Stan, with my different, more primitive approach to instumentation as previously described (electrical triple tach), I can add a few observations to this discussion. Relative to battery charging, (other than regen) it is not possible to isolate the function to one MG. The two work in tandem to control ICE revolutions as a primary function, and any charging activity is a seconday activity that may involve either one, or both, depending on the drive train dynamics.

When a sustained battery charge (as opposed to a high speed maintainence charge that I will discuss later) starts, the rotation and speed of MG1 is changed by the ECU such that the ICE to axle drive ratio is very slightly adjusted to a lower ratio. At the same time, I propose that some generated current is taken from either MG2 or MG1 or both as required to maintain the ICE to axle drive ratio set by the ECU. I can see this on my triple tach as a change in MG1 and ICE revs. Although I cannot see a rev change in MG2 (or the drive line), the fact that the ratio between MG1 and the ICE has changed implies a torque change to the drive line, and thus to MG2. (Unfortunately, I cannot record the triple tach yet, so I cannot show you any meaningful data.) A dead giveaway to the fact that this condition can exist is the fact that the MG1 can start the ICE at a stoplight when MG2 is stationary and then control its RPMs. This is only possible if there are holding forces on MG2 (brakes) that prevent (by exerting a opposing torque) its rotation. In both of these situations the torque exerted by MG1 to govern the ICE is the control here, any power that MG1 can produce from that effort is a by product. The only difference in the two cases is that when in motion and MG2 is spinning the ECU has the additional potential to either extract additional energy from MG2, or put energy into it as Assist.

What I do not know is whether the ECU "thinks" in terms of RPM, Torque or Current in the control program. One interesting thing to watch is what the triple tach does when the battery is at a topped out SOC. The ECU "looks" for ways to bleed off some stored energy. Just as it manages charging for a low or decreased SOC, it manages "discharging" for a battery with an SOC above some target value.

In the early days, we speculated that the car had an inclinometer or other way to sense a change in road gradient. It does, but it is done by sensing a change in torque, RPM or a departure from a steady state current situation (don't know which) when the drive train changes speed. That is why the battery will intermittently show an assist state in high speed cruise. The ECU is constantly switching from an under SOC target state to an above SOC target state and back again in an effort to maintain a target SOC.

The other advantage of this SOC "toggle" is that it results in a more constant RPM on the ICE with smaller rev changes because within certain limits the assist is taken as an alternative to a ratio change when more power is needed by the road conditions, and the charge is taken in the opposite situation where the road conditions permit some of the energy being produced to be taked as generated current. But in both of these situations, the ICE seems to be attempting to maintain the ICE revs at a constant.

Consider this little scenario and quiz -- I suspect not many people on this forum know the answer, although you and a few others might. What happens on a long downgrade when the SOC tops out? We know that the TCH does not just give up its simulated drive train drag. Since it can no longer regen because there is nowhere for the current to go, how does it maintain the electrical load necessary to maintain the artificial drag?

It is the ignorant among us that will eventually destroy us all.