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Agressive driving and battery life

Ok, so think of this scenario:

Bob and John are neighbors. They get a Model S 300 mile pack, sports version, at the same time. They both drive to and from work 5 days a week, 45 weeks a year. Bob is a hothead – he loves to floor it, he drives fast often going >130 km/h. John is careful and calm, drives “eco-friendly”, rarely goes fast, uses a lot of regenerative braking.

Bob has only 25 km a day to drive, while John drives 50 km per day. However, due to their different driving styles John manages to do this using the same amount of energy as Bob (i.e. twice as efficiently) so that when they come home every day both cars show 50% capacity. They both charge over-night and both start each day with a standard charge of some 80%.

Now, after say 3 years, who’s battery will be in the best condition (that is the least “worn”, best residual capacity, best performance)?

Mind you both neighbors have had the exact same total energy consumption during these 3 years – the exact same amount of charge-discharge to the battery.

Is it:

A) John’s – because he has driven more carefully and put less “strain” on the battery.

B) Both packs will have the exact same condition because they have had the exact same amount of charge-discharge.

C) Bob’s – because he has driven half the amount of km and done this in probably less than half of the time.

This is not a trick question by the way, and I don’t know the answer (that’s why I’m asking).

I'm also wondering about how my commute, that is mostly not flat and has at least one large hill, affects the range.

Going uphill will obviously consume more energy than going flat. This is noe different from a car that runs on gasoline and all in line with the laws of physics. Not sure I understand your question really?

If you go uphil on your way to work, you are probably going downhill when you're going home, right? If it's steep you might be able to recover some range there with battery regeneration though.

...good question. I've heard about a Roadsterguy who stated, the capacity of his accupack is stronger than it was, when he bought it. So, maybe there will be some surprises. Sometimes I'm the hot head and sometimes a smooth driver, maybe You'll get the best out of it with different driving styles- maybe not. What would be the analogy f.e. in comsumerelectronics?

I think, always keep the car as full as possible and put the pedal to the metal!

Hill is actually nearly exactly zero gain / lose situation until you need to engage regen braking. Without that it has pretty much same losses as flat + climbing uphill - going downhill. You get that whole potential energy back going downhill that you lost going up. With regen engaged however that means that you need to artificially slow down and that is always losing game. (though with regen you get something back, with ICE you would just plain lose that energy).

OP post, I would say that A) is right answer. Less strain in battery wears it down slower. Difference isn't large if both use same battery settings though (no performance mode for Bob). Bob would destroy his brakes and tires though, while John brakes and tires would be in much better condition.

2nd law of thermodynamics, no free lunches...

A.

Driving it harder requires both a higher C rate (discharge rate) from the battery, and also heats the battery up more due to internal battery resistance. Both of these effects are harmful to battery health.

However, the DEGREE to which it is harmful is an open question. it may not make a huge difference...or it might.

Depends on a lot of factors, including Tesla's battery cooling capability, and the C rate when flooring it compared to the max C rate capable by the battery.

Either way, A is the right answer.

@Peak Oil bruin, no free lunches, but for potential energy case is about as close to it as it can get. Car gains potential energy going up, so energy transfers from battery to car position. It loses it going down using same path but opposite direction. Energy cannot be destroyed, so it has to go somewhere.

If you look at it from force-POV you apply extra force going uphill and equal amount less force going downhill because for both situations gravity is the force you are dealing with and it doesn't change between two cases and force-vector stays same.

All that does matter is does that force exceed losses going downhill, in which case you need to start accelerating uphill in order to prevent car from accelerating. If this is the case you have to apply extra force in same direction going up and going down, and that is a losing game because regen is not as efficient as accelerating (otherwise it wouldn't matter).

Also there is factor of engine efficiency and power applied. In this case you could actually "gain more" going down than going up. Never more than losses (2nd law of thermodynamics), but saying it other way you might "lose less" going down relative to going up. That could give you actually better result from hills than from flat, but that is not very likely in reality.

@Timo: on that note, do you have any clue as to what the actual efficiency of the regenerative braking is? By this I mean that if we hypthetically think of the car moving i vaccum, with noe friction between the road and tires, and you say accelarate the car to 100 km/h and thus you have a certain ammount of kinetic energy "loaded" in the car and then you let go of the "gas" pedal and just slowly let the car brake regeneratively to a full stop how many percent of the kinetic energy that was present has now been converted back to electric charge in the battery?

@Timo, yes, if you look at it from force-POV (closed system) "nearly exactly zero gain/lose" statement is innocuous.

Yes, obviously I understand that climbing hills requires more energy than driving on a flat road at the same speed.

Sorry I know it's vague since I can't tell you the exact detail of every road I drive, but I guess I am just wondering how significant the impact is. As you mentioned, driving downhill could provide regen energy, and the electric motor in the Tesla is way, way more efficient than any ICE in other cars.

I'm not even expecting that someone know the answer I'm just wondering aloud. Putting it another way, say that in Nebraska, a someone gets 300 miles with his Model S. What can I expect to get? 280? 200? 100? Obviously it depends on many factors. Like I said, I guess I'm just wondering aloud.

