Optimal battery management

[Couscous19]

I bought my used S85 with the intention of having it for 15 or 20 years. That means that, while Tesla have done a good job at putting in systems to reduce battery degradation regardless of user habits, I am keen to do everything in my power to minimise the degradation.

Am I right in thinking that the following is a pretty exhaustive list of do’s and don’t do’s?

* If not using the car for several days have it left at ~50% SoC.

* Rarely exceed SoC over 80% and under 20%. Especially avoid very high SoC during high temperatures (not that we get much of that in the UK). Also avoid leaving the battery at a very low SoC during perpetually cold weather.

* Charge to 100% once every couple of months or so in advance of a long drive. Try and time the full charge to finish right as you’re about to leave so it doesn’t sit at 100% for long.

* In winter start charging the car a few hours before you depart so that it warms a bit before you get underway.

Any others I’m missing or have misunderstood?

 

[WannabeOwner]

I read something recently where someone prominent in Tesla Battery division (sorry, I haven't got a clue as to where I would, now, find a link) said that for maximum battery life the "daily recharge" should not exceed 70%

Other than that I agree with your points - although not sure about "If not using the car for several days have it left at ~50% SoC" - depends what your definition of "several days" is Perhaps if the car is left long-ish-term you maybe let the battery fall to, say, 40% and then recharge to 50% (and then repeat that cycle).

You might want to look at parasitic loss too. I have a couple of phone APPs and two internet logging applications that collectively cause some parasitic loss. The car normal does 80 miles a day, so I'm not particularly bothered (relative amounts of Parasiticriving are low), but for a low mileage car the parasitic loss could represent quite a lot of the recharging requirement.

I would always recharge on arrival if SOC was below 20%, in fact it makes sense to charge on arrival if the charge is at all low because otherwise the battery wouldn't be charged if you needed to use the car but we normally charge overnight on E7 (using car's Scheduled Charge), so I have to override charging if I want to charge during the day. Arriving home in the evening, >20%, then I'll let the overnight charge take care of it. < 20% I would force a charge straightaway.

In winter start charging the car a few hours before you depart

Not of as much benefit as you might expect. Firstly its hard to do - you have to set up scheduled charging to stop short, and then top-up charge an hour or two before departure. If you get a powercut in that interval then you are short-charged for your trip ... in practice I have found that the battery heater often doesn't come on (not cold enough) and in the UK it doesn't heat the battery much (before it cuts out). There's some relationship to Range Mode too - can't remember what it is but it might be that Range Mode allows a higher cutoff temperature and is therefore beneficial (but I might have that back-to-front). Either way I have reduced Regen for a good part of my journey (20 minutes?) on cold mornings, even with charge 2 hours prior to departure.

I do have a scheduled-charge-for-departure set up in TeslaFi but its unnecessarily fiddly. If you set it up as a "one off" then TeslaFi deletes it afterwards, and you have to set it up, next time, from scratch. if you don't then you have to remember to disable it (before "tomorrow")!

I have:

Increase Limit to 100% (optional, and on those occasions when I'm going a long way)
10 minutes later (allowing for several retries of the above) start charging
15 minutes before departure turn on climate (to use Shore-power)
At departure time +10 minutes turn off climate (i clearly didn't actually leave as planned!)
At some point change 100% back to 90%

Even if you are not charging to 100% then because you want to "part charge", to allow some further charging to occur, you would need to twiddle the charge-limits to leave space for further charging before departure, and then to set that back again afterwards.

I'm a Geek, so its fine for me. Not sure its fine for everyone. Most definitely NOT fine for my wife ... and I'm definitely prone to forgetting to do it.

Any others I’m missing

Don't Supercharge / Rapid charge unless you have to (although I wouldn't be too anal about that ...)

 

[Tozz]

Any others I’m missing or have misunderstood?

I think you're pretty much spot on

There are a few basics:
- Batteries last longer with a lower SoC than they do with a high SoC.
- Batteries last longer in the cold then they do in heat
- Batteries last longer when charged slow compared to being charged fast (C-rate)

This means that it is better to have a general low SoC. If you can do your commute in eg. 30% of your battery capacity it is better to charge to 50%, drive it to 20% and then recharge back to 50% compared to charging it to 90% and drive it to 60%

If you have time it is also better to set your charge amps down. This reduces the charging C-rate which prevents dendrite buildup in the cells. It is also better if you can charge in a relatively cold environment compared to eg. having your car sit in the burning sun when its 35 degrees outside. So maybe charge in the shade on a hot sunny day when possible.

