Driving on Rex only & not charging battery

Sorry if this is a dumb question, but I am still getting used to daily life with my 2019 i3S. Today I was very busy and did not have time or an opportunity to charge. I have been driving all day, multiple errands using the REX and 0% battery. REX works great!

Is it dangerous, or bad for the battery to do a day or two and be uncharged and use REX only?

Thanks!

A few things come to mind:
1. If it's going to get cold, especially below freezing while your battery charge is that low, you can damage the battery.
2. The REx is less than 40Hp. When your battery level is essentially zero, it's not going to have great acceleration, and may limit your speed. Think 1960's VW bug performance unless there's some battery capacity to augment it.
3. When the battery level is that low, it will disable most of the HVAC functionality. If you needed defrost capability, it might be degraded, and if it did turn on, you might notice your max speed goes down.

With my battery at 0% and driving with REX only....No reduction at all in HVAC and even at highway speeds of 70+ mph (up a substantial hill)...REX is strong and fast!

Is the '19 i3S with REX different from other years? Even off the line with REX only, it is shockingly quick!

I've seen figures that the 120 Ah models (42.2 kWh) have 37.9 kWh usable, so a 10% reserve (and about 15% reserve for the original 60 Ah batteries). So when your gauge says 0% SoC remaining, the car is still running on a modest little buffer. How the computer manages it from there, I don't know, but I'm sure it's actively working in the background with a hidden threshold that it's trying to stay above.

Deutsch100 said:

I have been driving all day, multiple errands using the REX and 0% battery. REX works great!

Is it dangerous, or bad for the battery to do a day or two and be uncharged and use REX only?

Click to expand...

Keep in mind that all i3 REx's are propelled by their electric propulsion motors only. The REx motor spins a generator that supplies electrical power to the circuit that includes the propulsion motor and battery pack. The propulsion motor doesn't care where the electrons that power it originate.

When driving below ~70 mph on flat terrain, the REx generator is usually able to provide sufficient electrical power such that no power is taken from the battery pack. If the charge level of the battery pack is less than ~6.5%, the REx generator's excess output (i.e., the power not needed by the propulsion motor) charges the battery pack up to a maximum charge level of ~6.5%. When driving faster than ~70 mph and/or up an incline, the REx generator output is insufficient to power the propulsion motor, so the power shortfall is taken from the battery pack which discharges it.

When you write that you were using "0% battery", I assume that you don't mean that the battery pack's charge level was 0%. If it were 0%, propulsion and climate control power would be reduced noticeably as jadnashuanh stated. Driving around town, the REx generator output would have been sufficient to power the propulsion motor, so the battery pack's charge level should have remained at ~6.5%.

Driving for any length of time with the REx motor running isn't dangerous. However, as jadnashuanh stated, leaving the battery pack at 6.5% or less can increase the battery cell degradation rate. You should have seen a warning displayed on the instrument panel to charge the battery pack. I have even received an email message containing the same warning although BMW might not send these messages now.

Hi,

Are you in Germany or outside the USA?

Deutsch100 said:

Sorry if this is a dumb question, but I am still getting used to daily life with my 2019 i3S. Today I was very busy and did not have time or an opportunity to charge. I have been driving all day, multiple errands using the REX and 0% battery. REX works great!
...

Click to expand...

In the USA we have different control laws. So many of us code our cars for EU rules and get a much improved car.

Bob Wilson

I coded my car so that I can use the REX as soon the battery level reaches 75%, this comes in handy for longer drives and you keep full power, ac, etc...

Thanks to everyone for ALL the responses, super appreciated. When I say 0% battery, I mean the battery display says ---% (but I assume it is between 6-7%, as this is when USA Rex models engage).

The 3x I have driven using Rex, where the battery display says ---%, when parking a message comes on the Infotainment screen shows a warning to prevent damage to the battery, the battery must be charged. I was not sure if this was just a BMW scare warning or actual truth. I charged my car last night to Full

Btw, I am in Calabasas, California (although ich bin Deutscher

The user display shows USEABLE battery capacity, so zero is way below when the REx would or should have turned on by itself.

Deutsch100 said:

When I say 0% battery, I mean the battery display says ---% (but I assume it is between 6-7%, as this is when USA Rex models engage).

Click to expand...

Then the charge level being displayed is incorrect which has happened with some older i3's. However, yours is new, so this is surprising. If I were you, I would ask your dealer to fix this under warranty.

