Long Term Battery Care and Charging Habits

[dave]

I plan to own my Model S for as long as possible. I do NOT plan on replacing the battery, and after it loses it's long distance capabilities, I plan to make it our "around town" car, and direct any battery replacement funds to the latest and greatest thing for our second vehicle. As such, I am keenly interested in understanding exactly how the Model S battery works so I can extend its usable life as long as possible.

Here is what I know for sure, from the owners manual:

• I should keep the car plugged in whenever possible
• Charging to maximum is worse for the battery than charging to 90%.

So this brings up a million questions for me:

• WHY do I need to keep the car plugged in all the time?
• Are there battery health benefits to keeping it as fully charged as possible, or is this just to prevent people from "Brodering" it and accidentally draining the battery?
• Does the battery management system behave differently if plugged in, vs. not plugged in?
• Is battery longevity affected by the state of charge while not in use? If so, do I understand that 90% is better than 100% when sitting in my garage? If 90% is better than 100%, is 80% even better than 90%? What about 5% or less, is that harmful? .
• What actually causes battery degradation? Is it the charging or the discharging? Does the SPEED of charging or discharging affect it? Is there an optimum charging amperage for battery health if the time involved was not a concern? I've heard that Superchargers are bad for battery life, but then I've also heard the opposite. Does anyone know for sure? Does the battery degrade based on time, or usage? Both?
• Does the length of a charge matter? Is it better for the battery to have one giant charge from 10% to 90%, or multiple tiny charges keeping it "topped off"?
• Does ambient temperature matter, considering that the battery has it's own internal temperature control mechanisms? Is it worse for the battery if I leave my car parked outside in the winter, or should I bring it in the garage?

Much of my driving is either long trips or very small around town trips, so I have the ability to leave the car unplugged for up to a week at a time, and would consider making small changes to my charging habits if it has the possibility to affect long term battery health.

Sorry for all of the questions... I have heard various conflicting reports while surfing around these forums and elsewhere, and I'm curious if anyone knows for sure the answers to some of these.

Thanks for your help!!

 

[Kevin Sharpe]

WHY do I need to keep the car plugged in all the time?

search on this site for the word "parasitic"... here's a start for you;

Large Drop in Charge When Parked in the Cold

Do you know that you must keep your battery charged?

 

[dave]

Thanks Kevin! Perhaps I should re-word or remove the first question. Obviously the reason to keep it plugged in is to keep it charged! To replenish loss either from vampire loss, or actual driving. I was more interested in long term health of the battery and what's best. If I don't care about the vampire drain and have the ability to NOT charge for a week or so at a time, is there any benefit?

 

[GeekGirls]

WHY do I need to keep the car plugged in all the time?

I don't have definitive answers to any of your questions, but I can at least speculate here. When the car is plugged in you aren't necessarily charging it - what you're doing is giving it the option to charge. At that point the battery management software decides what is best for the health of the battery. It's pretty evident that it doesn't just continually top off the battery because your range will cycle back and forth from 242 to 232 miles of rated range in an 85kWh model while plugged in. What's actually happening here? Are all cells being discharged 3% and recharged?

It's also possible that it's a balancing act between not wanting cells to sit for long periods with a high charge and not wanting to repeatedly discharge the same cells. A small group of cells could be used to condition the battery, provide power to key subsystems, maintain the 12V charge, etc. Once these have drained and recharged, you could engage another group to minimize the number of cycles for any individual cell, just like "wear leveling" a flash storage device. Pure speculation, of course, but giving the car the option to do what makes the most sense puts Tesla's engineers in control - and it seems likely they understand more than I ever will about proper care and feeding of their particular battery chemistry.

 

[dave]

When the car is plugged in you aren't necessarily charging it - what you're doing is giving it the option to charge.

Right, but the car doesn't realize that I'm only going to drive 60 miles this week. Tesla programmed it to charge to 90% whenever it is plugged in, so that's what it does. It doesn't have an option. I'm just curious if 80% is actually somewhat better than 90% for long term health reasons. Or is one big charge a week better for the battery than driving 5 miles a few times a day and constantly plugging it in and charging it to 90% again.

I totally understand that I'm overthinking this and probably being too obsessive about it. When I get home from the store with 220 miles left and no plans for a long trip, I just want to do what will be best for the battery over the next 15 years or so.

 

[ElSupreme]

There are 2 basic things that degrade your battery Time and Cycling. Below is horribly over simplified.

Time: There is nothing you can do about this type of degradation Having very high states of charge are worse than lower states of charge. I would imagine that 90% is probably low enough where going lower isn't going to make a huge difference. In effect 100-90 is a BIG change. 90-80 is not a big change. This degradation is basically the same if you drive your car 1,000 miles over 10 years or 1,000,000 miles over 10 years. This is caused by the breakdown of the electrolyte in the battery cells.

Cycling: Every time you charge or discharge the batteries they convert one chemical to another. This process will eventually produce crystals that don't convert back and forth. A 'cycle' is 100% discharge and charge. So you driving 3 miles and then recharging counts as a fraction of a charge. Also less full cycles are more damaging than many small cycles. Temperature has a large effect on how well the crystals grow. Hot is bad.