There isn't really any reason to charge to 100% when looking at battery lifetime except for when you actually need the range. HOWEVER! The BMS (battery management) "needs" a 100% - 0% cycle to "learn" the battery. Calculating the batteries remaining capacity is difficult. A 0-100% cycle fixes this (in software, its just the software that needs this to show the proper values). So, the charge to 100% is only useful for the car to "learn" the battery, doesn't do anything for the battery itself.

Now, your statements:
- There is nothing wrong with going under 20% as far as I am aware. There is plenty of spare capacity and brick protection
- Correct that leaving the battery at 100% is bad.
- You can also pre-heat the car in winter. That also heats the battery. Not really required to actually charge. It is however handy to have the car plugged in, as that will use "wallpower" instead of batterypower.

Now, from what I've read here on TMC taking all these precautions doesn't do very much in terms of battery degradation prevention, as Tesla takes very good care of the batteries. Maybe 1 to 3% in 10 years. Tesla batteries tend to have very low degradation in general, regardless of how owners have taken care of it.

 

[Tozz]

Don't Supercharge / Rapid charge unless you have to (although I wouldn't be too anal about that ...)

Especially "dont be too anal" is pretty much spot on. Please enjoy driving your car as well! Dont let your anxiety of battery degradation affect the pleasure of driving.

 

[arg]

So, the charge to 100% is only useful for the car to "learn" the battery, doesn't do anything for the battery itself.

It does also balance the battery (equalise the state of charge between the cell groups). Admittedly that has little relevance to battery wear except in extreme cases.

There is nothing wrong with going under 20% as far as I am aware.

No magic in 20% as such, but at lower states of charge the cell voltage is lower so you will be drawing a higher current to achieve the same power in the motor. So this has the similar effect to charging faster, or driving harder.

But the real take-away is that none of this really matters all that much. Experience with the oldest and the highest mileage cars suggests that even if you ignore all of this your degradation will be not that bad. So it's nice to bear these issues in mind and be kind to the battery if you can do so conveniently, but no more.

 

[Mark_T]

There are a number of battery myths that originate from ‘good practice’ when using hobby grade lipo packs, not going below 20% is one of those.

It only matters if 0% is actually 0 which it isn’t with these well designed and protected EV packs.

 

[WannabeOwner]

Since having EV I have started only part charging my cordless DIY tools, and then fully charging immediately before use. Difficult to part charge as no "button" for that, so I just charge on a timer for whatever I know to be "less than full charge time". No idea if it will make much difference to battery life, but it bugs me that these consumer-batteries are regarded as disposable

 

[Mark_T]

The degree of tolerance for remaining full but unused is also linked to the C rating of the cells.

Low C packs can remain full for a lot longer without harm, but of course this is never disclosed on the packs used in consumer equipment.

Also we,need to consider that 100% is also an arbitrary number so even when notionally full the packs may still have a little headroom left.

...but yes, I also avoid leaving equipment fully charged for more than a few days whenever possible.

 

[Tozz]

No magic in 20% as such, but at lower states of charge the cell voltage is lower so you will be drawing a higher current to achieve the same power in the motor. So this has the similar effect to charging faster, or driving harder.

Hmm, thanks. Point taken. Didn't realize that, but is indeed true.

This effect is mitigated somewhat though during navigation.. If your SoC is getting low your car will inform you to lower your speed in order to reach your destination. This has the side effect of lowering the current

 

[WannabeOwner]

If your SoC is getting low your car will inform you to lower your speed in order to reach your destination

I wonder if that is something "taken care of" by all EVs, or just another indication of Tesla's greater expertise in battery management?

 

[arg]

Hmm, thanks. Point taken. Didn't realize that, but is indeed true.

This effect is mitigated somewhat though during navigation.. If your SoC is getting low your car will inform you to lower your speed in order to reach your destination. This has the side effect of lowering the current

Indeed. Also, the car won't let you use full performance at lower SoC either - but that's an indication that driving hard at low SoC is a bad thing.

Like all of this stuff, not something to get to worried about as the car protects you from doing anything really harmful, but to bear in mind - it's an argument against keeping your regular charging level too low, which not only causes you inconvenience (and possibly unneeded fast charging when you suddenly need to go on a longer trip you weren't expecting), but also works the battery harder.

