Roadster Owner Based Study of Battery Pack Capacity Over Time

[benji4]

Your description of rarely if ever doing a range mode charge makes me think that this might be your problem. If it is, you can just do what I did. Be patient, though. While batteries that aren't terribly unbalanced can get to balance in an hour or so, you may take MUCH longer.

This looks like it's getting a bit off topic, but the above discussion has made me curious about a couple of things:

1) It's apparent that rarely or never charging in range mode leads to imbalances in the battery pack, but can this in itself cause damage or permanatly decrease range? What if the situation is left uncorrected for months or longer?

2) In order to get the batteries to rebalance, why is driving the car down to a low state of charge before charging in range mode neccessary? Wouldn't charging in range mode from any level of charge accomplish the same thing?

 

[richkae]

I think we are talking about 2 completely different animals and range mode charging solves them both for you.

The first is balancing the battery. Charging in range mode may not require any deep cycle to do this.

The second is knowing the capacity of your pack. If the pack is only exercised between 85% and 70% it loses track of what the full range is.
Charging in range mode from a low SOC to "full" SOC helps it to relearn this.

You may have both issues or only one of them.

 

[hcsharp]

...
1) It's apparent that rarely or never charging in range mode leads to imbalances in the battery pack, but can this in itself cause damage or permanatly decrease range? What if the situation is left uncorrected for months or longer?
...

It's possible but it wouldn't be much damage. Just like frequent charging in range mode shortens life, frequent deep discharge also shortens life. If your pack is out of balance then some of the cells will be deep discharged much more easily because they are starting at a lower SOC than the rest of the pack. Because those cells are stressed more than the others, they will eventually fail earlier, shortening range and the life of the pack. So a range-mode charge every couple of months or whenever you suspect there might be balance issues is advisable.

While range mode full charge and sitting provides the best balancing, the latest software does some balancing with only a std mode chrg.

 

[benji4]

It's possible but it wouldn't be much damage. Just like frequent charging in range mode shortens life, frequent deep discharge also shortens life. If your pack is out of balance then some of the cells will be deep discharged much more easily because they are starting at a lower SOC than the rest of the pack. Because those cells are stressed more than the others, they will eventually fail earlier, shortening range and the life of the pack. So a range-mode charge every couple of months or whenever you suspect there might be balance issues is advisable.

While range mode full charge and sitting provides the best balancing, the latest software does some balancing with only a std mode chrg.

That makes sense... so basically while it's 'better' to keep the battery pack balanced, since using range mode charging to correct an out-of-balance battery pack in itself is damaging to the battery pack, it's a tough call. Kind of like chemotherapy for the battery pack.

Then the second problem -- using deep charge cycles to get the range indication more accurate. That really should not be done all that often one would think unless the indication is pretty far off base.

 

[user497]

Last night I fully charged the car in standard mode and only got back to 164 miles. (When I charged in range mode last weekend I got 208 miles). So it doesn't look like charging in range mode really did anything for me. Unfortunately I wasn't able to fully charge in standard mode in one night so it was recharged over 2 nights (Friday and Saturday). I will try to get the car under 100 ideal miles and fully recharge in range mode to give it another shot. I am bringing the car in for its yearly service in March so I will have Tesla take yet another look then if I don't see more mileage by then.

 

[benji4]

Last night I fully charged the car in standard mode and only got back to 164 miles. (When I charged in range mode last weekend I got 208 miles). So it doesn't look like charging in range mode really did anything for me. Unfortunately I wasn't able to fully charge in standard mode in one night so it was recharged over 2 nights (Friday and Saturday). I will try to get the car under 100 ideal miles and fully recharge in range mode to give it another shot. I am bringing the car in for its yearly service in March so I will have Tesla take yet another look then if I don't see more mileage by then.

In my case I was down to 179 ideal miles for a standard charge after 6 months and 4000 miles on the odometer. One deep charge cycle followed by a range mode charge got it back up to 185 or 186 on a standard charge at 110V/15A. I may try this again and see if it improves it further. When the car was new I was getting 193+ with standard charging so still a bit off that, but not too bad. Tesla just checked the battery for me last week and said their were no problems with it and the range was still what they considered to be normal. I'm curious to hear what they say about yours as 164 definately seems too low.

 

[vfx]

Charge rate may have an effect here. Tesla told me that continually charging on my 120V 16 amp plug was getting my battery out of balance. The also noted my voltage dropped down to 100 on occasion and was not too happy about it's effect in the sheets.

I now have a 220V 30A option

 

[richkae]

shameless bump

 

[zack]

I had something interesting happen Saturday. I've left the car in storage mode for the last few months and took it out for a 30 mile drive. It started with a range reading of 101 miles, and ended at a range of 103 miles with an ideal range of 142.

 

[dhrivnak]

I had something interesting happen Saturday. I've left the car in storage mode for the last few months and took it out for a 30 mile drive. It started with a range reading of 101 miles, and ended at a range of 103 miles with an ideal range of 142.

That is entirely possible. The normal range is calculated based on the average usage of the last 30 miles. If you got onto it hard say 25 to 30 miles ago and then took it for an easy 5 miles then you average watts/mile could drop substantially and thus boost your range.

 

[richkae]

I had something interesting happen Saturday. I've left the car in storage mode for the last few months and took it out for a 30 mile drive. It started with a range reading of 101 miles, and ended at a range of 103 miles with an ideal range of 142.

