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40kWh battery vs. 60kWh battery

The vast majority of MSs on the road are 85kWh and I wish it was practical for me to have one. However, the realistic decision I face is 40kWh or 60kWh. I have been debating this over the last few months, but I think I have finally made a decision.

It took a lot of forum reading along with many questions of Tesla, but I am confident that the 60kWh is right for me, and I suspect right for many of the people planning about getting a 40kWh.

I apologize in advance for the long post, even though I promise I did edit out some of trail of thoughts.

For purposes of this discussion, I am ignoring the options as that is a value proposition unique to each individual. I will however state, that a bigger battery ended up being more important to me than some of the options.

It took me a long time to really wrap my head around the battery and what it really meant. At the very beginning, I started by using simply equations to compare the batteries before I found out about things like battery reserve to prevent bricking, range vs. standard charge, HVAC usage, etc. After figuring out many (I'm sure not close to all) of the nuances of the MS, I feel the following are very important to maintaining the quality and useful life of a battery: (1) you should not charge in range mode on an everyday basis - and really not more than a couple of times a month (2) you should also not the let battery consistently fall to below 20% before charging.

Even if you agree with the 2 above points, that info is meaningless unless you know (1) how far you drive, on average, and (2) WHAT RANGE YOU CAN EXPECT FROM EACH STANDARD CHARGE. I yelled (2) because it is by far the most difficult to predict and yet the most important factor.

If we take the 208 EPA rating for the 60kWh and assume 145 for the 40kWh (might actually be 150 when all is said and done), we can start to get some real world numbers. Because I plan to charge in standard mode almost all of the time, 208/145 are not realistic. Assuming standard is 90% of EPA, the new numbers are 187 and 130. Another way to look at this is that due to the approximately 5% reserve to prevent briking and the 10% below max standard charge, you really have 51kWh battery (not 60) and a 34kWh battery (not 40). These numbers now begin to tell us something about what the car expects your Wh/mile to be in order to reach these rated ranges. For the 60kWh battery on a standard charge, you really have 51kWh to use to go the estimated 187 miles, which implies a Wh/mile of 272 (51,000 / 187). For the 40kWh battery on a standard charge, you really have 34kWh to use to go the estimated 130 miles, which implies a Wh/mile of 261 (34,000 / 130). The question is how tough are these numbers going to be to achieve. Now I will admit that on average, I would expect the 85kWh to use a higher Wh/mile than the 60kWh or the 40kWh, but not a significant amount if you are looking at real world driving. Tesla has now said that the 40 weighs less than the 60kWh which weighs less than the 85, so this could also help to lower Wh/mile. I have reached out to some 60kWh drivers and they are saying that can get below 300 without a having to be crazy conservative as long as they don't gun it off the line every time. This initially looks pretty encouraging to be pretty close to the EPA numbers.

However, these EPA numbers and those provided by people in forums (mostly CA residents) almost totally exclude 1 extremely important factors that will affect range - use of HVAC. To give my example below some context, the EPA rating for the 85kWh assumes a Wh/mile of 305. However, most everyday drives in the 85kWh appear to be no less than that and for those using HVAC (specifically heat) those numbers are often 350Wh/mile or even 400Wh/mile. Because I live in Missouri, extreme heat and cold are common throughout the year, so the effect of HVAC is a real concern. If we assume you use an extra 50Wh/mile, the standard charge range for the 40kWh battery goes from 130 miles to 109 miles. For the 60kWh battery it goes from 187 miles to 158 miles. Using an extra 100 Wh/mile drops the 40kWh battery to 94 miles and the 60kWh battery to 137 miles.

So, realistically in the winter time and using a standard charge, we are talking about a range of 94-109 miles for the 40kWh and 137-158 miles for the 60kWh. This however, is the range the day you bring the car home. In 8-10 years, I think 15% degradation is a realistic number to use. Therefore, the winter range with a standard charge for the 40kWh drops to 80-93 miles and the 60kWh drops to 116-134 miles.