In general the degradation of Time and Cycling is NOT additive. Meaning that 10% Time and 10% cycling degradation does NOT equal 20% total degradation. It ends up being between 10% and 20%.

As for plugging in. I am speculating that the car will condition the batteries differently if plugged in, because it knows it can spend a little more power and not have a person get screwed out of some needed range. I would always plug in, I expect that it is the healthiest thing for the batteries.

I have done work for some home products manufacturers. I have always thought that the use 'X' amount on the bottles was so you would use more than necessary. But it turns out not to be the case. At least the producers I was working for, actually did tests and they recommended the least amount to use for maximum results (to the nearest usable unit obviously). I don't think Tesla would recommend something they don't think is the best.

EDIT: I said crystal buildup. That is common in batteries, but I believe that LiIon cells actually develop cracks on the electrodes. But the concept is the same.

 

[dave]

Also less full cycles are more damaging than many small cycles.

You lost me here. So in a week, 21 top ups (3 small trips per day) is better for the battery than 1 big charge at the end of the week?

I don't think Tesla would recommend something they don't think is the best.

Of course not, but they are making one blanket recommendation that would have to apply to all owners in all situations. The easiest and safest answer for them is just to say "always keep it plugged in". I'm just curious if there are more advanced behaviors those of us with OCD and too much time on our hands could employ.

 

[ElSupreme]

You lost me here. So in a week, 21 top ups (3 small trips per day) is better for the battery than 1 big charge at the end of the week?

Yes 21 top ups is much easier on the battery than 1 big charge.

Of course not, but they are making one blanket recommendation that would have to apply to all owners in all situations. The easiest and safest answer for them is just to say "always keep it plugged in". I'm just curious if there are more advanced behaviors those of us with OCD and too much time on our hands could employ.

Yes you are probably correct there is probably a 'better' solution for your specific case. But managing such a program is probably going to be less reliable than the 'plug in every night' system that Tesla has. But if you mess up you might induce more wear and tear than what would happen if you did it Tesla's way. But without knowing we are just guessing around.

I am also of the belief (lower currents during discharge for same power) that driving at lower SoC (State of Charge) is worse than driving at high Soc, and storing the car is opposite (to a point). I think the 90% charge level is the best compromise to driving at high SoC and storing at reduced SoC.

 

[Doug_G]

First of all, the battery life is maximized by maintaining a "medium" level of charge. Too high or too low will cause significantly faster degradation over time. Thus the Standard mode recommendation. It's okay to use Range once in a while, but using it constantly would degrade the batteries faster than normal.

Secondly, there is less stress on the cells when you draw lower current. If the battery pack is low while you are driving, the voltage droops, and the car has to draw more current to compensate. So there's more stress on the pack due to driving when the pack is low. Therefore it's better for your car to be more charged while driving rather than less.

So the recommendation to keep the car plugged in makes sense: the pack will not be very full or very low so the cells will not degrade quickly. Plus the next time you drive you'll have a higher voltage, and the cells won't have quite as much stress due because the current draw will be lower.

The second reason probably matters even more for the smaller pack sizes.

 

[waidy]

Does anyone know whether cell balancing occurs when SOC != 100% ? If so, how does it know the what is correct voltage to balance ?

 

[Hans (Amsterdam)]

From: Tesla Blog November 30, 2006

A Bit About Batteries

By Martin Eberhard

full article: http://www.teslamotors.com/blog/bit-about-batteries


The bottom line is that all batteries age, and they lose capacity as they do. This, in turn, shortens driving range.
Batteries age with use, and they age with time, even if not used.

We tend to look at two kinds of aging: aging from use, called "cycle life," and aging with time, known as "calendar life."
These two different aging mechanisms can be thought of as separate, overlapping forces.

The other factors affecting cycle life are tied to how the cell is used. In particular:

1. Avoiding very high and very low states of charge. Voltages over 4.15V/cell (about 95 percent state of charge [SOC]) and voltages below 3.00V/cell (about 2 percent SOC) cause more stress on the insides of the cell (both physical and electrical).
2. Avoiding very high charge rates. Charging faster than about C/2 (two hour charge) can reduce the cell's life.
3. Avoiding charging at temperatures below 0° C. (Our design heats the pack before charging at cold temperatures.)
4. Avoiding very high discharge rates. (Our pack has been designed such that even at maximum discharge rate, the current required from each cell is not excessive.)

There is a huge difference in cycle life between a 4.2V/cell charge (defined by the manufacturers as “fully charged”) and a 4.15V/cell charge.
4.15 volts represents a charge of about 95 percent. For this reduction of initial capacity (5 percent), the batteries last a whole lot longer.
Unfortunately, further reduction of charge has a much smaller benefit on cycle life.
Understanding this tradeoff, Tesla Motors has decided to limit the maximum charge of its cells to 4.15 volts, taking an initial 5 percent range hit to maximize lifetime of the pack.
We also limit discharge of our battery pack to 3.0V/cell and will shut down the car when the batteries reach this level

Calendar Life Li-ion cells lose capacity with time, even if they are just sitting on a shelf.
They lose the most early in their life (year one) and then continue to lose capacity gradually thereafter.
Two factors shorten calendar life considerably: lifetime average temperature and time spent at high states of charge.

