This is what the output waveform looks like between any two transformer outputs (except 4-5 which are shorted):

Measurements on oscilloscope (no load, directly on the transformer):

| Output 1 | Output 2 | Vmax | Vrms | | -------- | -------- | ---- | ---- | | 5 | 6 | 9,4 | 6,1 | | 4 | 6 | 9,4 | 6,1 | | 3 | 6 | 41 | 26,5 | | 3 | 4 | 49,6 | 32,5 | | 3 | 5 | 49,6 | 32,5 | | 4 | 5 | - | - |

I simulate the portion of the circuit that charges 9V batteries. V2 is the battery which is quite 'empty' at 8V. R2 is the internal resistance (no idea what that should be though...)

Interesting. It looks like the charge voltage is always a little higher than the battery voltage.