Transforming bike dynamo output to reduce diode bridge loss ratio
The bottle-type dynamo has a circumference of under 2 inches, and at
normal cycling speeds (12 mph), assuming it has two pole pairs,
will give AC at about 100 Hz.   This is compatible with an AC transformer
of the wall-wart type, but not ideal (because those
transformers typically have 120V-in, 12V out, and you want
circa 6V in...).
Power output is in the 3-5W range for the bicycle lights
and generators of old (with incandescent bulbs).
Another option is to use a voltage-doubler rectifier which lowers the
drop from 1.4V to half that, or less if you use Schottky rectiiers



simulate this circuit &ndash; Schematic created using CircuitLab
and use a DC/DC converter (probably a switchmode buck converter) to
deliver suitable charge current to the battery.   The light bulb is
just a dummy load for the simulation...