Most efficient charging method when traveling....

Mayhemm said:

Greetings all,

I was just wondering about the most efficient way to charge on a roadtrip.

Assuming your route has sufficient charging infrastructure with many equally-fast charging options (level 2), is it more efficient (ie: minimizing time spent) to charge frequently for short periods at many locations, or to stop only when range is getting low and charging for several hours?

I'm a couple weeks away from my first major roadtrip (~2000km) and was curious about how to minimize my time spent charging on the road.

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Well, since their really isnt any Superchargers around you, you do have access to Sun Country Highway's 70 amp Level 2's, so if you have Dual Chargers, your in for a treat

Charge as you need. Plan out your stops for when you think you will need to stop anyways. Potty break every 2 1/2 hours, well, as long as your stopping, turn it into a Charge stop. If you know it's going to be a extended stop, stuck up as much power as you can, especially if charging options are sparse on your route.
MOST OF ALL! DON'T BE AFRAID TO RANGE CHARGE! So many people seem afraid to range charge. It's really not Range Charging that is damaging the battery, it's Range Charging, and letting it SIT at 100% that is damaging. So if your on that road trip and you feel that it might be a stretch, DONT HESITATE!
Another time Range charging is a good idea, is say, if your next charging stop only has a single Level 2 to charge at. What happens if that Level 2 is offline or ICE'd, or something else has happened to it. With no other reasonable options for a higher power charging stop, having a 100% charge and squeezing enough into the battery, might shave HOURS AND HOURS off your charging if you find yourself in a emergency situation where 120v is your only option. I would also advise making a 120v standard plug to 240v adapter, that will allow you to plug into Hotel Room 240v Air Conditioner/Heater Outlets. That will effectively double your charging speed in emergency situations. Will it still be slow as hell compared to normal Level 2's? Yah, but if your flat dead, I'd rather take 33 hrs for a full charge over 66 hours , and lastly, the 120v 20 amp plug is a must. 33% faster then the 15 amp plug (16 amp charge speed as to 12 amp), and if you make that 240v adapter I mentioned, you've really increased your charging speed off a hotel AC unit's outlet!!!

I hate playing Devils advocate, but with close to 27,000 miles on my 60kW pack, in LESS THEN A YEAR!, and numerous road trips and getting stranded once due to a series of ****** unfortunate events, I've learned my lesson, and never can be too prepared. I see you do have the Twin Chargers, after my last road trip to Niagara Falls from Milwaukee, Wi, I immediately went to the service center and got upgraded. So far, worth every penny!!! I do for see the Twin Chargers being extremely useful for at least the next 2-3 years, until the Supercharger Network has Fully Matured.

In fact, It's Midnight, and I am currently at this very moment (well, not at THIS moment as I'm taking a lil break) installing the First of Two HPWC's at the Sandrift Resort in Wisconsin Dells. With how swamped with Travelers they got last summer on their 14-50, the dual Chargers will help considerably with getting folks charged and on their way (If so equipped).


I will post pictures of the Hotel AC Charging adapter I made. It is very simple, and cost about $8 to make using off the shelf components.

when it comes to Superchargers, you minimize your total travel time by driving faster between the stations. it will take longer if you drive slow to save. The optimal drive speed is equal to your available charge speed. if you have only slow chargers on your route, drive slow to minimize charge time. it will overall save you time. If you have Superchargers, drive as fast as you want.

islandbayy said:

I will post pictures of the Hotel AC Charging adapter I made. It is very simple, and cost about $8 to make using off the shelf components.

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I'm looking forward to seeing these pics. Thanks!

Canuck said:

I'm looking forward to seeing these pics. Thanks!

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I will grab it from the car in the morning and snap some shots. Heck, better yet, I feel one of my special "YouTube" videos coming on Tehe. Car is 2 minute walk away, but it's 1 amp and raining so I'm going to act like my 3 year old, throw a fit and go to bed

I did make use of that charger adapter in Niagara Falls. Came in handy. And Since I'm not forcing it to use a higher powered adapter on a lower powered circuit, You don't need to worry about turning the amps down (unless hotel has incorrect wiring or under-sized breakers).

