Rex4Rex
Posts: 5
Joined: Tue Jan 05, 2016 5:04 pm

Using range extender only for electric charge

Tue Jan 05, 2016 5:13 pm

I\ wonder if anyone does this regularly as i currently dont have anywhere locally to re-charge via a socket as i park on the street, im regularly running my new i3 Rex via many trips to the petrol station. IS there any long terms problems that can be caused by doing this - i hope to move to a home soon with a driveway !

additionally has anyone noticed that whilst driving with range extender on, the electric charge seems to drop also and lose range almost as fast as the rex range, despite only being power by petrol generated electricity. Does anyone know why this is or how this can be addressed?

jadnashuanh
Posts: 4784
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Using range extender only for electric charge

Tue Jan 05, 2016 5:22 pm

The REx is only a 34hp engine...you have probably NEVER driven a car with only a 34hp engine. Now, you certainly do not use all of the 170hp motor in the i3 all that often (well, if you do, do you see why the REx can't keep up!), but especially in the winter with the lights, heat, acceleration, climbing a grade all can contribute to using more power than the REx can provide. Stop and go around town, the REx can easily keep up. Cruising at a reasonable, steady speed on the flat, the Rex can keep up. Stress it, and it cannot, so the battery charge will drop. If you try to compare it to say the CHevy Volt, look at the hp of their motor...LOTS bigger so that it can always keep up. But, you'll also notice it has a much smaller electric motor, so it's not an apples to apples comparison. The REx will not recharge the i3's main battery to full...it is just not designed to do that, and will shut itself off once it reaches a certain level. That cannot be changed, and probably, even with a full tank, couldn't even if you could make it run constantly.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

Rex4Rex
Posts: 5
Joined: Tue Jan 05, 2016 5:04 pm

Re: Using range extender only for electric charge

Tue Jan 05, 2016 5:39 pm

i see, so then running off the Rex motor at say highway speeds etc may need some topping-up from the stored battery charge?
i suppose if the battery was already close to flat, then it would not be possible to drive the car to full capacity on the rex alone

Rex4Rex
Posts: 5
Joined: Tue Jan 05, 2016 5:04 pm

Re: Using range extender only for electric charge

Tue Jan 05, 2016 6:06 pm

i see, so then running off the Rex motor at say highway speeds etc may need some topping-up from the stored battery charge?
i suppose if the battery was already close to flat, then it would not be possible to drive the car to full capacity on the rex alone

eneka
Posts: 92
Joined: Thu Nov 12, 2015 1:57 pm

Re: Using range extender only for electric charge

Tue Jan 05, 2016 8:12 pm

Based on your replies, I'm going to assume you're not in the US. The rex is essential ly a backup. It will keep the charge but not charge more than the battery already has. In the US, we cannot turn on the tex manually, it will only come on automatically at 6% and if you stress the car at that time it will go into low power more
2015 i3 rEX Arravani Grey giga, parking, tech, 20"

jadnashuanh
Posts: 4784
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Using range extender only for electric charge

Tue Jan 05, 2016 10:08 pm

'Sane' driving of the i3 generally means the REx can maintain battery charge. But, depending on what you are doing and when it is engaged, you can get into a situation where you could run out of enough battery capacity to keep doing what you're doing! High speeds, climbing a grade, and the cabin heat on high (especially with the REx which doesn't have a heat pump) are probably your biggest power consumption sources. There is a power flow diagram that you can call up for the display to show you what's going on. Driving at night with the high beams on, the wipers running, the rear defogger on along with seat heater(s) just compound the energy use. A 34hp engine is about what the first VW bug had when introduced, or a 2CV...try to run up a grade and you'll not be able to fly up there once the battery reaches a certain low SOC. Once the battery charge gets critical, that's all you have to keep going. As you get close to that point, the car will warn you then start to shut down non-critical functions, but cross that magic threshold, and you'll be like you hit a wall, and slow down radically, not stop, but you wont' be able to maintain full speed while the load is still there. Pull over, let the REx rebuild some battery charge, and you could go again, or limp along until you reach the crest and can build some charge with regen on the way back down. Stay within the limits and capabilities of the car, and you may never notice. It is, after all, designed as a city car with a commute into the suburbs...not cross-country use even though people have and can do that with proper planning and understanding of how things work. The i8 and the next I-car can keep going, essentially without compromises under all conditions (but even the i8 has problems if you are using it on a track...even with over 200Hp, it can't keep up full performance while trying to keep the battery fully charged, but until the battery runs down, it goes really well!).
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

janner
Posts: 235
Joined: Sun Jan 25, 2015 5:24 am
Location: Bath, UK

Re: Using range extender only for electric charge

Wed Jan 06, 2016 1:00 am

The Range eXtending engine or ReX is a BMW W20 two-cylinder engine of 647cc producing 25kW at 4300rpm. The internal combustion engine drives a generator having an efficiency of ~94% giving 23.3kW at 4300rpm DC power at 330V. When operating, the electrical energy either goes directly to the electrical motor or to the battery with the aim being to maintain the battery state of charge SOC. Above a SOC of 6.5% (1.22kWhrs) the i3 behaves exactly the same whether on ReX or pure battery but has the capability to deliver an additional 21.6kWHrs of energy from the 9 litres of petrol.

