iPhones, iPad, etc, all have their optimal specifications for charging. A power source that does not meet those specification results in a slow charge. However, what happens when the specifications (volt/amps) increased & decrease (fluctuate)? Will the power fluctuations cause damage to the battery? Assuming there is a voltage regulator to keep it from going too high, will this still cause damage?
Connecting directly the solar panel to your iPhone will have negative influence on your battery.
Solar system that are design to charge the phone/GPS/what so ever, use a second battery as buffer. Do you use a system that has not one.
I was recently told by my landlords electrician that the voltage in my flat fluctuates quite badly, and that it can contribute to some devices having shorter lifespans.
But where I notice it most is in charging my phone, on a full charge at home my phone will last around 10 hours with a couple of hours of screen on time.
When I charge the same phone at my parents house, with similar power usage, wi-fi turned on, same apps syncing ect. It will last nearly 24 hours with 5-6 hours of screen on time.
This is all using the same charging brick and lead. The only solution the electrician told me about, was some sort of regulator that could be fitted to normalize the voltages. He said that it would cost a couple of hundred £'s :(