Starting REX with no charge?

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panamamike

Well-known member
Joined
Jan 25, 2020
Messages
121
If I run out of battery range, then leave the car off overnight with no charging cable, will I be able to start it up and go the next day?
Don't know how much battery the US version REX will preserve.
 
panamamike said:
If I run out of battery range, then leave the car off overnight with no charging cable, will I be able to start it up and go the next day?
Don't know how much battery the US version REX will preserve.
The REx generator will automatically turn on trying to preserve the battery pack's charge level at ~6.5%. It would be possible although not easy to park immediately after a very fast run up a steep incline with heat full on such that the charge level could be lower, but likely not lower than ~2% because the car would automatically reduce electric loads to preserve the charge level.

However, no electric load would reduce the battery pack's charge level overnight unless preconditioning were enabled. I'm guessing that preconditioning without an EVSE plugged in would not occur below a certain charge level. If the pack's temperature is considerably lower in the morning than it was when parked, the charge level would drop a bit until the pack warmed up.

The bottom line is that your car should be able to enter drive readiness state the following morning regardless of how you parked it the previous night. The REx engine would start and try to preserve a 6.5% charge level, so you should be able to drive as long as you don't exceed the REx generator's output long enough to discharge to below ~2% where propulsive power would start being decreased.
 
alohart said:
panamamike said:
If I run out of battery range, then leave the car off overnight with no charging cable, will I be able to start it up and go the next day?
Don't know how much battery the US version REX will preserve.
The REx generator will automatically turn on trying to preserve the battery pack's charge level at ~6.5%. It would be possible although not easy to park immediately after a very fast run up a steep incline with heat full on such that the charge level could be lower, but likely not lower than ~2% because the car would automatically reduce electric loads to preserve the charge level.

However, no electric load would reduce the battery pack's charge level overnight unless preconditioning were enabled. I'm guessing that preconditioning without an EVSE plugged in would not occur below a certain charge level. If the pack's temperature is considerably lower in the morning than it was when parked, the charge level would drop a bit until the pack warmed up.

The bottom line is that your car should be able to enter drive readiness state the following morning regardless of how you parked it the previous night. The REx engine would start and try to preserve a 6.5% charge level, so you should be able to drive as long as you don't exceed the REx generator's output long enough to discharge to below ~2% where propulsive power would start being decreased.

Thanks, i park it on a level road. Just didnt want a surprise if I try this out.
 
While you can precondition the cabin, the car won't warm the batteries unless it is plugged in.

Two potential gotchas if you have a very low battery and leave it. Temperature makes a difference in both the main and 12vdc batteries ability to provide power. BMW has a warning in the manual about leaving it at a low battery level without plugging it in.

If the 12vdc and main battery still have some usable charge, the car should run. All of the computers are run off of the 12vdc circuits, so if that battery is failing, it's more likely to strand you than the main one.

It's a good idea to at least partially charge the main battery if it is low overnight, especially in frigid weather.
 
There is very detailed info about this in BMW University technical training "W20 Engine" you can search online.
Page 54 and "I01high voltage components" page 182.

in summary, belo SOC 6.5% the REX will start but will auto stop if the SOC holds.
Below 3.5% the REX will runn continuously.
Below 1.9% performance is reduced
At 0.4% driving is stopped The battery has enough left to restart the REX or "not discharge fully over an extended period"
 
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