Hi.
In my experience (NX 450H+ PHEV). The battery will not fall below about 30% and will not charge from the engine above 80%. I have to charge from the engine occasionally to be able to access specific zones in some cities. I have noticed that charging on the motorway is a waste of time and fuel because it is so slow. At lower speeds, the charging is much faster and I can get 10 km electric in only 4 to 5 driving. I am assuming that there is much more "unused" energy available at about 40 to 60 km/h for charging.
Obviously all this is software controlled and I think the designers have made compromises to ensure that no matter what, there is always enough electric power and engine power for rapid acceleration or continuous 4-wheel drive.
I think the luckiest people are those who can charge every night and then drive mixed journeys with motorway and urban routes. Then the car will show its true value.

 

I use Hold Charge mode when EV battery is empty.
This when I'm driving on long Trips on Motorways, such to have 10 to 15 km of EV range when arriving to the final destination.
I notice that is much more convinient to use HGM when driving at 90 to 130 km/h: gasoline consumption is increasing by 1 to 2 km/l. You do not feel any lack of Power as speed is quite contant and the Atkinson Engine gives an higher torque.
I tried it at low speed, but gasoline comsumption is increasing in a relevant manner, with noticeable lack of Power, as the Atkinson Engine has not a good torque at low speed: the hybrid mode is helping a lot during such phase but not when in HGM.
Based on My experience i Would suggest not to engage HGM below 70 km/h, unless You do not care about higher gasoline consumption.
Than HGM self disengages at 71 km of EV range charge.
I had the chance to try it after a long downhill driving, as I wanted to complete the EV recharge, but a pop up message warned me that HGM was not available as EV battery was full and the EV range was at 71 km.

 

Just curious if you happen to know what percent of the HV Battery is at 71 km range? Seems high than 80%

 

It equals to full minus a fraction of kWh.
The rechargeable part of EV battery is 14,9 kWh, as 3,2 kWh are reserved to hybrid mode, or self rechargeable part while decelerating or braking.
So a bit below 80%.