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Battery Life Driver - Age or Mileage (Charges)

What do people think will be the driver for the life of the battery, its age or its mileage (or rather number of charges)?

I put pretty low miles on my car, about 6,000/yr., so that the battery lasts a long time is more important to me than that it lasts long miles/charges.

Put another way, 10 years into my 5-series BMW, I have driven 60,000 miles. In combined driving it averages a bit above 20 mpg, but lets use 20 and estimate I've bought 3,000 gallons of gas in that time. Let's say the average price over that time is $3/gallon (I think it's actually a bit less). So I've spent $9,000 on fuel. I haven't, nor do I expect to, replace the gas tank (storage compartment for energy) or engine (user of energy) anytime soon.

Now lets say I'll pay 10 cents per KWhr over the next 10 years, so it will cost me about $7/to fill up the 85 KW battery pack (the useable middle 80% of charge) and lets say I get 250 miles out of that. So electricity will cost $1,680 to drive 60,000 miles the next 10 years.

That looks like I'll save over $7,000 on fuel costs, and perhaps it will be more if gas keeps going up, but this is where my question comes in. Will my 85 KW battery back be at about half its expected lifetime (60,000 miles of 125,000+ estimated) or will it be over the hill (10 years old vs. 8 years promised)? Because even if replacement 300 mile packs are only $10,000, 10 years from now, the cost of fuel (electricity) + storage device (battery) will likely do no better than wash the ICE cost of fuel (gas) and storage device (gas tank). In this exercise, I have chosen to ignore the cost of other repair issues in the costs of ICE vs. EV.

So where this is leaving me is: Since I don't drive a lot of miles, I can't buy this car because it will save me money - I have to buy it because I love it.

Well, I'm OK with that.

Cattledog

Larger battery pack will have less discharge/recharge cycles for the same number of miles.

With the relatively low # of miles you are estimating, I'd say you'll have 70-80% of the original capacity left after 10 years.

My expectation is I'll be able to replace the 85Kwh battery pack with a 250Kwh mile pack in 10-15 years. So I'm looking at the car as lasting 20 years, maybe more.

I'm also looking at getting a GenIII in 4-5 years, so I'm hoping that Tesla does very well with the Model S.

Cattledog raises a good question regarding what are the most important factors in decreasing battery life: age, total mileage, number or charges, number of charges to near full, number of drives to near empty. I would appreciate anyone commenting on this who drives a Roadster or those who have heard directly from a Tesla rep.

I put on about 9,000 miles a year, most of it in trips under 20 miles between recharges but once or twice a week do trips of 90 to 130 miles. If the battery would hold its max charge reasonably well for 10 years I'd be fine with a 40 kwh pack. If not, I need to spring for the 60 kwh pack to account for degradation. The weighting of the variables accounting for battery degradation are currently a big unknown and $10,000 is not an insignificant sum for me to part with.

I expect others may also be unsure of battery pack size because of unknown battery degradation over time. Many buyers too, who don't follow these forums or who aren't savvy about battery degradation, may be dismayed after a few years should their battery no longer get them as far as they anticipated upon purchase. This is an excellent topic.

I also wonder if charging method (high speed v low speed) and temperature will impact the battery pack. Great topic!

If you consider the battery warrantee is 8 years and unlimited miles that as long as you use the car that the age of the battery pack will be the factor above all else?

Heh; which guarantee would you rather have:
1) 8 yrs unlimited miles, or
2) Unlimited years, 100,000 miles?

Ideal?
"8 yrs or 100,000 miles, whichever comes last."
or, stated otherwise,
"Both 8 yrs and 100,000 miles."

;-)

Ideal?
"8 yrs or 100,000 miles, whichever comes last."
or, stated otherwise,
"Both 8 yrs and 100,000 miles."

;-)

I'll take 100k miles because it may take me 10-15 years to hit that, my 2003 Accord had only 55k miles on it when I sold it recently.

Ideal?
"8 yrs or 100,000 miles, whichever comes last."
or, stated otherwise,
"Both 8 yrs and 100,000 miles."

;-)

Oops, that fake ERROR page plus "Under Maintenance" got me this time. The servers are limping -- again.