Was that ideal miles or estimated miles? I assume it was estimated.

 

[ChargeIt!]

I had something interesting happen Saturday. I've left the car in storage mode for the last few months and took it out for a 30 mile drive. It started with a range reading of 101 miles, and ended at a range of 103 miles with an ideal range of 142.

What's interesting here is (assuming you did not charge after taking it out of "Storage Mode") that the car, even if it was fully charged at the start of StorageMode, still had 142+ (+30? depending on how you drove) Ideal Miles left after "the last few months" sitting in StorageMode. So ... do you have more info ?

 

[richkae]

I have some interesting charts to share.
I don't know what they mean.
3 cars, all 2008 1.5 Roadsters with high miles:
46371 miles and 94% battery capacity remaining
49920 miles and 91% battery capacity remaining
35682 miles and 92% battery capacity remaining

Its worth noting that I have data for lots of other cars that have between 10000 and 21000 miles and most dont show any significant battery capacity change.
For example, my car is 33 months old with 21000 miles and still has 99% battery capacity.
So these 3 high mileage cars are the "worst" data I have in terms of battery capacity loss over time and distance.

Here are the charts ( red is the average amp hours of all the bricks, blue is the amp hours of the weakest brick in the pack )
Roadster 1: 46371 miles and 94% battery capacity remaining
This Roadster has not had any sheets replaced.

Roadster 2: 49920 miles and 91% battery capacity remaining
This Roadster had a couple of sheets replaced in january 2011.

Roadster 3: 35682 miles and 92% battery capacity remaining
Unknown if any sheets have been replaced.

Looking at the charts, it seems likely that early firmware did not measure the bricks in the same way, because the bricks didn't really change much until early 2010 when the firmware changed.
The big mystery is why does the capacity bounce back up? Is it just a firmware change that is interpreting the voltages from the battery differently? Was the firmware operating in late 2010 too pessimistic?
Is it seasonal? Does temperature have a big effect? Another 6-12 months of data will be telling.

 

[hcsharp]

Rich there are some good arguments for the firmware-changed-how-it-measured theory. But they don't all follow the same pattern. What about the theory that it starts bouncing up because the firmware started managing the batteries differently? Better cooling and/or balancing got them to perform better? And Tesla has a big incentive NOT to change the way they measure. It's unscientific. Makes it harder for them to do reliable R&D. And they have done a LOT of R&D looking at factors affecting battery life.

Then again we haven't decoded everything in the logs. Maybe there's a section with battery capacity and no changes to the measuring method?

 

[Alan]

Rich, I thought the firmware version was included in the logs? If so it would be interesting to see if they all got updated at the same time.

As for the temperature / seasonal effect this would carry more weight if the cars are geographically close. Eg if one is in the US, another in the UK and a third in Australia then we can rule out seasonal effects.

 

[donauker]

Looking at the charts, it seems likely that early firmware did not measure the bricks in the same way, because the bricks didn't really change much until early 2010 when the firmware changed.The big mystery is why does the capacity bounce back up? Is it just a firmware change that is interpreting the voltages from the battery differently? Was the firmware operating in late 2010 too pessimistic?

The answer I received from Tesla was yes to both of these items.

It was a firmware update that started the rapid drop in capacity from the original overly optimistic value. My Roadster is # 3 above, it has never had any battery work. As I saw my ideal range on standard mode charge completion rapidly drop into the 150's I had some discussions with Tesla. Several bleed tests were run and everything was reported as fine and it was indicated that the firmware was now overly pessimistic and they were working to correct it. A later firmware update did reverse the drop and brought my ideal range back up to 168-171 on charge completion with almost 40,000 miles and 35 months on the odometer.

I would consider this battery to have had a rather protected life. I have used performance mode very little (only 1 full performance mode charge). It had a very limited use of range mode in its early life and I have never used the lower range mode portion of the battery (never below the 0 mile standard mode level).

 

[richkae]

Thanks, thats good to know. I knew I should have plotted the changes in firmware revision on the chart. I'll do that next time.

It adds another level of complexity in trying to figure out how long our batteries will last.

I'd love to get more data from high mileage Roadsters and have it yield something meaningful to share.

 

[hcsharp]

Thanks, thats good to know. I knew I should have plotted the changes in firmware revision on the chart. I'll do that next time.

It adds another level of complexity in trying to figure out how long our batteries will last.

I'd love to get more data from high mileage Roadsters and have it yield something meaningful to share.

Yes, this "firmware factor" makes it hard to do any meaningful analysis. We don't even know if the current firmware is measuring accurately. There's a slight incentive for Tesla to make it a little optimistic. Owners will report to their friends "I still have 98% of capacity..." when we don't really know but are assuming we do.

 

[JRP3]

You can always do a full charge and see how far you can actually go at a steady speed, if you don't mind risking a tow. Nothing like a real world range test.

 

[hcsharp]

You can always do a full charge and see how far you can actually go at a steady speed, if you don't mind risking a tow. Nothing like a real world range test.

Not sure if you were joking or serious? Aside from the fact that this is hopelessly impractical, there are too many variables to control. Tires, air temp, humidity, battery temp, how it's driven, etc. You could improve the results by repeating the test enough times but for our purposes we want to know within 1 or 2% which would be pretty much impossible.