I have a 1.2 V nimh battery and a 3.7V li ion (no protection circuitry) battery, which would add up to 4.9V if put in series. However, I want to turn it into a temporary USB power supply (5V) so ignoring all the things regarding putting 2 batteries of different capacities together, is there a way to kick the voltage up a tiny bit? I was thinking something like filtering the output of a joule thief, possibly. I also have a solar panel (4.5V, ???A) and was wondering if that could help, but I don't want to rely on it too much.