Some more experiments with Torque:
How much charge can you gain driving down a slope?
Here is the data from a downhill road: the road is 1.4km long, I drove it entirely with the engine off, with the D gear, with only a slight braking, trying to keep the speed under 45 km/h
The blue line is the GPS altitude.
The red line is the battery % charge level.
Battery starts at 54.9 % at 238.5 meters.
At the end of the road, altitude is 159 m and battery is at 69.01%.
So a 79m height difference caused a 14% gain on battery level.
There's a thread about this on the Hybrid Synergy Forum.