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DaveEV
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Re: Which cell loses capacity fastest? Which retains it bes

Mon Apr 29, 2013 11:45 pm

thimel wrote:I did the test of running the battery down from full to nearly empty. As I normally only charge to 80%, two nights in a row I charged to 100% and after it was done, told it to start charging again. This presumably balanced the battery cells. I was surprised to find the SOC starts at only 92.08%. I then drove. I got LBW at 21.6% and VLBW at 10.72%. I then drove slowly home.
Very cool!

Here's my analysis at the data.

1. The overall shape of the 92% and 8.75% (or 100% and slightly below VLBW) appear to be very similar, leading me to believe that the relative health of your cells are fairly close to the same.
2. A SOC of 92% is low - that should read about 95% on a 100% charge.
3. At 8.75% you have a very wide spread in cell-pair voltages at 115 mV.
4. The 4 lowest voltage cell-pairs at 8.75% are also some of the lowest at your 100% charge - in particular #s 20, 26 and 35.

It seems fairly evident that more balancing would get you more usable capacity of your pack - and that if the BMS had the capability to shunt energy to lower voltage cells it might be able to do a

I think that when grabbing this data, also grabbing a screenshot when LBW and VLBW might be useful. For people that normally charge to 80%, a screenshot at 80% and a screen shot right after a 100% charge and after sitting at 100% for a couple hours would be interesting.

Can't wait for my BT adapter to get here!

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RegGuheert
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Re: Which cell loses capacity fastest? Which retains it bes

Tue Apr 30, 2013 4:08 am

thimel wrote:I did the test of running the battery down from full to nearly empty. As I normally only charge to 80%, two nights in a row I charged to 100% and after it was done, told it to start charging again. This presumably balanced the battery cells. I was surprised to find the SOC starts at only 92.08%. I then drove. I got LBW at 21.6% and VLBW at 10.72%. I then drove slowly home.
Thanks! Very interesting!

Can you tell us how many miles are on your car? (Perhaps I need to add a short questionnaire to the instructions for some vehicle details? MY, VIN, Purchase Date, location, miles, charging habits, driving habits, etc. OTOH, I don't want this to be too difficult!)

It seems clear that the stack of modules which stand on edge under the rear seat appear to be degrading faster than the ones lying flat under the rear floor and front seats. Even Cell-48 in the Module 24 on the far end of the rear stack is pretty degraded. Perhaps any module rotation scheme might see a swap between the modules up front and those in the back, among other possible changes.

I will admit that I had imagined that the cells in modules MD35 through MD38 (Cell-69 through Cell-76) at the tops of the stacks under the front seats might get the hottest due to long path down to the heat sink. In fact, these tend to do better than the cells in the rear stack and Cell-69 seems to be doing particularly well.

I suspect the vertical cells in the back develop a temperature gradient across the cell which might also result in a current gradient across the cell. If so, such a current gradient could cause the vertical cells to appear to have lower capacity than an isothermal cell, even before any degradation occurs. It could also accelerate degradation. It will be interesting to see similar data for a brand-new LEAF with all cells nearly undegraded. Alternatively, it might be interesting to see results comparing a car which is driven versus one that is discharged using the heater approach. With the heater, the cells should have a lower temperature gradient than driving and the results might be different.
RegGuheert
2011 Leaf SL Demo vehicle
10K mi. on 041413; 20K mi. (55.7Ah) on 080714; 30K mi. (52.0Ah) on 123015; 40K mi. (49.8Ah) on 020817; 50K mi. (47.2Ah) on 120717; 60K mi. (43.66Ah) on 091918.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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DaveEV
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Re: Which cell loses capacity fastest? Which retains it bes

Tue Apr 30, 2013 7:20 am

Reg, I don't see any evidence that the first 24 modules are significantly more degraded that the others, only that for some reason they all have a lower SOC than the other half of pack (they aren't charged as much!).

Weak cells will show up as having high voltage when the pack is charged and low voltage when the pack is discharged.

This pack simply looks out of balance.

Comparing to other cars will be interesting. It's way too early to draw conclusions from a single data point.

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RegGuheert
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Re: Which cell loses capacity fastest? Which retains it bes

Tue Apr 30, 2013 7:58 am

drees wrote:Weak cells will show up as having high voltage when the pack is charged and low voltage when the pack is discharged.
I guess I don't believe this since the pack is top balanced. They should all come up to the same SOC if the balancing is done perfectly (which it clearly isn't here). However, a degraded cell not only has lower capacity, but it also has a higher resistance. As a result, even if the top balancing were done perfectly, one discharge and charge cycle should result in the degraded cells being at a lower SOC at the end of the charge cycle.

Agreed that more data points will be interesting, particularly if we can get some new battery results.
RegGuheert
2011 Leaf SL Demo vehicle
10K mi. on 041413; 20K mi. (55.7Ah) on 080714; 30K mi. (52.0Ah) on 123015; 40K mi. (49.8Ah) on 020817; 50K mi. (47.2Ah) on 120717; 60K mi. (43.66Ah) on 091918.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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garygid
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Re: Which cell loses capacity fastest? Which retains it bes

Tue Apr 30, 2013 8:18 am

Yes, the equalizing looks suspect.

