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Battery Imbalance after 8 years
- Thread starter emil.aguinaldo
- Start date
Thanks for the hint, I now installed the beta version. Here's some new screenshots! Interestingly nominal kwh has gone up from yesterday, still quite crappy 68.6kWh though. This is 90kWh pack, you have almost the same in your 70kWh.
My CAC values are 210/215/226 Ah, so it's really high imbalance (16Ah). But since average is so low to minimum, my guess is that it's this exact "high voltage" cell that is also high Ah? Weird. Is it possible it has a broken balancing circuit so BMS can't drain it?
Everything looking really nice expect for this cell 88... And as I said it has been like that for at least a year, always showing about 70mV higher than the rest.
@wk057 care to comment?
SMT calculates the average cell voltage wrong, btw. It shows min 3.50V, max 3.58V, average 3.54V... But if you look at the actual cell voltages, pretty much every cell is at 3.50V, so the correct average is about 3.51V...
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Tesla replaces the MCU, not the harness. My 2014 Model S was upgraded with MCU2 but it still uses the Pre Sept 2015 cable.
I haven't had any issues with it.
Hello all,
Based on this interesting discussion I did a small experience two weeks ago, during a one day long trip, will appreciate your opinion.
Car: 100D, built 09/2017, 154 000 km, nominal full pack is at 89 KWh, DC charge 14548 KWh, Charge total 47085 KWh.
At start, after a full charge done during the night:
- SOC 99.5%, Cell avg 4.168 V (min 4.164, max 4.170), Imbalance 6.10 mV.
I drove during the entire day (was rather hot), mix of german motorways (achieved a small peak at 180-190 km/h, but most of the time speed is limited at 120 km/h) and slow B roads. No stop for charging needed, 470 km in total.
Just after being back home I connected SMT again:
- SOC 8.2%, Cell avg 3.226 V (min 3.196, max 3.250), Imbalance 54.3 mV
The same cell (number 7) is showing the highest voltage at the beginning and the lowest at the end, so interpretation is that it is in a slightly worse condition than the others.
Is there something to worry about on the long run? Or is this just usual ?
How accurate are these voltage readings?
Based on this interesting discussion I did a small experience two weeks ago, during a one day long trip, will appreciate your opinion.
Car: 100D, built 09/2017, 154 000 km, nominal full pack is at 89 KWh, DC charge 14548 KWh, Charge total 47085 KWh.
At start, after a full charge done during the night:
- SOC 99.5%, Cell avg 4.168 V (min 4.164, max 4.170), Imbalance 6.10 mV.
I drove during the entire day (was rather hot), mix of german motorways (achieved a small peak at 180-190 km/h, but most of the time speed is limited at 120 km/h) and slow B roads. No stop for charging needed, 470 km in total.
Just after being back home I connected SMT again:
- SOC 8.2%, Cell avg 3.226 V (min 3.196, max 3.250), Imbalance 54.3 mV
The same cell (number 7) is showing the highest voltage at the beginning and the lowest at the end, so interpretation is that it is in a slightly worse condition than the others.
Is there something to worry about on the long run? Or is this just usual ?
How accurate are these voltage readings?
Nope ScanMyTesla now report 64,7kWh Nominal and TeslaMate is semi stable around 340 km Range :-(
Slightly strange given that 'asking' car to charge to 95% from 65% cause the car to calculate charge duration of 2h:25m or 2,42 hours, which at 10 kW charge speed means that 30% added is 24,2 kWh or 100% is 80 kWh. That is of course not the case, but I do think that the BMS under estimates my battery capacity. Presumably the Nominal Capacity is an average over many charges/discharges and so the latest (more informative) charges does not immediately affect the Nominal.
The currently ongoing 'To 95% Charge' has these measurements, so charger pulls up to 3 x 16A at 229V == 10,992 kW. ('Up to, because we do not know the Voltage on all 3 phases only the one that the BMS reports)
My previous comparison between what kWh my 'Power Pusher' bills me and what the car reports Added, shows efficiency of 94%. 10.992 * 0,94 = 10,33 kW, so not unlikely that 10,0kW IS added.
Aha, looks to be AVVG(MIN(Voltages), MAX(Voltages))
But the CAC seems corretly 'averaged' as your Average number is not in the Middle of Min and Max!! Perhaps BMS calculates CACAverage, CACMin and CACMax, but Voltage Average is calculated by SMT ??
