I have been working on the 1500 year light and found that not only can you lite two LEDs in series with 3 volts but you can charge the battery with the same three volts. This is just preliminary results as the test is on going but I ran the cell down to 2.749 volts then changed the circuit and after six hours of run the current reading is 2.770 volts. I have taken the fall of the coil to send a about 7 volt spikes to the battery thus when it is not charging the driver coil the BEMF is charging the battery. I am sure that there is a loss but by feeding the battery with double the supply voltage we increase the power of the battery.
This is only preliminary info for non commercial use and is still in testing but I will need to post a photo of the coil for you to understand.