The following is a description of what happens to a 60 Ah REx as the charge level drops. I've not read that the behavior of 94 Ah and 120 Ah REx's is any different. The REx engine should turn on when the displayed charge level is 6.5%. When the displayed charge level drops below 3.5%, the REx engine will not automatically stop and start, so it would not stop running when the car is stopped at a traffic light until the charge level exceeds 3.5%. When the charge level drops below 1.9%, the gray power chiclets on the power (right) side of the power meter begin to disappear to reflect that the propulsion motor and climate control system no longer have full power available. When the charge level drops to ---%, all power chiclets would have disappeared such that there is no propulsive power available to drive.

@alohaart....When my battery gets to 6-7% then my REX turns on. Once the REX is active, then the battery meter always shows --%. I have never lost full climate power and my '19 i3 is always super zippy and fast using REX.

Perhaps the '19 model is different from the 14-18 models since the battery is "new" for 19. I asked the dealer when I had my CEL repair and they told me when the REX engages, the car will show --% battery on the display (although there will usually be anywhere from 3-7% of battery actually remaining).

The dealer told me the screen shows --% for the battery to let the driver know that there is no real "usable" battery left, the car is using REX and to encourage charging. Just what I was told.

Deutsch - My 2015 i3 does exactly the same - When my battery gets to 6-7% my REX turns on, and the battery meter then shows --%, the car has full climate power and is still zippy using the Rex.

Thanks MKH!!!

MKH said:

My 2015 i3 does exactly the same - When my battery gets to 6-7% my REX turns on, and the battery meter then shows --%, the car has full climate power and is still zippy using the Rex.

Click to expand...

That seems like a terrible user interface because we know that driving in conditions in which the propulsion motor requires more power than the REx generator can provide will gradually discharge the battery pack. I would certainly want to know when the charge level is nearing 1.9% at which point power to the motor and climate control system begins being reduced which will limit top speed. Knowing the charge level would allow me to modify my driving to prevent the charge level from continuing to drop. So you're telling me that when the REx engine is on, you have no idea what the charge level is! That's crazy!

When your REx engine runs out of fuel and stops, is the charge level displayed again? Wouldn't you want to keep the charge level at 6.5% when fuel is running low so that you'd be able to drive the maximum distance possible looking for a gasoline station or fast charger? However, if the charge level isn't displayed when the REx engine is on, you'd have no idea of the charge level so you could have a very unpleasant surprise when the fuel runs out when discovering that your charge level is only 2%.

No wonder so many 60Ah REx drivers have been surprised when their cars suddenly and occasionally dangerously decelerated to ~30 mph because the charge level had dropped below 2%. I guess they had no warning because the charge level wasn't displayed although they should have noticed the gray power chiclets disappearing one by one.

alohaart...the i3 with Rex is a strange bird, LOL. I am loving my new i3, but it is one quirky car

I just saw a photo of the instrument panel of a REx with the REx engine running. The charge level graph along the bottom of the panel shows the charge level although its resolution is likely insufficient to know when power loss might begin. However, the loss of a gray power chiclet on the upper right side of the power gauge would indicate when mild power loss begins which would allow a driver to slow down, turn off climate control, etc., to prevent further power loss until the REx generator increases the charge level. Still doesn't make sense to me that the numerical charge level as displayed by the onboard computer would show ---% when the REx engine is running.

Still doesn't make sense to me that the numerical charge level as displayed by the onboard computer would show ---% when the REx engine is running.

Click to expand...

To me, since my car is REx equipped, that last 4% - 6% of battery when the REx kicks in, is just the reserve battery-charge buffer needed by the REx to operate. Essentially, when my i3 REx hits 6% and the REx engine starts, I look at it as that I am 'out of juice', with the REx now supplying the electricity for propulsion.

The charge level graph along the bottom of the panel shows the charge level although its resolution is likely insufficient to know when power loss might begin.

Click to expand...

That seems like a terrible user interface because we know that driving in conditions in which the propulsion motor requires more power than the REx generator can provide will gradually discharge the battery pack. I would certainly want to know when the charge level is nearing 1.9% at which point power to the motor and climate control system begins being reduced which will limit top speed. Knowing the charge level would allow me to modify my driving to prevent the charge level from continuing to drop. So you're telling me that when the REx engine is on, you have no idea what the charge level is! That's crazy!

Click to expand...