 

[Daniel Scherer]

One other thing to consider is charging at the lowest amp rate you can to achieve only the charge you need. If you have 6 hours to put 100 miles worth back into it, it makes no sense pumping 40 amps into it at 25 mph. Step it down to 30 amps. 20 amps gets you 10mph. This is all based off the on board 10kWh charger and a 14-50 outlet. The On screen charge estimate is pretty accurate if you set it and let it stabilize for an hour then check it by the app.

 

[ElSupreme]

One other thing to consider is charging at the lowest amp rate you can to achieve only the charge you need. If you have 6 hours to put 100 miles worth back into it, it makes no sense pumping 40 amps into it at 25 mph. Step it down to 30 amps. 20 amps gets you 10mph. This is all based off the on board 10kWh charger and a 14-50 outlet. The On screen charge estimate is pretty accurate if you set it and let it stabilize for an hour then check it by the app.

But is there any evidence that this will increase/decrease your battery life? You might stem some 'vampire load' by not stopping charging earlier but you will still use that power.

 

[Dave EV]

Yes 21 top ups is much easier on the battery than 1 big charge.

Of course, that advice completely depends on the start/end SOC and amount of time it's spread over.

Since calendar life losses tends to be the dominating factor, it's apparent that one should be able to get significantly better battery life by keeping the average SOC of the pack closer to 45-55% rather than 75-85%.

Ideally you'd always cycle the battery around 50% (the actual ideal % might be higher or lower), you'd only charge up high enough so that the planned driving would take you no further down than the amount over 50% you charged and you'd also charge the battery right before you leave (to minimize time spent at high SOC).

Of course, that's not very practical, so most EV manufacturers simply recommend you plug in all the time so you have plenty of range when you might need it as that's "good enough".

Edit: Numbers above are representative and not meant to precisely portray reality.

 

[Elshout]

Dave,

You have asked all of the questions I have been puzzled by. By the way, I believe TM recommends charging to just 80% of capacity, not 90%.

 

[gregincal]

Since calendar life losses tends to be the dominating factor, it's apparent that one should be able to get significantly better battery life by keeping the average SOC of the pack closer to 45-55% rather than 75-85%

I don't think it's apparent at all the the battery life would be "significantly" better. The blog above definitely implies that as long as the charge isn't at one of the extremes, the difference in battery life depending on the charge isn't that great.

 

[dtich]

Of course, that advice completely depends on the start/end SOC and amount of time it's spread over.

Since calendar life losses tends to be the dominating factor, it's apparent that one should be able to get significantly better battery life by keeping the average SOC of the pack closer to 45-55% rather than 75-85%.

Ideally you'd always cycle the battery around 50% (the actual ideal % might be higher or lower), you'd only charge up high enough so that the planned driving would take you no further down than the amount over 50% you charged and you'd also charge the battery right before you leave (to minimize time spent at high SOC).

Of course, that's not very practical, so most EV manufacturers simply recommend you plug in all the time so you have plenty of range when you might need it as that's "good enough".

Edit: Numbers above are representative and not meant to precisely portray reality.

this is my thinking too, based on a lot of research, and martin e.'s post seems to support it as well. my system has been to drive a day, and if significantly less than 200mi range (85kWh pack), i plug in. if i have close to 200, i leave it until the next day. that way i keep the battery closer to 75 or even 60% for a longer period of time. i also charge at 24A, rather than 40, because i have the time. i plan to almost never get below 50mi range. this is a no-brainer for me and works fine with my driving needs. obviously, each will need to tailor to their own habits. i don't expect my system to make a huge difference, but whatever smart things i can do to make any difference is good in my book. fwiw.

 

[Doug_G]

Dave,

You have asked all of the questions I have been puzzled by. By the way, I believe TM recommends charging to just 80% of capacity, not 90%.

No, a Standard Mode charge is 90%.

The confusion is probably because Roadster charges to 90%, but only shows 80%. That's because it hides the bottom 10% unless it's in Range mode.

The Model S doesn't do that - it also charges to 90% but always shows you the full 90%. Less confusing.

 

[LazMan]

This thread could be called "Best Practices in Battery Maintenance".

Quick question, is it better to limit the charge rate of a HPWC if you have all night to charge?

 

[DriverOne]

One other thing to consider is charging at the lowest amp rate you can to achieve only the charge you need.

This may help keep the battery cooler during charging? The charging cable itself gets warm at 40A. Cooler = happier battery and ill take any cooling help during the summer. But since the battery is cooled, maybe the effect of charging rate on temp is immaterial.

This cold weather may be hurting people's ranges, but on the bright side the cold may be better for the battery's life!