Cold Weather Regen Limiting and Accessory Draw

I'm starting this thread new rather than post in the existing "Cold Weather Regen" thread, because I'm explicitly discussing the accessory draw here.

We all know that the pack can't accept significant charge if it's too cold. I often will have regen limited in the cold, and occasionally it will be disabled altogether. Yesterday was such a day for me (along with the first time I've ever seen the dash "range snowflake") where, after sitting out all night in nearly 0 degree (F) weather, the battery was completely cold-soaked and regen was disabled.

As I began driving, my accessory power draw was somewhere on the order of 10kW. This was cabin heater on, both seat warmers on full, and the pack heating obviously engaged. I determined the draw based on the the dash gauge when the car was stopped at an intersection.

So, we know the battery pack can't accept charge when it's too cold. My question is: Why can't the car offset the current draw for accessories with regen power, even if it doesn't provide current to the pack?

Even 10kW of regen helps slow the car when you are approaching an intersection. And when the pack heaters are running full-bore it would be nice to recover as much energy as possible.

My guess is that the accessory draw can be instantly variable: The heaters could go off at any time, thus forcing regen to instantly disable during deceleration, thus causing a safety concern. Of course, regen can instantly disable when traction control detects a slip too, causing the same safety concern... but in that circumstance there may be little other choice, and halting regen may be the lesser of two evils.

Thoughts?

FlasherZ said:

This is a great point. My guess is that the drivetrain doesn't have the instantaneous power draw information from the pack to make the right decision on regen. Even when plugged into the mains, it seems the car reactively adjusts amperage to balance out an average draw from cabin heating, etc. If you sit in the car while it is using shore power to heat, and you turn up the heat, it doesn't suddenly jump up, it ramps up to match the average over a few seconds.

That would be my guess, uneducated WAG though. Perhaps they could instrument it somehow and get the messages to the drivetrain.

Click to expand...

Hmmm, the pack teardown threads have revealed current shunts in the pack allowing for current draw measurement... and the dash obviously displays power draw with a sub-second resolution... so the car knows what the pack is putting out....

But I wonder if the accessory draw is essentially a parallel load behind the DC-DC converter, thus making it impossible to avoid supplying HV from the drivetrain during regen to the accessories without also applying that voltage across the pack terminals. Your point about the accessory behavior on shore power might support that...