2000 mA. For fast charging, the current should be around C/3 or higher, up to, say, C. You know they are full when the voltage levels off or dips (dV/dt), or the temperature starts to climb more rapidly (dT/dt). The voltage of the cell at this point varies depending on many factors. It could be as high as 1.6 V, but it will drop off rapidly when charge current is removed.
If the batteries are charged at a lower rate, for example C/10, it may not be possible to detect the dV/dt or dT/dt signal at end of charge. Usually, when charging at this low rate, the charge is terminated by a timer instead.
If you use a NiMH cell until it is "dead" or "flat," you will probably see that the open circuit voltage is around 1 V/cell.
So, when charging NiMH cells, current is what is important, not voltage, per se. You can expect the voltage to vary from low (could even be zero, in theory) to 1.6V. For a battery pack with 6 cells, the nominal voltage would be 7.2V. When discharged, the pack voltage might be around 6V (open circuit). And while being charged, the highest voltage might be around 9.6V (while charging).
I hope this satisfies your curiosity about NiMH. Most of this information is available on the internet. You can look at guidelines published by battery vendors, and also the people who make chargers and charger IC's.