The ReX can be started and stopped manually at a SOC below 75%. At <6.5% SOC the ReX will be started automatically.
Once started the ReX has a warm-up period (for the engine and catalytic converter) of about 6 minutes, depending on ambient temperature, when it runs at 2,200-2,400rpm – this is the engine idle-speed. Stop/Start is inactive during the warm-up period. During this idle period you aren't producing much power.
If SOC charge is <0.7% then the ReX runs at idle up to about 12mph and then increases gradually to 4300rpm at about 40mph.
If SOC charge is <1.5% then the ReX runs at idle up to about 25mph and then increases gradually to 4300rpm at about 56mph.
Below 1.9% power is restricted.
If SOC charge is <2.5% then the ReX runs at idle up to about 35mph and then increases gradually to 4300rpm at about 62mph.

Between 1.9% and 3.5% stop/start will be deactivated.

Between 3.5% and 6.5% SOC the engine is stopped at speeds of less than 6mph. Above that it runs at idle between 6mph and 35mph when it increases gradually to 3600rpm at 56mph and above.

Above SOC of 6.5% and below 75% the ReX will stay at 3600rpm above 35mph delivering up to 20kW and will increase to 4,300rpm (23.3kW) if additional power is necessary to hold the designated SOC.
2015 i3 ReX Solar Orange, Loft interior world. BMW professional, 19" turbine-spoke 429 wheels, DC rapid charging, driving assistant plus.

psquare
Posts: 497
Joined: Wed Oct 15, 2014 5:57 am
Location: i3 REx, 225xe Hybrid UK
Contact: Website

Re: Using range extender only for electric charge

Wed Jan 06, 2016 4:41 am

After over 10 months of ownership, we use our REx for about 9% of our journeys. I occasionally travel 800-1000 miles from England to France, Belgium, Netherlands and Germany and the REx is perfect for this sort of thing.

BMW might have advertised the i3 as a city car, but it can easily be used on longer journeys when fitted with the REx and a reliable charging network is in place. Experience shows that REx models tend to travel more electric miles, as these can also be used on journeys where pure EV owners would have to opt for an ICE vehicle. Unless battery capacity significantly increases, this will be the reality.

panamamike
Posts: 63
Joined: Sat Jan 25, 2020 10:21 am

Re: Using range extender only for electric charge

Wed Feb 12, 2020 10:36 am

janner wrote:
Wed Jan 06, 2016 1:00 am
The Range eXtending engine or ReX is a BMW W20 two-cylinder engine of 647cc producing 25kW at 4300rpm. The internal combustion engine drives a generator having an efficiency of ~94% giving 23.3kW at 4300rpm DC power at 330V. When operating, the electrical energy either goes directly to the electrical motor or to the battery with the aim being to maintain the battery state of charge SOC. Above a SOC of 6.5% (1.22kWhrs) the i3 behaves exactly the same whether on ReX or pure battery but has the capability to deliver an additional 21.6kWHrs of energy from the 9 litres of petrol.

The ReX can be started and stopped manually at a SOC below 75%. At <6.5% SOC the ReX will be started automatically.
Once started the ReX has a warm-up period (for the engine and catalytic converter) of about 6 minutes, depending on ambient temperature, when it runs at 2,200-2,400rpm – this is the engine idle-speed. Stop/Start is inactive during the warm-up period. During this idle period you aren't producing much power.
If SOC charge is <0.7% then the ReX runs at idle up to about 12mph and then increases gradually to 4300rpm at about 40mph.
If SOC charge is <1.5% then the ReX runs at idle up to about 25mph and then increases gradually to 4300rpm at about 56mph.
Below 1.9% power is restricted.
If SOC charge is <2.5% then the ReX runs at idle up to about 35mph and then increases gradually to 4300rpm at about 62mph.

Between 1.9% and 3.5% stop/start will be deactivated.

Between 3.5% and 6.5% SOC the engine is stopped at speeds of less than 6mph. Above that it runs at idle between 6mph and 35mph when it increases gradually to 3600rpm at 56mph and above.

Above SOC of 6.5% and below 75% the ReX will stay at 3600rpm above 35mph delivering up to 20kW and will increase to 4,300rpm (23.3kW) if additional power is necessary to hold the designated SOC.
I wonder how they calculated the generator efficiency to be 94% giving 23.3kW. A gallon of gas has about 33.7 kilowatt-hours of energy.
Comes to 69.1% efficiency?

alohart
Posts: 1909
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: Using range extender only for electric charge

Wed Feb 12, 2020 12:11 pm

panamamike wrote:
Wed Feb 12, 2020 10:36 am
I wonder how they calculated the generator efficiency to be 94% giving 23.3kW.
I interpreted 94% to be the efficiency of the electric side only (i.e., the generator, rectifier, battery cells, etc.).
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

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