MurphyDog
Posts: 7
Joined: Wed Aug 06, 2014 7:04 pm

Re: Why is the roof black?

Tue Sep 23, 2014 9:39 pm

I highly recommend getting your windows tinted. I live in the California Bay Area and we normally see temps in the 90-100's all summer long. I too found the AC being very lack luster until I tinted my windows (35% back, 50% front). Ever since the tinting the I only need to run the AC on low to cool the entire car. Best $250 I ever spent.

MikeS
Posts: 713
Joined: Thu Jan 16, 2014 8:29 am
Location: Brighton UK

Re: Why is the roof black?

Tue Sep 23, 2014 11:47 pm

Boxbrownie wrote:
MikeS wrote:What makes you think the a/c is marginal? Mine works as well as the ones in my other cars.
Mike...........Southern Calfornia vs Brighton

He may have a point........ :lol:
Not really - the a/c marginal statement must be a comparison on different car’s a/c effectiveness as I doubt that the OP has tested the car in UK and California :lol:
BMW 3 Series 2005 - Aug 2014
BMW Z4 35i 2009 - Mar 2014
BMW X1 Mar 14
BMW i3 Sep 14

Boxbrownie
Posts: 85
Joined: Sat Sep 06, 2014 10:21 am
Location: Pastyland, UK

Re: Why is the roof black?

Thu Sep 25, 2014 11:58 am

MikeS wrote:
Boxbrownie wrote:
MikeS wrote:What makes you think the a/c is marginal? Mine works as well as the ones in my other cars.
Mike...........Southern Calfornia vs Brighton

He may have a point........ :lol:
Not really - the a/c marginal statement must be a comparison on different car’s a/c effectiveness as I doubt that the OP has tested the car in UK and California :lol:
Well it seems he has compared it to other vehicles there, a Smart no less......

I can understand how the AC could be marginal in some circumstances as it has to be running a lower power system to most cars, I am sure it works fine but just takes a bit longer to get into the comfort zone.

There was a big difference in cool down temps on three of my cars during our summer and all cars are running in tip top regards AC, funnily enough the oldest a Lexus RX was best with the Audi Hybrid second and an '10MY ML trailing, but all worked fine after a while and kept nice and chilly, but I wouldn't be surprised if when we get the i3 the AC is not exactly "the best" as comprimises have to be made.
Best regards........David

If anything in my post offends you please let me know, I may wish to offend you again. ;)

AlanfromBigEasy
Posts: 19
Joined: Wed Feb 10, 2016 11:54 am
Location: New Orleans
Contact: Website

Re: Why is the roof black?

Sat Jan 11, 2020 7:51 pm

I painted my roof white. Dramatically cooler in summer and looks better too with my white 2015 i3 !
Alan Drake
-----------------
2015 BMW i3 REx Giga[/i]

tjsean0308
Posts: 8
Joined: Sat Apr 28, 2018 6:30 pm
Location: MIchigan, USA

Re: Why is the roof black?

Mon Jan 13, 2020 4:10 pm

To echo comments here. The roof is just clear coated CFRP. I'd imagine they made the choice for both a bit of weight savings (all be it very minor) and styling. I love the roof and the fact its literal cut-offs from the layup of the rest of the car. If you wanted you could wrap it in a silver or light gray to help reflect some heat.

The BEV does have a heat pump, which generally are more efficient in cooling. I haven't had my car in desert heat, but 100 degrees in the midwest was not a problem with the heat pump. It easily condensates the outside of the windows.

eNate
Posts: 438
Joined: Mon Sep 09, 2019 5:33 pm

Re: Why is the roof black?

Mon Jan 13, 2020 4:49 pm

tjsean0308 wrote:
Mon Jan 13, 2020 4:10 pm

The BEV does have a heat pump, which generally are more efficient in cooling. I haven't had my car in desert heat, but 100 degrees in the midwest was not a problem with the heat pump. It easily condensates the outside of the windows.

That's not quite right with respect to the heat pump.

The battery will always produce heat that needs to be stripped away (or "managed," I guess one could say), but will never produce cold.

So the system pumps coolant through the battery tray to carry off the excess heat, just like the coolant system that runs through an engine block.

