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Stuck in a snow drift

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heater snow
6K views 30 replies 18 participants last post by  DBMandrake 
#1 ·
After hearing an article on radio 4 about precautions for driving in the current snowy spell my wife asked me how an EV would fare if stuck in a snowdrift miles from anywhere.

The official advice is to drive (your ICE) with at least a half full tank of fuel. Assuming a half full battery, how long would this keep you alive in an EV in extreme conditions? I am not sure how much power the heater alone would consume.

I suppose heated seats may be the answer to keep body core temperature up without draining too much power.

I have a Rex but that wouldn't help because it only cuts in when the car is in motion (I think)
 
#2 ·
SWMBO got stuck behind an accident on the main road during the last snow storm. She was worried the same, and only turned the car on for brief periods as she had packed extra clothing and a shovel just in case. She was only delayed for 2½ hours, but it was terrible for the range on the GOM - it said she didn't have enough to get home even though the battery was actually more than half full. The next day the range was still low, only showing as 50 miles, but a brisk 50 miles on the motorway later, it was still showing 27 miles.
 
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#5 ·
At a rough guess I'd say you need about 500W continuous to keep you warm inside an EV in winter.
... just warm enough ...

But as you don't know how long it might be then that has to be the way to go. Not much benefit in turning heat on and off from an engineering POV, just run at the lowest fan speed and adjust the temperature according to your needs (Note; this is not the way to drive an EV because low fan and adjusting temperature will lead to misting up, but if you are stuck then it is not an issue.)
 
#7 ·
It's an interesting comparison - EVs don't "idle", so from that point of view it's a lot less inefficient to sit with just the heater running and of course there's no danger of poisoning yourself with CO. But the guess of about 500W to keep yourself from freezing is probably not far off, for an EV with a heat pump, so how long you can last depends massively on battery size and whether or not you have that heat pump.

Teslas with big batteries are not quite as obviously to be preferred as you might think just looking at battery size, due to the lack of a heat pump.

A Leaf 24 isn't ideal due to the small battery, but even so will last a good 10 hours without using much more than a third of a charge - but if you do have to travel it would be worth charging to 100% to add a small safety margin.
 
#19 ·
As you sit in the snow watching your battery level dropping, you can at least feel happy that with an EV you can keep warm without needing to worry about carbon monoxide poisoning which can be a real danger if you run an ICE while snowed under.
 
#22 ·
The second it starts snowing, cancel all plans to the leave the house, and sit down with a nice drink and the TV on.

If somewhere else, panic and leave immediately even if you're a vicar halfway through a marriage ceremony. You should beat the weather.

If all else fails, consider yourself lucky that life has been good to you so far. Alternatively, if life hasn't been good to you so far, which given your present circumstances seems to be more likely, consider yourself lucky that it won't be troubling you much longer.
 
#24 ·
Or, in my case...if snowing:
  • Plan more time for journeys so one can walk instead of taking the car
  • Pause the running training plan - purchase gym passes instead
  • Immediately drive the car to a charger and charge to full so there's plenty of energy for pre-heating!
  • Park the car on destination charger even if it means walking further to destination, to maximise remote pre-heating time and minimise battery usage ;)
  • Enjoy practically empty, well gritted roads :)
 
#25 ·
The Leaf does give me a dilemma in snow.
I like to keep the battery as full as possible, in case I'm stuck in traffic jams.
But a high level of charge reduces regen braking and makes slippery downhill sections as scary as I've experienced in any car, ever.
Actually speeding up down a slippery hill and no option but the friction brakes is bloody terrifying. :confused:
My winter tyres make it better, but still not the best.
 
#26 ·
There is a very logical reason for this.

Let's suppose you had a charge of 20 kWh in the battery when you start. You travel 20 miles at 4 miles per Kw. So you have used 5 kWh, and have 15 kWh left. The GOM is going to suggest that you have 60 miles left ( 15 x 4 miles per Kw).

You then get stuck for 2 hours in the snow, and using some heater, you use maybe 2 Kw, but don't increase your mileage at all. So you have now used 7 Kw to go that 20 miles, which means that the 20 miles have now been at an average of 2.85 miles per Kw. As you now have only 13 kWh left the GOM is likely to suggest that you now have 37 miles left instead of 60 ( 13 x 2.85 miles per Kw). So those 2 hours and 2 Kw used reduces the GOM figure by 23 miles.

The poor old GOM can only go by what you have been doing, and you really have now used 7Kw to go the 20 miles.
Only the poor old GOM in the Leaf might be fooled that easily. The GOM in at least some other EV's calculates the range effect of the heater separately and adds it in at the end in real time.

So for example on my Ion the GOM when calculating the average miles/kWh consumption excludes the energy consumption of the heater (and A/C) and adds it back in separately when the heater or A/C is turned on.

So sitting around for a long time going nowhere with the heater on does not throw the estimate out like you describe. The range remaining will tick down due to heater use but the miles/kWh figure that it calculates the remaining range from will not be skewed by sitting stationary.
 
#28 ·
why are people terrified of braking on ice, but think regen/engine braking is somehow better?

regen/engine braking only applies to the driven axle, the hydraulic brakes applies to all four corners, thus you've got 4 wheels trying to stop the car rather the two (usually).

Clearly, if you slam the brakes on your going to crash, but braking lightly with the hydraulic brakes is NO different to using engine braking. The tyres dont care about what is causing the braking torque. Its all about fine control and understanding what the cars doing.
 
#29 · (Edited)
I think the point is that engine braking and regeneration are both a lot smoother and more controllable than friction brakes, where pushing the pedal a little bit further can quickly go from light braking to locking the wheels on slippery surfaces.

Also think about how regen differs from friction braking - when a friction brake has constant force applied to the calliper the friction increases when the wheel slows down and comes to a stop - static friction on most sliding surfaces is always greater than dynamic friction.

So if you are already creeping along slowly and you loose traction the wheel will tend to lock and stay locked even though you didn't modulate the brake pedal.

Regen braking has the reverse effect - as you slow down and come to a stop regen braking naturally eases off and is thus less likely to allow the wheel to completely lock. In fact when you are stationary regen braking applies no braking at all.

So if the wheel tried to lock on ice with regen braking as soon as it stops turning there is no longer any regen so it will be free to start spinning again. So a complete lockup like you can easily get with a friction brake actually isn't possible from regen alone as any tendency for the wheel to lock instantly removes any regen torque.

So it will naturally find a balance where it can slow you as much as possible without skidding or locking the wheels. That was certainly my experience when I experimented with regen vs friction braking during the recent snowpocalypse.
 
#31 ·
For sure. Regen has the potential to do a lot more of our braking on a 4WD EV than we get now with a 2WD EV. It's not just because you have twice the number of wheels but because its unsafe to brake a car hard using only two wheels.

There is a limit to how hard you can brake using using only the rear wheels because braking throws most of the cars weight forward onto the front wheels - and off the rear wheels, limiting traction at the rear under braking... So regen from a RWD EV is always going to be limited.

Meanwhile braking hard only on the front wheels is certainly effective at stopping you, but without at least some braking applied at the rear to pull it back the car can become unstable and not keep a straight line. Also some rear suspension geometries like trailing arms rely on the rear braking torque to hold the rear of the car down. Braking hard at the front and not at all at the rear on a car like that can cause the car to lift up at the rear under braking which is not ideal.
 
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