Best way to charge for battery health?

[lmore]

Top-charging (100 %). I never top-charge my Roadster. A Tesla employee where I live rented the Roadster from me, although I told him (and wrote in the contract) to never top-charge the car and never charge at higher rates than 30 Amp, he top-charged at 60 Amp... Sometimes I don't understand what goes on in peoples minds. He was fully aware of that I could monitor everything he was doing (via log-files and OVMS). This rent was obviously ended because of lack of trust.

He claimed that this charging was not harmful for the battery. He let the battery sit at 100 % charge only 2-3 hours before driving. And I have read on the Model S forum that it is better for to charge at high speed. This was a bit of a shock for me to read, is that so for the Roadster also? Ambient temperature never gets much higher than 20C (room temperature) where I live and is half year around freezing point.
What about top-charging, is that not harmful at all if it stays 100 % only 2-3 hours?

This summer I am planning to rent the Roadster to a very enthusiastic woman for 50 days (I have several other EV's that I need to focus on fixing). She plans to drive a lot, 4000 km, several long trips (>400 km) but she can stop to charge after 200 km. I want the battery of my Roadster to live as long as possible. Would the optimal for this be to:
* always keep the car in Standard mode
* not go below 30 %

This will limit her usage of the car somewhat, but she told me she has plenty of time. She has already installed the OVMS on her phone and I remember that a notification can be set there when the limit gets at a certain point.

I plan to charge around USD 3000 for the rent, what would you estimate the maintenance/degradation cost of the Roadster to be per km/mile?

 

[djp]

This summer I am planning to rent the Roadster to a very enthusiastic woman for 50 days (I have several other EV's that I need to focus on fixing). She plans to drive a lot, 4000 km, several long trips (>400 km) but she can stop to charge after 200 km. I want the battery of my Roadster to live as long as possible. Would the optimal for this be to:
* always keep the car in Standard mode
* not go below 30 %

The Plug In America Roadster Battery Study shows the main factor in degradation is mileage. The occasional range charge doesn't seem to hurt capacity.

http://www.pluginamerica.org/surveys/batteries/tesla-roadster/PIA-Roadster-Battery-Study.pdf

The danger is in leaving the pack at 100% for a long period of time, especially in the heat. The BMS chills the battery during a Range charge to protect it, and limits the actual max SOC to 95%. The effect of a single quick range charge is minimal, but to be safe you should time the charge to finish within a few hours of departure to minimize the amount of time at full charge.

Keeping a buffer of 30% is not a bad idea, but more for unplanned events like unexpected headwinds or broken charger stations on the road trip. Going below 30% won't noticeable degrade the battery - the risk is in going to absolute zero.

Don't worry about charge rate, even the max current from an HPWC is nowhere near the max C rate of the battery.

In terms of fair price for the degradation, as a rough guess you can take the rate from the PIA study of 0.15% loss per thousand miles driven. That works out to 0.37% over 4000km. If you assume the pack is worth zero after 30% capacity loss and costs $40,000 to replace, then the 0.37% loss is worth $493.

 

[Doug_G]

+1. 100% agree with everything djp said.

The first renter was correct that you really don't need to be concerned about an occasional range charge or the charge rate, but since it is your car you have every right to decide how it is used.

 

[bart513]

I had my car parked in the garage at about 95% SOC. It sat for almost 2 weeks without being plugged in and held a charge quite well going down to about 80%, possibly higher. Because I had a 135 mile drive to make I plugged it in the night bofore, drove to NJ and charged it to 62% SOC getting a CAC of 154.86 on April 2nd. The CAC on March 13th was 155.13 when I had the car plugged in on a regular basis.

 

[hcsharp]

... And I have read on the Model S forum that it is better for to charge at high speed. This was a bit of a shock for me to read, is that so for the Roadster also? ...

I pretty much agree with everything djp said. The charging rate at 70A is still very gentle on the battery. Regarding charging speed and the comments in the Model S thread, I don't think the readers were interpreting the study properly WRT how it applies to these cars. When somebody later clarified with one of the authors of the study, he responded that it's probably marginally better to charge at a slower rate, but not much difference. I'm not sure why the "charge fast is better" myth continues to propagate. Tesla's charge management tapers down at the end of a charge in such a way that charging faster does not keep the battery at a higher state of charge for a shorter period of time the way it might with a computer or cell phone BMS.

If I were writing the Roadster rental agreement, I would not put any restrictions on charging rate. As far as top charging, I would include a limit on the number of times she can do it during the rental period. A reasonable max number might be 3 to 5 over 4000 km. That might actually be better than not doing any at all because it will help keep the battery balanced.

My guess is that you will wear out tires at almost the same rate that you wear out the battery.

 

[lmore]

Thanks for the great help (again). Tire wear is also a good point. I usually drive with 16" back wheels which reduces tire wear, they might last as long as the battery. The 17-inch standard tires didn't last more than 10000 miles.

I might add that what I am not that worried about the battery capacity dropping a few percent. The big fear is the battery going bad and needs to be replaced. So the car usage should reflect that. From what I have understood, the risk for a defect battery is higher if the battery is deeply discharged (often) since the risk for single battery cells will go bad is higher if they are dropping to a low voltage.

