The effect of rain on range

[SparkyB]

I just took my first road trip in the TESLA and noticed that my range was less than expected. It meant a little additional time charging... Conservative planning, so I started thinking about why.

If you walk down the street there might be some wind that pushes back or helps you along. Likewise, if it is raining and the rain is coming down straight more or less, you might assume some minor impact to milage with a Tesla. Not so fast, though. My travel was in some Texas spring rain storms. The windshield wipers had a hard time keeping up. My actual milage was reduced by 20% to 25%... non-trivial when on the road.

So imaging that your car is moving through still air... the energy to overcome the resistance is 249 Wh/mi. Now we know that water is heavier than air, every time I hit a raindrop it takes a little bit of extra energy in the Tesla. When you have millions of them for a long period of time that adds up to a lot of extra resistance. Therefore we are going burn a lot more power... 319 Wh/mi. to cover the same distance at the same speed.

When planning road trip take weather in to account. Planning to arrive with 30 miles left on the car may seem doable... but it might not be close enough if you encounter rain or strong headwinds along the way.

 

[joelliot]

So imaging that your car is moving through still air... the energy to overcome the resistance is 249 Wh/mi. Now we know that water is heavier than air, every time I hit a raindrop it takes a little bit of extra energy in the Tesla. When you have millions of them for a long period of time that adds up to a lot of extra resistance. Therefore we are going burn a lot more power... 319 Wh/mi. to cover the same distance at the same speed.

I don't think it's the raindrops slowing you down. The thin layer of water on the road has to be displaced or you would be hydroplaning. You probably never noticed this in an ICE car, but you have to pump water out of two strips the widths of your tires that are a mile long for every mile you drive.

 

[Foxhound199]

I've been noticing that really wet, cool air (regardless of actual rain drops) seems to take a bigger toll on my efficiency than frigid, dry air. Not sure why.

 

[woodguyatl]

I just took my first road trip in the TESLA and noticed that my range was less than expected. It meant a little additional time charging... Conservative planning, so I started thinking about why.

If you walk down the street there might be some wind that pushes back or helps you along. Likewise, if it is raining and the rain is coming down straight more or less, you might assume some minor impact to milage with a Tesla. Not so fast, though. My travel was in some Texas spring rain storms. The windshield wipers had a hard time keeping up. My actual milage was reduced by 20% to 25%... non-trivial when on the road.

So imaging that your car is moving through still air... the energy to overcome the resistance is 249 Wh/mi. Now we know that water is heavier than air, every time I hit a raindrop it takes a little bit of extra energy in the Tesla. When you have millions of them for a long period of time that adds up to a lot of extra resistance. Therefore we are going burn a lot more power... 319 Wh/mi. to cover the same distance at the same speed.

When planning road trip take weather in to account. Planning to arrive with 30 miles left on the car may seem doable... but it might not be close enough if you encounter rain or strong headwinds along the way.

Wet roads increase energy use because the tires are constantly pushing water out of the way.

 

[GHammer]

Definate increase in rolling friction on wet roads.

 

[Sophias_dad]

IMHO, all the answers yet are correct...

Definite rolling resistance increase from wet roads, and definite continuous load increase due to accelerating zillions of tiny raindrops from 0 to N MPH. It might be even worse for raindrops hitting the windshield, since you are simultaneously stopping their vertical momentum and accelerating them horizontally.

 

[HawkPingree]

I think these two explanations (pushing raindrops and tires pushing water) are both incorrect. After all, they apply equally to IC vehicles and I haven't heard of that issue for them. And Foxhound1999 reports experiencing the effect on cold, humid days where neither is present.

I suspect that it's evaporative cooling from the water on the road hitting the battery case on the bottom of the vehicle. At speed (and I notice the effect at freeway speeds, but not particularly at low speeds), the rush of air speeds evaporative cooling, which then cools the battery inside the casing, and forces the vehicle to use some energy just to maintain proper battery operating temperature.

Anything wrong with this logic?

Hawk