REX automatically reverts back to battery after gas tank is empty?

Two fold question:

1. I've coded the i3 with the battery hold feature here in the US. It didn't occur to me until this evening while driving that when I run out of gas, the car may not revert back to battery automatically. I was trying to switch back over to battery manually, but the function didn't engage. Does anyone know if the car with indeed go back to using battery on its own once the gas tank is empty? Pretty important to know as I don't want the car to stop midway on the freeway.

I noticed that after I did get gas and restarted the car, it went back to using battery. I don't know if that was a result of:
A. The car restarting or
B. It just happened to take that long for it to switch back to battery from gas. Sometimes I press the button and it's not always immediate.

2. What is the purpose of people setting their i3's to go to battery after 75% of the battery is depleted? I realize the battery hold is to be able to use up the entire tank of gas, but why 75% and not any other number and why not just do it manually as needed? I'm sure there's plenty of good reasons. I'm just too new to the i3 to really know. Your thoughts would be appreciated. Thanks!

Yes, it reverts to EV. Enabling range extender is a manual process. I don't remember seeing a coding for making it a default.

Bob Wilson

Regarding #3, IMO starting the REx at 75% is useful for long trips. And yes, it's a manual process.
The REx is not capable of holding the battery charge when driving more than about 70 mph. So if you're on the interstate and want to drive, say, 75, the charge on the battery will continue to decline even if the REx is on. If you get down to around 5%, you'll go into limp mode. So it's best to start the REx at the highest state of charge that you can (75%) to get maximum range. This is assuming you don't want to stop and charge for an hour or so during your trip. Of course, if you stop and charge, then you don't need the REx anyway. And, when you get to your final destination, you'll need to charge to 100% for the next day. Otherwise you'll finally hit that dreaded 5% state of charge.
FWIW I've had my car for about a year and use it for short trips around town. I haven't coded my car because I'm able to charge overnight. I really didn't want the REx but the boss said if I wanted the car, it had to be a REx. OK....compromise

One concept that may not be obvious to everyone is that the i3 ALWAYS runs on electricity, there is NO physical connection between the REx and the drivetrain...the REx drives a generator. This is called a serial hybrid as opposed to the much more common (I think there's one more serial hybrid either out or coming out soon) than the parallel hybrids out there where they can run either off of their electric motor(s), the ICE, or both, depending on conditions. The i8 is a parallel hybrid.

So, If you're stressing the i3, the battery gets low, the only power you have is from the electricity that the REx can generate, and think about this: the i3's maximum electric motor can produce 170Hp...the REx is about 34...you WILL NOT have full capacity without augmenting power from the battery pack, which can't happen when it's flat. Now, luckily, you rarely use the full power of the electric motor (and the batteries are also powering your lights, computers, HVAC, seat heaters, rear window defogger, stereo, etc.), so even less of that 34Hp is available to drive the car. When your batteries are low, your flashlight won't be as bright...the i3 can't be 'bright', either when it's battery is nearly depleted. Unlike an ICE running off of a gas tank, which is either running fine, or abruptly shuts off when the gas tank is empty...the i3 at least starts to slow down, and doesn't immediately halt like an ICE would. Seems like a good compromise for safety to me. The car at least warns you when this is about to happen. An ICE may not even have a low fuel warning light, and fuel gauges tend to be less accurate (have you ever seen one where when the needle was centered on E that it was actually empty?). The i3 is pretty good about warning you, if you're paying attention.

sipabit said:

Two fold question:

1. I've coded the i3 with the battery hold feature here in the US. It didn't occur to me until this evening while driving that when I run out of gas, the car may not revert back to battery automatically.

Click to expand...

As jadnashuanh stated, an i3 always runs on battery power, so when the REx engine runs out of fuel and stops running, an i3 should continue being propelled by battery pack power. However, some REx owners have reported the propulsion of their cars suddenly stopping when the REx engine runs out of fuel even with remaining battery energy (i.e., a charge level >0%). Some have reported pulling off the road, shutting down the car, restarting the car, and then continuing to drive on battery pack power. This behavior is almost certainly due to a software bug that has been fixed in later software versions. If you experience such behavior, ask your BMW to update your i3's system software.

sipabit said:

I was trying to switch back over to battery manually, but the function didn't engage.

Click to expand...

As stated, your i3 is always powered by its battery pack, but the REx engine can be turned off manually when the charge level is >6.5% just as it can be turned on. However, its shut down is not immediate and can apparently take several minutes.

sipabit said:

I noticed that after I did get gas and restarted the car, it went back to using battery. I don't know if that was a result of:
A. The car restarting or
B. It just happened to take that long for it to switch back to battery from gas. Sometimes I press the button and it's not always immediate.

Click to expand...

The answer is 'B' except that you are just turning the REx engine off rather than switching back to battery pack power.

The engine management system likes to run the REx at a lower RPM to get it warmed up before then ramping up to whatever level may be required, and, prior to turning off, depending on how hot it is, it may not shut down immediately, but idle a bit. This helps to minimize wear. It's never a great idea to rev an engine before things warm up, and the computer won't let it. At that low RPM, it won't be making a lot of electricity, so during that time, your SOC is likely to continue to drop. That's partly why it doesn't wait until the SOC is lower...that gives it a buffer to warm up before it can crank out maximum juice and the SOC doesn't hit rock bottom.

It appears that the SOC logic needed to be tweaked to help it match reality better, which is why things may not have been reported properly...the automatic trigger to turn on the REx may not have actually had the charge, and thus the time, it thought it had, leaving some people stuck until the level could be built back up. LiON batteries discharge voltage curve makes it tough to determine the exact SOC. It's a gradual drop in voltage and then a sharp drop-off when close to the end of its capacity. Being off just slightly can mean the readout isn't close.

I have a 2017 REX and ran it out of fuel on the freeway just the other day. I heard the motor stop, but there were no extra warnings or anything. Driving seemed unaffected. I think it was at 4% battery when this happened. I was able to drive about 1 mile to gas station, and the battery said 1% as I pulled in a nd shut down.

But I'd say the the computer shut down the motor, not that it ran out of fuel. At the gas station it only took 2.1 gallons.

Thanks for this info. This is contrary to what I was expecting which is actually good news.

To check my tank coding, I ran it dry yesterday after enabling REx at ~55% SOC. When it ran out, the car continued to drive as before only a little quieter. When I filled up, 2.296 gallons, $5.94, Costco premium at $2.589/gal. Then I found the cap tether was cut.

Bob Wilson

At least on some other BMW vehicles, the in-tank pump uses fuel to help lubricate itself...as a result, the computer won't actually let it run dry. That may be the same thing on the i3. IOW, while the tank might hold a bit more, you cannot use it all to protect the pump and prevent filling the fuel injectors with air, making it much harder to restart once you've refilled the tank. As with any fuel tank, a slight tilt when parked at the pump can affect how much you can fill it. When dealing with a small tank, that probably exaggerates that issue.

I would add that sometimes on a down slope, the fuel level can get an early 'empty' signal and shutdown the REX. This results in getting a slight variation on how much gas can be put in the car.

Bob Wilson