Also, there is an unusual difference between the cells
on the left half and the right half of the graph.
See SOC/GID-Meter and CAN-Do Info
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FairwoodRed
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Re: Which cell loses capacity fastest? Which retains it bes

Tue Apr 30, 2013 8:48 am

To start off, no I did not follow the instructions properly, but when I found myself at VLB last night, and I finally had the ELM327 working, I decided to snap a photo or two. I hope they are of use.

On this day, I used a charge timer set for 100% and then commuted to work. Commuted home and charged for 10 min while I got ready for a movie, then drove to/from the movie. Plugged in and let my timer charge overnight again (it finishes about 1 hour before I leave). Typically, I get home with LBW about 25% of the time in winter and rarely in summer. I use a 100% timer every day of the week, with midday charging about 4 times a week, including weekends. I drive about 60-65 on the highway often, but only sometimes get into spirited driving. I have 40,000 miles in Seattle since I purchased my 2011 Leaf in late June 2011. My car is one of the “pollen cars”, produced on March 8th and languished at the Long Beach port for three months before delivery to me.

It seems to me that my pack could use more opportunity for cell balancing, so this morning I have disabled my charging timers. I’ll try to take some readings again at the end of the week and see how it goes.

VLB after driving:
Image



Full after 100% timer charging:
Image

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RegGuheert
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Re: Which cell loses capacity fastest? Which retains it bes

Tue Apr 30, 2013 8:53 am

FairwoodRed wrote:To start off, no I did not follow the instructions properly, but when I found myself at VLB last night, and I finally had the ELM327 working, I decided to snap a photo or two. I hope they are of use.
Thanks! Yes, very useful!

I will note that there is not a big difference between the rear stack (first 48 cells) and the front stacks like we saw with the other data.

BTW, I recommend using L1 to try to achieve the best balance possible in the LEAF, at least for the last couple of bars of charge.
RegGuheert
2011 Leaf SL Demo vehicle
10K mi. on 041413; 20K mi. (55.7Ah) on 080714; 30K mi. (52.0Ah) on 123015; 40K mi. (49.8Ah) on 020817; 50K mi. (47.2Ah) on 120717; 60K mi. (43.66Ah) on 091918.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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DaveEV
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Re: Which cell loses capacity fastest? Which retains it bes

Thu May 02, 2013 12:22 am

Here's my pic - started out at 27 gid parked in the garage, then ran it down to 24 gid (VLBW) with the heater/defrost on max. Will get 80% charge tomorrow and if I have time, 100% charge, too.

Drive to get to this point - started at 80% charge, mix of freeway/surface streets, LBW around 48 miles, pulled into garage at 55.4 miles with 27 gids at 4.5 mi/kWh. Interestingly, my Ah reading is basically the same as thimels earlier - perhaps not surprisingly given our near identical age of vehicle and driving habits despite being situated on opposite ends of California.

Looks like modules in the mid-20s like to read significantly lower than the rest of the pack when low like other posts - the animation shows how quickly max-min climbs - in 4 gits it went from 40-87mV. And cell-pairs 49-96 are significantly higher on average than cell-pairs 1-48 like others.

Other notes of interest: VLBW went on at 13.30% SOC compared to thimel's 10.72%. Despite my battery temp readings of 74-78F, I only showed 5 temp bars on the dash. Wow - it was pretty hot in Sunnyvale on 4/29! Pack temps a few degrees over ambient correlate well with my pack temp today and today's max around 75F.

Image

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JeremyW
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Re: Which cell loses capacity fastest? Which retains it bes

Thu May 02, 2013 7:29 am

It may be a bit early to draw conclusions, but looking at drees' neat pic as well as thimel's. It sure looks like the middle of the back stack (modules 1-24, cells 1-48) is where the weakest cells are. Not suprisingly, these are the modules that are getting the least exposure to the outside world and are probably running pretty hot compared to the rest of the pack.

To add annother confusing data point, I've noticed when doing heavy quick charging, the first temp sensor is often the highest by a few degrees C. As this is located in the front of the pack, it contradicts the above paragraph. :roll:
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DaveEV
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Re: Which cell loses capacity fastest? Which retains it bes

Thu May 02, 2013 8:58 am

JeremyW wrote:To add annother confusing data point, I've noticed when doing heavy quick charging, the first temp sensor is often the highest by a few degrees C. As this is located in the front of the pack, it contradicts the above paragraph. :roll:
I have to wonder if the BMS is reducing voltage on areas of the pack that tend to be hotter on purpose...

It seems pretty consistent that when reading the 4 temp sensors the first two are higher than the latter two by a couple degrees.

Anyway, here's what my pack looks like this morning after charging to 80%. Just eyeballing it, relative balance appears to be similar to the pics from VLBW. Not going to charge to 100% since it's supposed to be a scorcher today will be interesting to see how hot the pack gets. The pack didn't seem to cool off much from last night. Garage temps a steady ~70F with an overnight low of 59F.

Image

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