After looking at these numbers and remembering I don't want to consistently bring the pack below 20% charged, I just don't think the 40kWh option is practical for anyone wanting to drive more than about 70 miles between charges. Although that is still a lot relative to other EVs, it is significantly less than what the 40kWh looks like on the surface. This analysis holds significantly less weight if you live in a more temperate climate, but it could be even worse for those living in Chicago, Detroit, Canada, etc.

Again, I am sorry for the long post, but I wanted to pass along my thoughts and hope to either have enough holes poked in my numbers to make me rethink the 40kWh or help others get the battery they will be happy with now and in another 5-10 years.

David Dennis | 24/01/2013

Intuitively, I always felt the only viable model is the 85kw, but that certainly deprives a lot of people of the opportunity of Tesla ownership :(.

Northern California, where the car was founded, seems like about the most friendly environment possible for range, since HVAC requirements are always modest.

I wonder how things would go in here South Florida. Right now, the need for HVAC is minimal, during our rigourous, tough winters. (This is a joke. Temperatures average right around 75degF now). But during summer, temperatures climb to 95degF or so and at that level, air conditioning is essential.

Here's an interesting thought, though. I would expect most people to take road trips more frequently during times when the weather is good. So if you need 150 mile range for those road trips, you might still be able to use the Tesla since HVAC loads would be minimal and occasionally you could run the battery below 20% without harming it permanently.

D

myhanhhoskin | 24/01/2013

I have the 85 kwh performance. I charge it on Standard. Every morning the battery says I have 239 to 241 miles range. I drive it pretty fast 90 miles round trip on mostly freeway. 75-85 mph. What I've noticed is that I need 130% distance available to cover my trip. In other words when I get back home it says I have 124 mile range even though I started off with 241 range and went 90 miles. Be sure to take that into consideration when you're calculating your commute, especially if you don't plan to drive 55 mph. Hopefully you have somewhere to plug into at work. I don't so I had to get the biggest battery.

dsetia | 24/01/2013

I am in Northern California (Bay Area) and my daily commute is about 80 miles roundtrip. On most days the traffic is heavy so I don't expect to do more than 55 mph on average. However on weekends, it is possible to go at 70-75 mph. Can I get by on 40kWh battery? Or is it cutting it too close?

schoendp | 24/01/2013

Dsetia - you should be able to do it but I would not want to have that commute everyday with a 40kWH battery. The amount of miles you drive would also be helped by the better warranty on the 60kWh.

With colder weather and a traffic jam you could definitely run out of range on your way home. Plus if you wanted to do anything at lunch or after work, you would really be pushing it if you weren't really conservative with your driving.

schoendp | 24/01/2013

David - I agree that most trips are when weather is generally nicer but that is just one more restriction a smaller battery places on the owner. For some that are willing to drive more conservatively, my analysis might be less of an issue, but if you want to drive like you do now in an ICE and/or have fun in the MS, I think people may overestimate the range fairly often.

sayidreddy | 24/01/2013

@Dsetia, I agree with schoendp that the 60 kWH would provide the added assurance if you had to do anything beyond your daily commute, but the 40 kWH pack should still be adequate especially considering the Bay area climate. If you could find a way to charge your car at work or at a nearby location, the 40 kWH pack would certainly get you by, even at 70-75 mph highway speeds.

jat | 24/01/2013

The AC doesn't really seem to use much battery life (admittedly I only ran it a few days when it got into the upper 70s, and not anywhere like a hot July day), but the heater draws quite a bit more. Even then, it isn't that bad unless you are making lots of short trips where the heater runs wide open most of the time you are in the car (I was averaging over 600 Wh/mi when doing that for several days).

Also, if I were going to base my decision on whether the car met my daily needs on it, I would use 30% for battery degradation at 8 years rather than 15% -- you would really hate to find out that you have to trade cars because the degradation is higher than you planned, even if that is considered unlikely given the experience of Roadster owners.

nickjhowe | 24/01/2013

@dsetia - it depends. :-) 85 kWh has 300 idea miles in range mode, and 240 projected miles in normal mode. Real life usage seems to be c. 10% worse than projected miles (varies massively based on driving habits, elevation speed and weather)

The fact that the 40kWh has an ideal range of 160 miles suggests that it is lighter than the 85 kWh variant (rolling resistance is proportional to weight). If we assume the same delta between ideal/range and projected/normal we get to c. 128 miles. Allowing 10% worse gives us c. 110-115 miles.

