To integrate the new stuff into the mower, I needed to get power to the mco and imon boards.  And I needed them to be powered when mowing, and when charging, and otherwise power down.  I have also drawn and added the top level schematic on how the bits fit together into the mower to the github repo.

The mower has a keyswitch on the control panel. The green wire from the keyswitch was  an easy place to find 48V when mowing. But how about when charging?

The charger socket has 3 pins. This is what it looks like fromthe back of the mower.

 The bottom 2 pins are battery plus 48V on the lower left, and battery -  on the right. Always directly connected to the battery. Always on.  The top middle pin is used by chargers for thermal sensing and signalling it seems. 

The back of the socket looks like this. Its upside down. You can see the back of the 2 power pin barrels between the 2 philips screws. The middle top (when right side up) pin is behind that kludgy green wire hex screw head.

This took a bit of investigating.  The heavy red and black wires go to the battery. The grey wire has a 2 pin plug on it, and connects to a NTC thermistor, about 11K at room temperature, which bolts thermally to a nearby battery terminal post. I suspect the original charger uses the thermistor. The thermistor appears to connect via 2k resistors moulded into the socket to the middle pin and to the ground pin. 

The sky blue wire seems to connect via a diode to the bat+48V pin , and ends in a 1 pin barrel connector. The other end of the barrel connector appears to run the mower contactor via the keyswitch - at least the mower won't run without it connected. The blue wire didn't buzz out so there is something moulded into the socket, I assume a diode. 

I found online descriptions of different model chargers, suggesting the charger plug should jumper 48V+ to the middle pin to make charge lights work.  

I added the green wire shown. Its  a short flying lead from a lug to a anderson powerpole connector. Lug goes over a steel self tapping screw , about 6mm. I made a brass nut which tightens the lug to the screw head. (5.5mm hole)    The screw then gets driven into the barrel  of the middle pin from behing the socket, a couple of turns until it nips up nicely.   And the charger plug gains a wire jumpering the middle pin to the Battery +48V pin.

Result is the green wire through that powerpole gives me battery +48V when the charger is plugged in. So I can power the electronics when charging or when mowing.  The electronics can then wake up and turn the charger on to charge. And turn the charger off again when charged.   I need to unplug the charger from the mower the next morning.