I don't know the actual exact numbers, but you can calculate the extra energy you need to climb a hill just by calculating the potential energy you gain and that is surprisingly low number even for Model S. Real life experience with Roadster tells that you get very close to that theoretical number with it.

To make that even better is that you gain some of it back going back down.

In fact if climbing hill requires less speed (by speed limit), lets say 55mph instead of 75mph flat you might actually benefit from it because then in flat you would have bigger losses.

Formula to calculate potential energy is U = mgh which gives you energy in joules. One kWh is 3600000 joules.

m = mass of object in kilograms
g = acceleration of gravity in m/s^2
h = height difference of the climb in meters

Lets say that Model S weights that assumed 4000 lbs and you climb one kilometer high hill.

That's then 1814kg*9.81m/s^2*1000m = 17795340 kgm^2/s^2 which is joules. 17795340 / 3600000 = ~4.9kWh.

If we assume you get 60% back going downhill that's 1.9 kWh extra lost caused by that hill during your round trip to that hill.

If then we assume again that hill was 10 degree angle average you travel about 5.8 km for each climb, so twice that is 11.6 km.

...this starts to sound worse than I expected. 300Wh/ mile in flat = 480Wh/km + 2000Wh/11.6km = 407 Wh/mile. If 300Wh/mile gives you 300 mile range that's 90kWh. 90kWh/0.407kWh/mile = 221 mile range.

That should not be too far from reality.

If you didn't notice this previous calculation actually gives you rough estimate on how much it affects your driving independent of hill height, as long as average angle of climb is 10 degree and you end up back where you started. Isn't math fun :-)

Nice calculation and probably roughly correct in the real world. You stated "If we assume you get 60% back going downhill..." Where did you come up with 60%?

I seem to recall reading that the regen was about 65% efficient, though this is just hearsay.

Friends with a Prius tell me they get the best mileage using an accelerate/coast/accelerate/coast cycle. If that's true, then one may get better range in gentle hills. (This doesn't accord with my intuition, but ....)

I saw it somewhere. Don't remember anymore.

90% engine + 95% PEM + 95% battery would indicate higher result though. Around 80%.

Does anybody actually know the real regen efficiency? I just have vague memory that it isn't as efficient as normal acceleration.

On real mountain roads things are complicated and particularly - your driving habits matter a lot (evidently). Maximum acceleration after each hairpin e.g. would consume a lot of additional Energy.

When you have to drive relatively slowly on winding and narrow mountain roads, consumption is lowered by reduced drag.

Driving prudently in range mode I ended up in one longer drive across three passes with about 160 Wh/km (260 Wh/m). On another shorter run in standard mode I logged about 180 Wh/km (290 Wh/m). Of course always measured once down in the valley again.

The effect resembles a bit the "ice and snow effect". On icy roads you have to slow down and that tends to offset some or even all of what you might spend on heating and defrosting.
- Alfred
http://web.me.com/alfredar/Alfreds_Pages/Blog/Entries/2010/7/4_Tesla_Roa...

To Johan on battery life (OP): It may depend very much on what that aggressive driving precisely is. Sporadic bursts of discharge (and charge) are well tolerated by some current Co Li-Ion cells. As the details of the cells planned to be used are not known, it is not possible to answer your question as yet with any certitude.

If I assume that Tesla will size the A/C such that "Standard Mode" sporty driving will not push battery temperature into an "accelerated aging" zone, I would consequently also not expect any easily noticeable effect. Battery life would remain roughly proportional to charge throughput and calendar life.

- Alfred

As I understand it, the difference between the Sport model and Regular is the ability of the car to cool the batteries faster. (Please correct me if I'm wrong - I'm a bit hazy on this..)

If that's the case, and if a driver were to drive either the Sport or Regular the same way (hah!), would the Sport's coolant capability make for a longer lasting battery? I doubt it's much of a difference, though.

Robert,

The 65% is what is recall also. But I suspect that the more gentle the regen, the more efficient it becomes. E.g., lower regen rate over a longer distance should not require as much battery cooling, etc.

I am Bob the hothead!

This brings to mind a related battery/regen issue. I live on a high hill, so pretty much anywhere I drive starts with 800 vertical feet downgrade. What happens with regen when the batteries are already "full"? Or more extreme, leaving the vacation cottage at the pass I can no longer afford ;) it's over 3000 feet downgrade.

I'm quite curious. I understand normal "full" charge isn't really max capacity, but under certain circumstances....

Teoatawki, there really is only one way to deal with this: Regen is automatically deactivated to protect your battery, and you have to apply regular brakes.

I think I read something along the lines that when you live uphill, you can configure the car to charge less such that you can fully exploit the energy that comes "for free" when driving downhill, and end up with a full battery at the bottom.

("For free" is in quotes b/c it isn't really for free, obviously. You have to use energy to get uphill in the first place. But at least you can recover some of that when going down again, and that will reduce your electricity bill.)

The regen will only be disabled if you fully charge IN RANGE MODE. If you are just charging in standard mode you'd have a few thousand feet of regeneration before the battery would start to limit regen.

Thanks Volker.Berlin and ggr. Totally makes sense. Seems like a lot of battery information is not very intuitive.


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