I've had the experience of driving a BMW X1 this week, as a loaner car.
My own car was in the body shop for some damages, so the insurance
company sprang for the X1 as my loaner. I was taken a little off-guard
by the engine auto-stop feature when you come to an idling situation,
like stoplights, parking, etc. I eventually got used to it, and actually
think it may have helped save a lot of gas for me, as I am mostly doing
city driving. But even though I liked saving the gas, it seems to me
that this would be hard on the batteries, as I noticed that all of the
lights and AC remain open during this time. My question is, how does BMW
achieve this? Does it have some special batteries, or are they just the
Heavy duty batteries, combined with regenerative brakes in my car
(Mini). Mine's 60,000 miles/5 years and still going.
I have read that the system is only of benefit if the stop period is
above a certain time. There's some research out there somewhere - 30s
and up ISTR.
Actually, nothing like the regenerative braking as utilised by electric
vehicles, it is just "Intelligent Alternator Control". All this does is
turn on the alternator during "over-run" or whilst braking, instead of
charging all the time.
It can be disconcerting to watch the battery voltage fall below 11.8
volts on a longer run, such as on the motorway using cruise control, but
this can be counteracted by turning on the side-lights, which then
maintains 13+ volts across the system.
The stop/start system only functions when the battery has enough charge,
the engine is warm and the Air/ Con is not on.
Initially, this was only with 4 pot manual cars, recently rolled out
across the ranges.
The batteries are known as Absorbant Glass Mat, still lead acid, but
these are now known to not be the optimum chemistry for this system.
There are many cases of the batteries not lasting for more than three
years in certain circumstances.
There is research into carbon / lead acid chemistry for the battery.
So does the alternator somehow produce a braking force on the engine
during this condition?
I haven't noticed the voltage gauge yet, but I don't think I'll have
this problem anyway, since I'm in Canada and we have daytime running
lights up here. So it'll be producing a constant drain on the electrical
system all of the time anyway.
Actually, I found it was still working while the AC was on. At least the
AC light was on when I looked at it.
However, today, it suddenly got colder again, hovering around 0C, it's
going to snow heavily later tonight, so I noticed that it's
automatically disabled the auto-stop feature. I think the manual
mentioned that it disables the feature during outside temperatures of
less than 37F/3C.
It starts up extremely quickly too, just let go of the brakes and it
automatically starts with barely any delay.
It can be felt as a minor deceleration. A bit more than engine braking
The alternator is not turned on instantly, but softly over a period of a
few seconds. You wouldn't want a jolt that would strain the belt and
other mounts if it suddenly came on.
There isn't as such a voltmeter on the car. The main ECU voltage can be
brought up into one of the displays, if you know how to ...
The engine drives the A/C compressor, so if the engine is not running,
then the A/C compressor will not run. There are some variables as to
whether the A/C operates with regard to outside temperature, internal
temp, humidity and 'misting' of the screens et cetera. There may be
different programs in different countries.
The latest BMW I have, a 2011 120i, has a different 'Auto' program to
the six BMWs I had before it.
Over this this of the pond, Stop/Start would initially only work if the
transmission was taken out of gear (remember at first it was on manual
shifters only) and the handbrake applied.
This may have changed on the very latest cars.
Now, this is a little strange to me, it was my understanding that the
alternator is always on, while the engine is running, so that all
electrical activity is run through the alternator, including basics such
as ignition through to optionals like lighting and entertainment.
Including the recharging of the battery. Are you saying that the
alternator is not always being used?
Ok, insider knowledge.
Here in Canada, you can get every climate test available to you within a
matter of days, if not hours. A couple of days ago, it was a mild 15C
outside. Yesterday, it hovered up and down around 0-5C, and clear.
Today, it was below 0C and there was a snowstorm! (How's that for Global
During the mild days, the auto-stop worked consistently.
Yesterday, when bouncing between 0-5C, especially once the temperature
fell to 3C, you'd hear warning sound inside the car about cold temps,
and it would automatically disable auto-stop. You'd see throughout the
day while driving when auto-stop was working if it was above 3C, but
then disabled again at 3C. As I said, in Canada you get to see in front
of your eyes how the computer adjusts to its environmental parameters.