Mayhemm said:

I was just wondering about the most efficient way to charge on a roadtrip... charge frequently for short periods at many locations, or to stop only when range is getting low and charging for several hours?... curious about how to minimize my time spent charging on the road.

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I can't add much to this since I only got my car recently and I have only done range charges a few times. But what I have learned is that it takes a lot longer for the battery to get to the full charge than the car estimates (but only at the very end of charging to full). I also noticed that as it gets really full, the amps drop way down. So, your charging time will be faster until you get close to full. Then it drops right down and seems to take forever to finally finish.

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islandbayy said:

I will grab it from the car in the morning and snap some shots. Heck, better yet, I feel one of my special "YouTube" videos coming on Tehe. Car is 2 minute walk away, but it's 1 amp and raining so I'm going to act like my 3 year old, throw a fit and go to bed

I did make use of that charger adapter in Niagara Falls. Came in handy. And Since I'm not forcing it to use a higher powered adapter on a lower powered circuit, You don't need to worry about turning the amps down (unless hotel has incorrect wiring or under-sized breakers).

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Sounds great. No rush... I don't have any hotel trips planned until the summer!

While they may "look" the same, the 240v plug is exact opposite blades of te 120v plug. Since the UMC accept both 120 and 240 without prejudice, no current "work arounds" are done. This is a simple pass through to allow 240v through the 120v plugs. You can use either 15 or 20 amp 120v plugs with this adapter. and Since their is no "legal" 240v 15 amp plug currently in the USA, they should all be 20 amps, so once again, 15 or 20 amp 120v adapters will work no problem in between these is just a 5 inch piece of Romex that I had laying around, of course sized appropriately for the amperage.

A few points:

1. Not all Level 2 stations are created equal. The stations range from 24 amp to 80 amp (delivered) and could be either 208V or 240V. Power = Current * Voltage, so Level 2 could mean anything from 5kW to almost 20kW. Look for CS-90 (70A) Sun Country Highway stations, or better yet CS-100s (80A). The CS-100s are rare and need a secret handshake to enable 80A (hit the Start button twice). Stations wired on 240V will charge 15% faster than an equivalent station on 208V. Check the comments on PlugShare for available current and voltage.

2. As @canuck mentioned, the current tapers off as the battery approaches full charge. Keep an eye on the current drawn and stop charging when the rate drops below the rate of the next station (given that you have enough range to get there!).

3. Try to time your full charges with overnight stops.

I just finished a Los Angeles to Connecticut road trip, and I think there's very good advice here. I was on superchargers most of the time, but the two times I left the trail it's critical to be charging overnight. Level 2's can range from 15mph to 32mph. If SC's are nearby, definitely drive fast and charge often. Besides the efficiency, it helps to preserve your sanity on road trips if Corollas and Fiestas aren't flying by. Try to make sure the car is charging whenever you stop.

David99 said:

. The optimal drive speed is equal to your available charge speed. if you have only slow chargers on your route, drive slow to minimize charge time. it will overall save you time. If you have Superchargers, drive as fast as you want.

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I have seen this statement a few times on the forum.

Is this guideline the result of experience or is there a theoretical reason for it to be true?

It's not a bad rule of thumb, but not exactly true. I built a spreadsheet to calculate the optimal driving speed based on distance and charge speed. It turns out to be roughly 90km/h for the Roadster when charging at 70A.

The total distance changes the result - with a shorter distance you spend more time driving on the initial overnight charge so you can get away with driving at a higher speed.

I haven't done the calculations for the Model S, but you can get the data from the chart in this blog post.

Model S Efficiency and Range | Blog | Tesla Motors

simonog said:

I have seen this statement a few times on the forum.
Is this guideline the result of experience or is there a theoretical reason for it to be true?