With the heat pump, the i3 system resembles an ICE car where that heat is then exchanged with the interior system and warms the cabin air, but doesn't cool it. Without this option, that stripped-away heat is shed via a radiator to the outside airflow and "lost."

When the car and battery are parked and cool, this same system will use resistive heating to warm the "coolant" (which instantly becomes an inappropriate name), to bring the battery up to a friendlier operating temperature. But even though this is the reverse of what's described above, it doesn't exchange any cool air with the cabin -- particularly because if the battery is cold and needs to be heated, so too does the cabin.

Put another way, the only thing that cools an i3 interior is the battery-driven AC compressor, and it's identical on BEV and REX.
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tjsean0308
Posts: 8
Joined: Sat Apr 28, 2018 6:30 pm
Location: MIchigan, USA

Re: Why is the roof black?

Mon Jan 13, 2020 5:43 pm

eNate wrote:
Mon Jan 13, 2020 4:49 pm
tjsean0308 wrote:
Mon Jan 13, 2020 4:10 pm

The BEV does have a heat pump, which generally are more efficient in cooling. I haven't had my car in desert heat, but 100 degrees in the midwest was not a problem with the heat pump. It easily condensates the outside of the windows.

That's not quite right with respect to the heat pump.

The battery will always produce heat that needs to be stripped away (or "managed," I guess one could say), but will never produce cold.

So the system pumps coolant through the battery tray to carry off the excess heat, just like the coolant system that runs through an engine block.

With the heat pump, the i3 system resembles an ICE car where that heat is then exchanged with the interior system and warms the cabin air, but doesn't cool it. Without this option, that stripped-away heat is shed via a radiator to the outside airflow and "lost."

When the car and battery are parked and cool, this same system will use resistive heating to warm the "coolant" (which instantly becomes an inappropriate name), to bring the battery up to a friendlier operating temperature. But even though this is the reverse of what's described above, it doesn't exchange any cool air with the cabin -- particularly because if the battery is cold and needs to be heated, so too does the cabin.

Put another way, the only thing that cools an i3 interior is the battery-driven AC compressor, and it's identical on BEV and REX.

I wasn't saying the battery cools the cabin. I was saying heat pumps are generally more efficient at cooling than heating do the thermal operating range of the refrigerant. The i3's heat pump is effective at heating down to 14f, but I notice the resistive heater starting to come on around 22f. This performance is on par with most home heat pumps making it a quite good system.

The i3 doesn't use the heat pump to heat the battery. The i3's battery pack is directly cooled by the HVAC refrigerant not a traditional liquid coolant. It is heated by a resistive battery blanket which was optional on 2014 models and only installed with heated seats as a pseudo cold weather package.

The battery driven AC compressor is the refrigerant pump for the heat pump. A heat pump is essentially a vapor cycle cooling system with a reversing valve to allow the evaporator and condenser to swap roles. That enables the conditioned space to be heated and cooled with the same refrigerant and compressor. There is bit more involved, but that's the basic concept, an air conditioner that can heat and cool. The i3 takes that to the next level with multiple expansion valves and the ability to control which evaporator gets priority.

Here is a video from the Munro labs tear down where the talk specifically about the refrigerant cooling on the i3's pack. Tesla's and others have liquid managed packs, but the i3 is refrigerant only in the pack. https://www.youtube.com/watch?v=pgu6mkK ... u.be&t=329

Here is a pamphlet from BMW showing all the heating and cooling system components. You can see on page 37 the battery in particular and the refrigerant lines that are in the pack. The only coolant is for the electric heater and the drive motor itself because liquid is much more efficient to heat with a resistive heater.
https://www.speakev.com/attachments/08- ... pdf.79217/

There was a very useful FAQ from a BMW engineer on the sadly dead now, BMWiForum that clarified all this and confirmed the HV battery is refrigerant cooled and resistively heated. That is why the battery preconditioning lead time is a minimum of three hours because it's only a 1kW blanket of sorts that heats the battery. It's pretty plan to see when you have cold soaked the car and are on reduced power. Even driving the wheels off it, it takes about 30-40 minutes to start recovering the power back and usually I'm still reduced by the time I get home from work when it's been a cold night. (I work 24 hour shifts so the car is cold soaked overnight a few times a month)

Edit: Clarity.

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