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What factors to consider when choosing a battery pack?


I've just received my invitation to build my Model S but I'm having trouble deciding on which battery pack to get. I originally thought 230 miles was enough but seeing how $10k in savings will be realized I chose the 160 mile one, I'm having second thoughts. Below are some facts:

- Live in downtown Washington DC
- Use the car about 3 days/week at approximately 20 miles roundtrip.
- About 5 times a month, I'll go visit family in the suburbs or have business there. On average, the DC suburbs are 10-20 miles away so assume 50 miles roundtrip.
- I would also like the ability to occasionally make a road to Baltimore, Philadelphia, Richmond, etc. all of which are within 200 miles. Obviously the 160 won't cover NYC but I haven't heard much of charging stations on the East Coast.

Any thoughts/ideas on this from you guys would be much appreciated. Hope you're all enjoying your Model S's as I'm quite excited to get mine soon.



schoendp +1

Real World Mileage Range - Current and 8-10 years

Use standard charge, which is 89% of EPA rated mileage
80% of standard charge is real world driving experience
20% battery degradation over 8-10 years

Standard Range (Current) = 105 miles
Standard Range (8-10 years) = 84 miles

Standard Range (Current) = 148 miles
Standard Range (8-10 years) = 118 miles

Standard Range (Current) = 189 miles
Standard Range (8-10 years) = 151 miles

I know we have all read stories of people getting 230+ miles on the 85kWh, but those are typically range/max charges. If you think this should be its own thread, let me know and I will start a new one.

Personally, I think you can get much closer to rated range than that if you want to - I get 80% or so without driving conservatively in the slightest, but maybe if you live in a very cold climate that might be reasonable. Also, I think 70% after 8-10 years is a more realistic worst-case capacity loss to plan for.

Other than that, I agree with your point, and that you shouldn't buy one that you will have to do max-range charges for your daily usage.

jat - you're probably right that 80% after degradation is not conservative enough. The good thing is that if you assume 85% of standard charge is normal driving and that 75% is a realistic degradation number, then the 8-10 year numbers come out the same.

If you assume 85% of standard charge is normal driving and 70% after degradation, the 8-10 year numbers only fall by 6 miles for the 40kWh and 11 miles for the 85kWh.

I also think resale is going to be a factor when considering battery size. I would expect the P85's to be more sought after when used.

You probably shouldn't assume it, but Roadster owners are seeing lower rates of degradation, and the MS battery is improved. The rule of thumb is that the engineers actually expect half the losses "advertised", but add a safety margin. Check back in 7-10 yrs, and I bet the actual losses will be 10-15%.


As a program manager, if you tell me we need a safety margin of 50% I'm assuming there is a LOT of risk with the numbers being identified (new configuration, lack of data in all intended environments, etc). Thus I would be prepared for worse numbers - not better.

tsx - I like the cynicism, I just hope your wrong.

No, they expect X degradation, but tell customers to expect 2X. It's the engineer's version of "under-promise, over-deliver".

Look at it this way: the 40 'will do for now', but the 60 with SC there is no caveat: either it has the range itself, or the supercharger network will give it the range you need, whatever that range might be.

I look at it as future-proofing. Whatever life throws at you, travel wise, you should be good to go.

the 60 WITH the SC option. $2K up front. And 60kWh cars have to pretty much stuff themselves on range mode, which takes much longer, to make it to the next station. Not ideal.

You'll still get there B. that's the point.

At the expense (time) of 85s waiting for much faster 150-mile fillups? (Speed of charging is a function of battery size and SoC.)

YUP, that's how it works Brian. The first person to the charger goes first. The 85kWh driver that pulls up just as another 85kWh person is plugging in still has to wait. That is life. Hopefully Tesla will keep adding chargers as needed. So far I haven't heard to many complaints about people waiting in line. I am certainly not going to spend an extra 10K so someone else has to wait less. They can give me 10K to speed up their wait time if it is that important to them.

Today’s news battery packs are not upgradable:

I finalized months ago, but of course we just find out today that upgrades to the battery pack are not possible, kind a sucks, and would have been nice to know this in advance. If I extend the basic warranty, both the battery and car will be out of warranty at the same time, which I might consider an after market solution.

I always assumed a battery upgrade at the 10 year mark would be an aftermarket solution. I'm sorta counting on it.

I am more concerned about resale value in 10 years when Tesla has the 120 & 160 kWh packs. Who's going to want a car with a new 85kWh pack much less a 40 or 60 then?

I agree with ChasF that there will probably (hopefully) be an aftermarket third party manufacture of battery packs by then.

On Tesla's facts section of their website it says "while technically possible to upgrade to a larger battery, we recommend configuring your Model S with the battery that meets both your present and future needs".

This seems to contradict what was posted by Tesla on the bulletin board today. There's a big difference between saying they recommend something and saying that a change CAN'T be done.

@DFibRL8R +1

I predict at least 10 policy changes on this in the next 10 years.

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