@Brian H, it is because the gerbils went on strike again. Seriously Tesla, you need to update this forum!

I find myself posting more lately on the TMClub site but still post here on subjects that I would want TM to know of. Heck, who am I fooling, I'm probably over active on both of them! ;)

Well, let me try to challenge your approach.

First of all, an old battery does not render your car unusable. After 10 years, you are probably left with around 70% of the original capacity. This assumption is based on Tesla offering an 8-year warranty for "near-new" performance of the battery, which can be assumed to translate to something like "better than 70% of specification" in fine print. They should have calculated with some margin to avoid a flood of warranty claims. If you don't need more than 70% of the original range, you are still covered for some time after that.

Secondly, you could argue that when you buy a new battery pack after 10 years, this cost should not be contributed to the cost of using the cars for its first 10 years, but for its second 10 years. Assuming that you buy the car in 2012 and replace the battery in 2022, then you have a brand new battery that will have you covered at least until 2032! To compare the battery cost to savings in gas prices, you must take the average gas price from 2022 to 2032. That's all guesswork of course, but I think it's pretty safe to assume that you'll be better of with an EV than with an ICE. (Every calculation can be tweaked to yield the desired result ;-)

Thanks for the post VB, so based on what you posted: How would you calculate which battery pack to purchase?

For me, as one example, I can get by with 160-mile range battery for the day-to-day driving. Weekends, the 160 probably would be too limiting, but I had planned to purchase the 300 mile battery because I never drive 55 MPH anyway(my average is about 70-85 MPH actually or more if I had a wanted to be really honest) and I would like the original battery to last as long as possible. This means I don't mind if the battery drops down to under a 50% charge, I would still drive on it until it was no better then say the Leaf with an 80 to 100 mile range.

How would you(using fair and real world scenarios) calculate how long the battery would last and assuming the battery technology doesn't change a whole lot. Battery technology is the slowest to evolve at this point in history over everything else?

I also don't want an excuse not to drive my Model S because I plan on loading it up with all the creature comforts such as the Technology, Audio, and Pano packages.

Another reason for the 300-mile pack is access to the SuperCharger network for when I'm ready to take my car on that road trip. I am trying to justify the extra $10K or $20k based on the fact that I will drive my Model S for at least 10 to 12 years and find in my mind too many factors to accurately guess a best and worst case scenario.

Teslarumors depreciates the car at about standard industry rates to take account of battery replacement, and TOC still comes out far ahead, making even conservative assumptions about future fuel costs. The difference is that the battery replacement comes in a "lump" at some point.

Considering inflation (discount rate) and probable improvement in costs per kwh storage over 8 yrs., however, some mild version of Moore's Law is likely to apply, and a straight-up replacement is likely (IMO) to cost about ¼ of current pricing.

The question of residual value of the battery is interesting; it can both be recycled and "demoted" to some other less demanding use, such as home back-up/buffer/UPS. 20-50kwh is still a pretty substantial capacity!

I see I didn't even come close to answering the original question.

"Will my 85 KW battery back be at about half its expected lifetime (60,000 miles of 125,000+ estimated) or will it be over the hill (10 years old vs. 8 years promised)?"

The warranty is not the total lifetime of the battery pack, as Volker pointed out too. Using the batteries less (60k total miles vs 120+k) will cycle the batteries less and they'll have a larger capacity after that amount of time. From what I understand the number of charge/discharge cycles the batteries go through is most of the degradation, so less miles = more capacity after a set period of time. So for your example, assuming the 8 years/125K miles is 70% capacity minimum, you might be looking at 80-85% at 10 years/60K miles.

Regarding the calculations used versus the fuel cost of an ICE, I think VB covered that. E.G. look at the battery cost as a pre-payment of fuel costs, in the same way solar cells are a pre-payment of electricity costs. You are locking in a fixed price for X years where X is the point that it becomes advantagous to pre-pay again because of capacity loss or low price points.