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It's actually not a complicated mathematical problem. It's pretty straight forward.

assume you travel 100 miles (which is the average leg between Superchargers). It's just a random number, you can extend it to longer trips, but it doesn't change anything. It just repeats each driving and then charging cycle. So it's fine to just look at a single drive/charge cycle.

Going at 65 mph: drive time is 92 min, charge time is 17 min = 109 min total
Going at 75 mph: drive time is 80 min, charge time is 24 min = 104 min total
Going at 85 mph: drive time is 70 min, charge time is 30 min = 100 min total

You are using more energy to travel the same distance, hence the charge time is longer, but since the Superchargers are so fast the time lost in driving slow outweighs the extra time spent charging.

Level 2 chargers are slower. Since they charge slower than you can drive the times are opposite.

djp said:

It's not a bad rule of thumb, but not exactly true. I built a spreadsheet to calculate the optimal driving speed based on distance and charge speed. It turns out to be roughly 90km/h for the Roadster when charging at 70A.

The total distance changes the result - with a shorter distance you spend more time driving on the initial overnight charge so you can get away with driving at a higher speed.

I haven't done the calculations for the Model S, but you can get the data from the chart in this blog post.

Model S Efficiency and Range | Blog | Tesla Motors

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Is this guideline the result of experience or is there a theoretical reason for it to be true?

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I do not believe that driving the charge speed is a theoretical optimum. However, after doodling with a lot of travel/charge time spreadsheets, I can tell you that for travels where charge time is a significant part of total trip time, then driving the charge speed is a good, approximate rule-of-thumb for fastest total travel time.

For Superchargers that can charge at 100's of miles per hour (if they are spaced closely enough), then you can drive as fast as you want and reduce total travel time, but increase charge time.

@David99 Thank you - I understand that there is a trade off between travel time and recharge time.

It's the statement that optimum speed = charge rate possible that I think cannot be analytically correct. It might be a useful rule of thumb as @djp says; it is so dependent on the specific equation for the energy use vs speed of the car (which is all about efficiency and air resistance inter alia) that when I wrote even approximate equations I couldn't get to it!

Something based on measured experience might be an eventual input into the ultimate trip planner, which not only optimises the route between charger but also estimates the optimum speed for each leg.

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@Cottonwood
Thank you - exactly what I needed to know. Now I can stop worrying why I couldn't make the physics work!

Simple math:
Driving is 60-90mph
Charging is 340mph
No matter how fast you drive, Supercharger is still faster than you
So Supercharger will offset the loss of speeding

Based on this curve from Tesla's website I did some calculations. It shows the energy usage vs speed.

Based on it I made a very simple spreadsheet to find out what the ideal drive speed is depending on the charger speed available. It is based on driving 100 miles. I used three different chargers: a typical 7 kW public L2 station, a 20 kw HPWC with dual chargers and a 100 kw Supercharger. I used 100 kW as it seems a reasonable average considering tapering. All times are in minutes.


For the 20 kW chargers the ideal speed is almost identical with the charge speed (20 kW gives you a little over 60 miles per hour)
For the 7 kW charger the ideal speed is slightly above the charge speed. It's because the efficiency curve flattens out below 40 mph.
For the Supercharger the ideal speed is pretty much 'as fast as you can go'.

IOW, the slower your charge speed is the slower you should drive to safe time overall. The faster the charger you have available the faster you should go to minimize total travel time.

Now add cost

plus heat, A/C usage, terrain, and weather.

Also keep in mind most high amp L2 stations on Sun Country Highway are 208V 70A (14.56kW) which will increase charge times by 30% compared to 240V 80A.

The spreadsheet clearly shows the need for SuperChargers on road trips. With high amp L2 it's counterproductive to drive faster than 60mph - you'll spend more time charging than you save driving. High amp L2 is great as a destination charger, but doesn't cut it for long distance road trips.