Today, with the snowstorm, I also got the chance to see how the Xdrive
AWD system works, and compare it to my Subaru.
Yeah, this one was on an automatic transmission car, and it goes well
beyond just auto-stopping for parking. I read in the manual that it'll
go into auto-stop when you've come to a full stop for at least 1 second,
as long as you've got the transmission in regular Drive (D) mode, and
not in Drive Sport (DS) mode, or manual mode. Also if you've got your
steering turned steeply, then it won't go into auto-stop: if your
steering is cranked like that, then it determines that you're trying to
make a turn at that moment, making it dangerous to auto-stop. And of
course as mentioned before, it'll determine the temperature outside and
decide whether to activate auto-stop or not too.
Anyways, I picked my own car (a Subaru) back up today, and gave the BMW
loaner back, so it was fun getting to know it. Some features I liked
about it (like this one), others that I didn't.
Indeed he is.
This is a good feature - less strain on the alternator, very slightly
greater deceleration on over-run and less power drained during acceleration,
although IIRC, even in a 7 series it only takes about 1kW.
So while the alternator is not running, is it running back on the
Well, it's not running to the extent that's it being used to cool the
environment inside the cabin, but I know that most cars use the AC to
create de-humidified air to blow through the windscreen to defog it.
Again, that probably only refers to the cabin A/C which is used to cool
the cabin. But the A/C is also used to dry the air going into the
windshield defogger. Most car companies do that with their A/C.
Especially when the full-defogger is selected, i.e. where all of the air
is forced through the windshield rather than just partially through the
windshield and through the cabin.
Recirculating the air inside to defog would actually cause more fog to
appear inside. In the snowy climates, as you enter a cabin, and after
awhile the snow melts off your boots and your clothes, more moisture
becomes available inside the cabin. When the air is being recirculated,
this moisture can't escape, and starts coating the windows and
windshield. The solution here is to make sure dry outside air comes in
and that it gets dried further by the A/C.
However, I can see it might the opposite situation in warm tropical
climates, where the outside air is moister than the inside air. Then you
might want to recirculate the inside air instead.
Lets try this again shall we. On all five 7 series I have had the AC will
not operate below an outside ambient of 2.5C, even if the "defogger" is
selected, UNLESS you select recirculation and even then the intake air has
to reach 2.5C.
Rubbish - the inside air IS above 2.5C so once the cabin reaches 2.5C then
AC will switch on and condense all that water out of the recirculating air.
You can't dry near freezing air, because it will ice up the condenser.
"A defogger, demister, or defroster is a system to clear condensation
and thaw frost from the windshield, backglass, and/or side windows of a
motor vehicle. For primary defogging, heat is generally provided by the
vehicle's engine coolant via the heater core; fresh air is blown through
the heater core and then ducted to and distributed over the interior
surface of the windshield by a blower. This air is in many cases first
dehumidified by passing it through the vehicle's operating air
conditioning evaporator. Such dehumidification makes the defogger more
effective and faster, for the dried air has a greater capacity to absorb
water from the glass at which it is directed."
"The defroster works two ways: first, it blows warm air on the
windshield, this warm air can hold more water, so some of the water
evaporates back off of the windshield. with luck the defroster can pick
water up faster than it condenses. In a car with Air Conditioning, the
air conditioner also has a component that dehumidifies (sucks the water
out of) the air that comes out of the vents. so those cars blow warm
dry air across the windshield to draw more condensation off of the
Not entirely sure of your point. In most cases AC shouldn't operate at
or below freezing because moisture will freeze, and ice will build up,
on the evaporator coil. Compressor damage will likely follow.
It was okay, but it really wasn't enough snow accumulation to test for a
difference, most of the snow was wet or started melting to water within
hours. Also, the X1 was fitted with just normal all-season tires, but my
Subie has full snow-tires on it, so it may not have been a truly fair
test. Wish I had this to test during January or February, even early March.
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