My 2c? At 55 you'll definitely be OK. At 70 you should be OK with room to spare.

If you do 75, and it is cold (less battery capacity and you have heated seats), and then you get stuck in traffic, you might be OK, but it wouldn't hurt to have a list of locations along your route where you can plug into an outlet for a while.

If you could plug in at work you have nothing to worry about.

dsetia | 24/01/2013

@nickjhowe, @sayidreddy, @schoendp, and @jat - thanks for your comments. I think I should be fine 80% of time (80 miles roundtrip compared to 110-115 available miles, mild bay area climate, average weekday speed around 55-60 mph, some outlets at work). For planned events (lunch meetings, doctor visits) I can take regular car to work. It is the 20% that bothers me. 60kWh would be ideal if budget wasn't a concern :-)

syddent | 25/01/2013

@dsetia - one last vote for the 40kWh. In the bay area, IF you need an extra charge, there are a HUGE selection of charging stations. I plan on bookmarking http://www.recargo.com/ on the browser and I am hoping recargo will release a Tesla friendly app for finding nearby charging stations.

For me, I think 95% of my trips in the bay area (work or fun) won't need an extra charge, but the option exists if I do

RedShift | 25/01/2013

For me, the decision to go with 60kwh came down to the ability to supercharge.

Brian10 | 25/01/2013

My 1999 Lexus has 78,000 miles. I don't have a long commute so the decision to go with the 40kWh was easy for me. On our longer family trips, we'll just take the minivan. And when Tesla announces that they are making a minivan with supercharging, I'll be P1 on that list.

Brian H | 25/01/2013

Brian10;
What does the Model X lack that the minivan has?

Brian10 | 25/01/2013

Love the Model X but can't get it in my garage (freakishly low ceilings) and can't put my surfboard (9'10") on the roof. And before you ask, I'm not putting roof racks on my beautiful S. That just wouldn't be right.

cmeyers | 26/01/2013

The worst case mileage that I computed for the 40KW battery, the fact that the battery only normally charge to 90% and watts per mile figures from forum members and extrapolating the power curve vs. speed up to 75 mph is also about 95 miles.

This number is worst case covering all conditions, except for extreme cold since I live in the Bay Area.

Now one thing to consider is that when the car is plugged in you can cool or heat the cabin. Also the battery should maintain it's operational temperature while plugged in. So when you leave your house in the morning you will save energy there if the car is programmed to be ready but heating and cooling don't consume much relatively speaking.

As for battery aging, these cells should be able to retain a large percentage of their capacity of at least 2000 cycles. That is 5.5 years of every day driving with nearly 100% discharge cycles.

If you aren't in the extreme discharge range driving every single day or extreme weather the 40KW battery seems to be viable for a lot of people.

schoendp | 26/01/2013

Cmeyers - very well stated if 2000 is accurate. I think that is where a lot of questions come in. If I new degradation would be 10%-20% then a 40kWh would easily be doable. I guess I am very worried about degradation - maybe too much.

Amped | 26/01/2013

@Schoendp, I arrived at the same conclusions here in Denver for the 60
& 15 within 8-10 is fair, degradation is expected to be around 1.7% a yr

ChasF | 27/01/2013

Is that degradation estimate based on current Roadster numbers? Don't forget, the Model S batteries are babied a lot more (liquid cooled = more stable environment) so degradation should actually be less.

Even assuming he worst case ( 95 mile standard range after 8 years), the 40kWh still works for me in Central Florida. No extreme winters. Daily commute is 30 miles round trip. Weekend driving averages 70 miles/day. 100 miles of driving maybe two times a year. On those rare occasions, I plan to do a range charge to cover the longer trips. In 10 years I'll upgrade to a bigger battery (aftermarket probably) or buy a new car.

dave5609 | 27/01/2013

Definitely planning on the 40kwh battery and have been since my reservation back in 3/2009. it's been a hard long wait made tougher by seeing Model S on the road for the last few months, but really happy for all the people that have already taken delivery!! I think the 40 is a great option for people that don't need the longer range. This battery should be fine for 99% of my driving though it is close..I have a 85 mile round trip commute to work..though in pretty ideal EV conditions: (LA climate, flat, frequent traffic). I'm figuring that even with all the knock downs (over 55mph driving, standard charging, degradation over time, etc.) I should still be fine for the next 8-10 years. Work is also considering putting in 14-50 outlets for EV charging which would help with the range anxiety on days I need to do a few extra side trips.

