As I have mentioned once or twice, when towing my aim is to keep an as high as possible SOC. Typically, this means I will hit the Charge button as soon as I drive out of our street. I thought I had noticed a couple of times, pressing Charge would not (immediately) start the engine if I did it to quickly. I assumed it was because SOC was too high at that moment. For this reason, I was curious about your experiences.
Yesterday, we towed our caravan over approx. 80 km. Before I got a change to engage Charge mode then engine had already started because I had "floored" it (*). As soon as I heard the engine, I remembered to press Charge and I did. At that time, SOC was around 83%. The engine kept running in serial mode and produced just enough power to drive the car and increase SOC bit by bit. It kept running in serial mode even after we entered the motorway (doing 60+ MPH). Don't know why it does / did that, but it started doing this about a year ago. Either way, I like it, as it means I can maintain high SOC for when I need it.
Funny enough, presumable because SOC is that low, the car does not feel the need to increase SOC very fast. So, although RPM is not fixed with speed, the very high revving stays out. Not ideal, but much better than towing with low SOC, where the car switches between parallel mode and high revving serial mode every tow minutes or so.
(*) Due to the poor state of health of my battery, the battery will not release more then 45 kW unless it has to. Not nearly enough for towing.