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Limited Regen behaviour

Does anyone fundamentally understand how the limited regen works as it seems very random.

Some days I’ll charge shortly before leaving the house and will have limited regen.

Other days no charge throughout the night, similar outside temperatures and no limited regen.

I’ll then drive for 15 or so miles at higher speeds with limited regen and on arrival the limited regen continues as if the car has not warmed up.

It seems quite random. Colder days no regen limits on occasions and on warmer days you experience a limit.
Regen means the energy captured has to go somewhere - and just as a full battery limits charging speed, so regen is limited with full or close to full batteries. A full battery has literally no regen although as soon as you start driving the battery level goes down, but the speed you can charge the last 1% is still really slow.

Secondly, cold batteries also limit the charge rate, and regen is effectively charging. It can take quite a while to warm the battery up and a car warming up quite quickly in the cabin because the abient has warmed up doesn't mean the battery will be quick to warm up..

On an MS/MX you can bring up a display which shows this and full regen is over 50kw and if you plugged your car into a supercharger when you have limited regen you may find its topping out at 20kw and 20kw of regen feels a lot less than 50kw of regen.
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Variables at play include big variations in night time temperature and the thermal mass of the battery might mean its temperature lags hours behind outside temp. Combine that with different timing of charging and driving, much less heat wasted than in an ICE and some bits of road using much more power than others and it stands to reason you might get a lot of differences between one day and the next. I too find sometimes that limited regen goes much further into a journey than other times with a lack of obvious correlation with current outside temperature. Plus the effect goes much further into a journey than I'm used to it taking to warm up an ICE.
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I charge to about 75 overnight, bang it up to 80 shortly before I go out, then charge a bit more, unplugging as I leave. It reduces or eliminates the problem at present, but car is garaged and it's not very cold yet. There does seem to be a bit of hard-to-explain variation though, I agree.
I've come to think that perhaps the best way to explain this is to get people to think about their cordless drill or battery leaf blower or some other appliance that uses lithium batteries (or any battery, really). You plop that battery into a charger when you are not using it. The battery has temperature sensors built into it (that's why there are so many pins) and so the charger is informed about the battery temperature. The charger also has circuits which measure the voltages of the individual cells (another reason there are more than 2 pins) and so it knows about the state of charge of the battery as well. It has a microprocessor in it that knows how much charge it can, based on temperature and voltage readings, send to the battery to charge it as fast as possible without damaging it. It commands an internal power supply to send that much current to the battery.

It's exactly the same in your car. The BMS knows the charge status of the battery and it knows it's temperature. From that it can calculate safe charging current. It does this and, if you take your foot off the pedal, commands the power supply, which in the car is the motor acting as a generator, to supply that much current and no more. If battery temperature is low or the battery is nearly charged it will tell the generator to back off. Recognize that there is another factor here and that is how rapidly you want to slow down. If you take your foot off the pedal you clearly want to slow as much as possible and the limit will be what the battery can safely accept. If you only back off a little and the current command will be limited by the desired braking torque as well.

The idea that regen is limited when near full charge "because there is no place for the energy to go" seems to be popular. But it is incorrect. The energy does have someplace to go and that is the battery. The problem is that when the battery is cold or nearly fully charged sending energy to it will damage it. Thus the controller does not send energy under those circumstances.
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