You get a warning when the REx starts, that you may run into situations where there will be a loss of power - but that is going to be exceedingly rare. When the range-extender engine turns on, it initially runs in the lowest of the three power levels. If you continue to drive at speeds higher than 40 miles per hour, it will kick up to the next power level. If you are driving at highway speeds, it will jump up to its highest (28kW) power output. At that output you can run at 70 mph all day long until your fuel runs out, with no power loss, even on moderately hilly roadways. The REx turns off when you slow down to less than 15 mph, unless your state-of-charge is lower than 3 percent. If you exceed the range extender’s capability, it will slow down to 40 mph. At that speed, it can maintain just about any climb. Now if it is 20 degrees out, and you are climbing a miles long, steep grade, yes, your speed may drop,

When your REx engine runs out of fuel and stops, is the charge level displayed again? Wouldn't you want to keep the charge level at 6.5% when fuel is running low so that you'd be able to drive the maximum distance possible looking for a gasoline station or fast charger? However, if the charge level isn't displayed when the REx engine is on, you'd have no idea of the charge level so you could have a very unpleasant surprise when the fuel runs out when discovering that your charge level is only 2%.

Click to expand...

If you are 'out of juice' and running on the 'reserve' power supplied by the REx, if you let the REx run out of fuel too, then you are simply out of gas, and stopped, just like if you let an ICE car run out of gas. Why would you expect an extra reserve source of power? Keep your eye on your REx gas gauge, and you shouldn't be unpleasantly surprised - after all, that's why the REx is there, to give you additional range to get you to a charging station, when the distance you are driving is beyond battery-only range.

Great reply MKH! Depending on speed and outside conditions, one can really hear the different speeds or levels of the generator, aka BMW Motorcycle engine powering our batteries.

When my car switches to REX, there are NO warnings or messaged that come on (not main instrument cluster or infotainment screen). When REX activates, the usually light gray fuel bar graph indicator switches to bright and easily legible white, instead of faded grey. The REX is so good that the first time my REX activated, I did not even know I was using the generator!

When my car switches to REX, there are NO warnings or messaged that come on (not main instrument cluster or infotainment screen).

Click to expand...

Yeah, I was miss-remembering when you see that message. It only comes up if, while using the REX 'strenuously', your state of charge drops below 3%, then you get an audible alert, and on your instrument panel a message pops up that says: "Low Battery. Power Reduction Possible." Also, you can always see the percentage of battery remaining, top left (accessed by pressing button on end of signal stalk),

View attachment dash.png

MKH said:

If you are 'out of juice' and running on the 'reserve' power supplied by the REx, if you let the REx run out of fuel too, then you are simply out of gas, and stopped, just like if you let an ICE car run out of gas. Why would you expect an extra reserve source of power?

Click to expand...

There is no extra power reserve if the displayed charge level is <1%, except that when the Rex engine is running, apparently there is no displayed numeric charge level, so the driver wouldn't know that there might not be any remaining charge to allow her to drive to a charging or gas station. This would be a problem only when driving at fast highway speeds, up steep inclines, in cold weather with the cabin heat on high, etc., which could be a common situation for some REx drivers. It's also a bigger problem for i3's with smaller battery pack capacities.

Indeed, there is (was?) a lawsuit against BMW for sudden power loss when driving with the REx engine running. I thought that this was due to the driver not paying attention to the charge level, but if the numerical value isn't displayed, the driver is somewhat more justified to blame BMW. Of course, the linear charge level gauge running along the bottom of the instrument panel provides low-resolution charge level information, and the loss of gray power chiclets from the power gauge would be a strong indication of the need to reduce power requirements so the REx generator can keep up. However, I've never read of the loss of power chiclets in the i3 Owner's Manual, so most owners are probably unaware of this indication.

MKH said:

Keep your eye on your REx gas gauge, and you shouldn't be unpleasantly surprised - after all, that's why the REx is there, to give you additional range to get you to a charging station, when the distance you are driving is beyond battery-only range.

Click to expand...

Many REx owners use the REx engine to drive long distances instead of BMW's intended use that you describe. If I owned a REx instead of a BEV, and I were in a situation in which I was running low on gasoline, I would appreciate knowing the charge level so that I could better plan how to charge or refuel. At 6.5% charge level, a 60 Ah REx could drive about 5 miles after the REx engine runs out of fuel. I still can't understand why BMW chose to hide the charge level.