Charging phone battery with a lower voltage, coming from a bike charger?
First of all, the USB is not being fed directly to the phone’s battery. It’s actually being fed to the battery charger circuitry inside the phone. You cannot safely directly connect a normal regulated voltage source to a lithium ion battery without the distinct possibility of an explosion and/or fire! The charger circuitry adjusts the voltage and current fed to the battery depending on the state of charge.

Then, your question becomes what happens when you feed incorrect voltage into a phone’s charger circuitry. This would depend on the specific model, but we can generalize. With too low voltage, such as 3.5 volts, the phone will probably ignore it and refuse to charge. With too high voltage, such as 9.3 volts, the phone is likely to either shut down to protect itself or be damaged by the excessive voltage.

Your circuitry, which you have not really described, is totally unsuited to charging a mobile phone! You have a good chance of seriously damaging its circuitry. You should probably find a commercially produced buck/boost module, capable of taking in the maximum voltage from your dynamo and outputting a stable 5 volts at 500 ma, or whatever your phone needs.