I'm excited to see the EPA data in the next month or so and hoping the 40 continues the trend of the 60 and shows the best (lowest) kWh/mile ratio of the bunch. Would help make up for the lower acceleration, reduced warranty, and no supercharger access that became realities for the 40 as Tesla released more info over the past few years.

It's been a long road to get to this point and this forum has been an incredible place to get information and keep the excitement level high reading about other people experiences. I couldn't be more excited to take delivery in the next couple months and hoping with a low reservation number (R75) to be one of the 1st off the line. I'll definitely post timing of delivery button activation and expected delivery window as these become available.

Congrats again to all the 85 and 60 owners that have taken delivery and posted their experiences. We're are excited to join the party!!!

Brian H | 27/01/2013

Dave;
Thx. You appear to be an ideal representative of the "majority" of drivers that the statisticians keep telling us about, who drive 30 mi/day and less, and do road trips rarely. Could be that as experience of others like you accumulates and becomes better known, that more will feel comfortable with a choice like yours.

It is worth remembering, of course, that Tesla's lowest capacity battery is the match of the highest that anyone else offers!

D Vo | 27/01/2013

I also have an excellent 40kWh situation:

1. Two early teenagers.
2. Three car family.
3. 24 mile round trip commute.
4. Drives > 100 miles = 6/year.
5. Drives > 1000 miles = 2/year
5. Hybrids handle the longer drives.

Some think 40kWhs are the minority. I'm still looking for the facts/numbers to support that. Until then, I frugally think 40kWh is the majority. Hoping for delivery on my 2-year anniversary - P3565. Can't wait to hear from you on your 4-year anniversary, DaveR75!

PS - I do look forward to the technology and infrastructure to replace the hybrids. I know there are plans, but Texas to Michigan, Georgia, or Florida isn't EV convenient yet.

cmeyers | 27/01/2013

I think that the degradation should be very minimal but we can use a Panasonic datasheet as a reference. In the car the charge is rarely ever allowed to come close to the upper or lower limits which makes a LiIon battery last much longer. As ChasF commented the batteries are temperature controlled which also helps.

No one would probably say that a NiMh battery is a good choice today but I work for someone that drives 10+ year old (it might be a 1999) electric Rav4 with the original NiMh batteries and it is still running. Granted it is starting to loose some significant range but it only had a 27kWhr battery to start with.

I wouldn't worry about the batteries in a Model S unless their battery management system does something to shorten their life. They probably worked those bugs out on the Roadster where they did initially have battery software problems.

dmunjal | 28/01/2013

I agree with you. My 2004 Prius has over 100k miles with Ni-MH batteries and still gets 45mpg as it did when I first got it. I'm sure climate has a lot to do with it but NorCal is pretty mild generally. My two-year old Nissan Leaf still hasn't lost any battery capacity either.

I'm hoping my 60kWh meets my needs for the next ten years.

cmeyers | 28/01/2013

It appears that Panasonic doesn't publish cycle life data good enough to predict the life of an individual cell. Also so much of the cell's life is determined by the actual charge and discharge profiles and temperature of the cell that is is nearly impossible to predict without Tesla actually sharing their knowledge.

I went to a presentation at Stanford that was about the Tesla battery system and I know they have done a lot of individual cell testing over the years. I wish I had a copy of the presentation.

The destructive tests where impressive.

schoendp | 28/01/2013

What a difference a short time can make. It looks as though i will have some charging options during the day so a 40kWh battery is once again viable, even after potential degradation. Now it's a cost benefit analysis of the 60 vs. the 40 with regards to warranty, performance, cost, etc.

GLO | 28/01/2013

Get the 60kw with Supercharger access. LOVE it!