 

[wiztecy]

Thanks for the great help (again). Tire wear is also a good point. I usually drive with 16" back wheels which reduces tire wear, they might last as long as the battery. The 17-inch standard tires didn't last more than 10000 miles.

I might add that what I am not that worried about the battery capacity dropping a few percent. The big fear is the battery going bad and needs to be replaced. So the car usage should reflect that. From what I have understood, the risk for a defect battery is higher if the battery is deeply discharged (often) since the risk for single battery cells will go bad is higher if they are dropping to a low voltage.

How do 16" rear wheels lower tire wear over 17"? 16" should wear faster actually due to a quicker snap of the tire off the line.

 

[jerry33]

How do 16" rear wheels lower tire wear over 17"? 16" should wear faster actually due to a quicker snap of the tire off the line.

The main reason would be that the tires aren't the same from 17" to 16". Even if they are the same brand and model tire, they will likely be different. In addition, the taller sidewall allows the the 16" tire to absorb road and alignment irregularities better.

 

[wiztecy]

The main reason would be that the tires aren't the same from 17" to 16". Even if they are the same brand and model tire, they will likely be different. In addition, the taller sidewall allows the the 16" tire to absorb road and alignment irregularities better.

Thanks. So with a higher sidewall you're gaining side to side slop in the rear. I think I'd take more tire wear over side to side slop, especially where its being rented, pushed most likely hard around turns, and possibly could end up in an accident. The car wasn't designed for 16" rears. If it were me I'd put the best performing tires on the car for the car's safety as well as the renters and increase the rental rate. They are renting it for the "Tesla Roadster" experience and not some scaled down version. Again increase the price, state the car is a performance vehicle andclearly state to use/appreciate it safely, and lastly keep close to the specs the car came with. If it was your daily driver you'd learn the way the car drives with the 16's, but your audience is not and assuming the car will be setup close to the way Tesla sent it out of the shop. If I did anything close to this I'd make sure my good lawyer has spent a good bit of time reading over the contract to protect myself as well as my assets in the worse case scenario, which do occur.

 

[smorgasbord]

OK, back to battery charging.

I don't know why I can't find it, but I remember reading about how every now and then one should do a Range charge and then drive without turning the key off until the battery SOC is below some number (like 20%?), and then charge back up to full (Standard mode?) in order to get the pack balanced.

Can someone explain precisely what the procedure is/isn't?

 

[Doug_G]

I've never been convinced that it is necessary to do this without turning off the key, but that's what "they say".

 

[smorgasbord]

I've never been convinced that it is necessary to do this without turning off the key, but that's what "they say".

So, should I take a second key and if I need to leave the car for a bit, put the car in Park (but still on), and use the other key to lock/unlock the door? Assuming I'm in a safe area, of course.

Unfortunately, for the trip down, the best thing for me is to Range charge, drive 60 miles to Monterey, get breakfast and some power at a ChargePoint, then take off down the coast. So, if I don't charge to Range Full again in Monterey, I can't do this, right? Or, does the drive from not quite Range Full to almost empty in SLO count?

 

[wiztecy]

So, should I take a second key and if I need to leave the car for a bit, put the car in Park (but still on), and use the other key to lock/unlock the door? Assuming I'm in a safe area, of course.

Unfortunately, for the trip down, the best thing for me is to Range charge, drive 60 miles to Monterey, get breakfast and some power at a ChargePoint, then take off down the coast. So, if I don't charge to Range Full again in Monterey, I can't do this, right? Or, does the drive from not quite Range Full to almost empty in SLO count?

Taking a 2nd key is a good idea, its a pain not being able to leave the Roadster. You have OVMS, so you could also lock your doors that way too and not feel as worried since you can track your Roadster as well as shut it down if someone broke a window out then drove away.

> So, if I don't charge to Range Full again in Monterey, I can't do this, right? Or, does the drive from not quite Range Full to almost empty in SLO count?

From what I heard you don't need to do a Range Mode charge to have the pack recalibrate the ideal miles to CAC. I heard the pack needs to do a 70% pull down to redo the calulations. That being said you could charge in std. mode to 84% SOC and pull the pack down to at least 14% SOC on one key turn, then your charge needs to be at least a full std. mode charge. I just feel like doing a Range Mode charge helps out with the calculation better and logically to me makes sense if I have the time. But I've never read the specs nor seen the code, just did lots of shop talk at Menlo Park with managers and techs to make sense of it all. I even caught a few of them getting confused with this I Tried to get them to pull and review the spec sheet / documents from the Roadster Development Project their SVN repository, came close, but no luck. That would have told us exactly what to do. Unfortunately the specs / docs are pretty scattered, I just hope someone will preserve and ensure this data gets backed up and stored in one central repository for legacy cars, otherwise if its lost then all we have is word of mouth and human memory which I know is not the best way of tracking / keeping data in sync.

This would be a great request to make at the next TesLive. To ask if Tesla could assign an engineer to validate and create a Wiki on these tips we've pulled together on maintaining the packs, etc.... I don't think that would be too much to ask nor take that much time and money.