Hence I would argue that for most people a regular charge of 80% is more sensible than 50% - since the minor gain from 50% can be wiped out by occasionally dipping very low and/or extra fast charging. Of course, if you are prepared to think about your charge level every night and put in just the right amount (and your life is predictable enough that you don't make mistakes) then that's ideal - but most people wouldn't consider that worth the effort.

 

[Mark77a]

The rule I use is:

Never use full power below ~25% SOC.

.. simply because the cells voltages are getting low, and so when you are pulling full power the weakest cells will have their voltages pulled down most, and whilst the BMS will do its best to keep balance, tolerances mean that there will always be weaker cells that will get damaged first.

Keeping the weakest cells as healthy as possible (the weakest links in a chain) to maximise the long term battery health.

FWIW My Roadster battery is 8 years old and at ~90% original capacity, and I want to keep it and the MS's battery as healthy as possible, for as long as possible (whilst still having fun ... in the 80-25% SOC range )

 

[Peteski]

There are a number of battery myths that originate from ‘good practice’ when using hobby grade lipo packs, not going below 20% is one of those.

It only matters if 0% is actually 0 which it isn’t with these well designed and protected EV packs.

I think the same chemistry basics apply to both, but as you say, Tesla batteries have far more inbuilt protection than hobby grade lipo packs (which basically have none). I'm happy to stick to the following practical regime:-

1. Only charge to 100% when needed and don't store the car above 90% for any length of time
2. Daily charge 80-85% unless I know more is required for a trip
3. Try to stay above 20% as a minimum unless needing to use full range
4. Only supercharge as and when required
5. Don't stress about not following those rules religiously!

So far I've been typically using the battery in the 85-30% range on a daily basis, which I'm more than happy with in terms of degradation. Rarely need to Supercharge, so that's a non-issue. Lowest charge I've seen so far is around 15% and it was still driving fine down at that level. Haven't needed to charge above 90% yet, but certainly will when I need it. The battery only has to make it to 4 years and 50-60K miles so not going to over pamper it.

 

[Peteski]

Hence I would argue that for most people a regular charge of 80% is more sensible than 50%

Especially so if you have a smaller battery size. I wouldn't run my X75D at 50% daily unless I knew I wasn't going anywhere. I did however leave it parked and plugged in at 50% when abroad for 3 weeks.

 

[WannabeOwner]

Lowest charge I've seen so far is around 15%

I got home on 1% once ... not sure if that's worse for the health of the battery ... or me!!

 

[Paul73]

AFAIK for short trips, if you can daily topup to 60% this is good (parasitic reactions increase at higher soc, and oxidisation at high soc causes sudden capacity loss after fewer cycles when surface area is reduced by solids, even if gradual degradation so far has been low [37:00 in video] -- a lower voltage e.g. 3.9 V per cell or ~60% means much more cycles/years before this failure)

The shallower the depth of discharge the better e.g. 60%-40% (20% dod) would be good [20% of 300 miles = 60 miles]

Charging near 100% won't help the battery, and probably not needed for balancing if this is done at all voltages (I wish Tesla offered charge to a selectable voltage (3.9 V x 96 = 374 V) instead of 60%, then any error in estimating what the predicted range is when only using shallow cycles wouldn't matter, only actual low battery voltage would matter. Hopefully min/max brick voltage vs time can be graphed on the car's screen in the future)

Slow charging at home (7kW is approx C/10 for an 80kWh battery) is probably better than supercharging at 120 kW.

Avoid fast acceleration below 40% soc?

Jeff Dahn at the end of his presentation says heat is the worst actor, "Keep it in a fridge", good that Tesla is thermally managed

For a little extra gain, could schedule the topup to start in the early morning when the battery has cooled after driving -- let it cool before charging if possible, especially in the summer. Lucky in the UK the average annual temp is approx 10 degrees C.

I try to keep my phone between 75% and 45% (Accubattery on Android shows 0.12 cycles wear for this 30% dod), but I don't worry too much since I don't expect to keep it 20 years

A Bit About Batteries

 

[.jg.]

Another point to consider is that, as battery technology improves and the take up of EVs gathers pace, the price of batteries has been decreasing. The price per unit energy has dropped by about an order of magnitude in the last decade. As the number of older EVs on the road increases, it is likely that a market will develop for refurbished battery packs and the cost for a replacement new or refurbished batteries will be dramatically cheaper than the prices we see today. This is already seen for older hybrids, where a failing battery pack can be replaced with a refurbished one for under £1000.