Best case scenario as I have it calculated. 10 years at about 10k miles per year and gas prices no less then $4.00 a gallon (usually more where I live) and best case scenario is 25 MPG on my car on average but probably worse. That all works out to 100000/25*4 which equals about $16,000 just in gasoline over 10 years. I estimate that gas would go up and that makes up the difference in what I would pay to fill my Model S up in electricity. The last few months I have pair between $4.25 and $4.55 per gallon of gas and it hasn't been near $4 for a while. Based on that, and the fact that I want a higher performance from my car, have a lead foot and want to hold off replacing the battery as long as possible, hopefully 12.5 years, then that would be a break even gas vs. electricity if the price of gas stayed at today's rate.

Do you see any fault in those calculations or my logic? In reality my car probably get's about 19 miles per gallon as well, but this again is worse case scenarios.

Variables:
- price of gas per mile (PGM)
- miles per year (miles)
- price of electricity per mile (PEM)
- price of batteries (Battery$)
- number of years before battery replacement (time)

So figure (PGM - PEM * miles) * time is the Gas price differential (GPD). And then compare that versus Battery$. If Battery$ > GPD then you aren't ahead on just that part of the TCO.

@BYT - Do you see any fault in those calculations or my logic?

I think I do. When you buy your car, you are also buying the battery capacity for the first 100K+ miles. When you replace that battery pack, you're buying for the next 100K+ miles. So, in 12-13 years, you're basically prepaying a portion of your "fuel" costs for the second 100K+ miles. So you'll be buying a battery, presumably at a significantly lower cost than today's price against gasoline costs 12-25 years out. I have a hunch that gasoline prices in 25 years will be higher, but I'm not an economist, nor do I play one on TV. ;)

Assuming I keep the car beyond the 12 years, my guess is I will want to upgrade by then for a change of scenery, you know, get a 2026 Apocoliptic Green Tesla Model Q Super Sport... ;)

Assuming Tesla continues with the revision designation rather than model year, more likely an Apocalyptic Green Tesla Model Q 3.0 Super Sport, I'd expect. Probably have a 0-60 that would require a pressure suit like fighter pilots wear to keep from blacking out in sport mode.

@Teoatawki, that sounds awesome! When do the reservations on that start? ;)

BYT

Lets say after 10 years the battery delivers 70% per VB. Keep in mind TM recommends you charge to the 80% level. So………….

Miles per Charge
100% 100% -20% 70% 70% -20%
55mph 160 128 112 90
230 184 161 129
300 240 210 168
75mph 116 93 81 65
169 135 118 94
225 
 180 158 126

When I first started my Tesla journey I was convinced the 40Kwh was the right one for me. 65% of my daily trips are 50 miles or less. However, the 60 Kwh gives me that extra cushion plus the performance (0-60) is significant. Also, you never know what your future needs are going to be.

sorry about the spacing

Thanks petero, that is very helpful!

Petero, My impression was you charge to the 90% level then drive down to the 10% level vs. charging to the 80% level then driving down to 0%. Is this correct? It makes quite a difference because in the first case you always have that 10% buffer you can dip into in a jam whereas in the second you don't. This was the impression I got from a Roadster owner comment. Definitive clarification would be helpful.

Ddruz. I do not own a roadster and I do not have any first hand experience with an EV/BEV (except a test drive in a roadster). My understanding is, charge to the 80% and either top it off nightly or depending on your range and needs, every few days.

I have never heard the 90% ‘charge to ‘% only that TM recommends charg up to 80%. I doubt any roadster or “S” will drive theirs down to 0% unless it was an emergency and few will run it down to 10%. My guess, that most owners will charge to 80% and recharge either nightly or when they are at 40-50% -Just my guess. Further, I understand when the charge on an “S” gets critically low you probably go in to a “limp mode” or it will shut down to protect the batteries.

I would appreciate hearing from roadster owners about this subject or the many knowledgeable forum readers.

      Miles per Charge

       100% 100%-20% 70% 70%-20%
55mph   160    128   112   90
        230    184   161  129
        300    240   210  168
75mph   116     93    81   65
        169    135   118   94
        225 
   180   158  126

Test of 'pre' tag.

Petero;
As you see above, the <pre> tag takes your spaces and linefeeds etc. exactly as typed ("preformatted") and allows simple tables. I checked all of the "allowed" HTML, and this seems like the best way to do it.

Different perspective on ROI and ICE vs EV: http://evworld.com/currents.cfm?jid=236


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