I cancelled the 'to 95% SoC' charge at 82%, which was enough for our trip. Because stressing my battery JUST to get a nicer number makes little sense..
Today OBD2 reports Nominal = 64,6 kWh, but TeslaMate reports Projected Range of 346 km! So Nominal kWh is Down and Typical Range km is up :-( Possibly, the Nominal Capacity is a very slowly moving average and Range is more based on last charge?
The charge from 45% to 83% added 24,95 kWh == 38% which corresponds to 100% being 65,7 kWh, which is a Netto Number. Some of the 65,7 kWh is heat loss from internal resistance, but newer the less it corresponds to a Brutto of 69,7 kWh. Nominal 64,6 kWh is as well a Brutto, but does does not match Range of 346km ~ 65,74 kWh
With Average voltage over SoC range as 3,6V the CAC numbers argues 220,4Ah * 3,6V * 6 Bricks * 14 Modules = 66,65 kWh. Seing the Voltage Versus SoC of my Imbalance Versus SoC graph argues that the Voltage span is rather 3,1 - 4,2 V (and not 3,0 - 4,2) and with average Voltage of 3,65V the GAC suggest 220,4Ah * 3,65V * 6 Bricks * 14 Modules == 67,57 kWh. Whether the CAC is a Netto or a Brutto I do not know, but I think the BMS is very conservative regarding my Nominal capacity ?
I'm puzzled!!! (But the discrepancies are small enough to not call for yest at +90% SoC stress tests!)
(I am Danish and so do not normally participate in conspiracy theories, but perhaps the BMS does not want to UP my Nominal, because that means that Tesla will newer be able to offer me a refurbished battery on warranty, because it will have far less Nominal capacity than my old battery - just kidding!!
Following up on this thread. Have an update and some "guesses" since any detailed BMS programmed logic is invisible to us. But now have experience helping @mr_hyde to fix his battery pack and having been inside to see what hardware exist and how it works, we can see what hardware sensors is visible to BMS so can make some "guesses" at what the BMS is doing/reporting.
First on my "successful" rebalance to reduce imbalance from 21mV to 17mV @ 70 SOC ( post #71 ). It went back to 21mV in a few weeks. So net nothing was gained afterwards.
@Zuikkis post #77 may be showing a problematic BMB (board on each module) Each BMB has 6 voltage sensors on the 6 bricks in the module, have bleed resistors for balancing and 2 temp sensors. BMBs can have corrosion simply from the moist air (through air breather and equalizer) flowing into the pack and condensating. @mr_hyde BMB corrosions ( post link has detailed failure mechanisms ) are towards front of the pack where air breathing vent and air equalization valve is mounted. @Zuikkis car in Finland maybe have these issues. It could also be some seals in the rear of the pack (umbrella valves and hole seal caps) got compromised causing corrosion since Module 15 (near front of rear passenger tire) shows the imbalanced brick.
===
Regarding CAC. I think this is just a calculated number and probably similar to SOH (State of Health) in other EVs. I think of the pack in the following way
New pack has new cells and lowest resistance. Overtime, worn cells have growing resistance and reduced capacity. How does BMS tell what capacity is lost? A couple of papers below shows how to calculate battery resistance and calculate SOH
cellsaviors.com
www.e-motec.net
I would guess Tesla BMS calculates these with a load. Basically, take voltage reading before load, apply load (say a complete charge / drive discharge cycle ), take voltage reading after load. calculate. This is probably why we hear people getting a reman/rebuilt battery needing to fully cycle it a few times to improve maximum range.
Beyond just cell capacity wear, BMS also need to account for imbalance. Can't drive the car beyond lowest voltage bricks emptying its capacity first.
Therefore, I'd imagine improving range or capacity requires more balanced pack and less worn pack.
===
How to trigger rebalance in BMS is quite the mystery. I'd imagine Tesla's high conservatism to avoid battery fires would like BMS to collect a full spectrum of statistics (at minimum all bricks voltages on all SOC % over large temperature range window) and maybe some windows of high charge and discharge rates. BMS probably want to see the cells can behave safely over a wide range before trigging rebalance logic. As I understand, each module's BMB has the tiny bleeding resistors and BMS tell each BMB each brick's bleeding to voltage target. So BMS is very much the decision maker even though BMBs do all the work.
There are probably also other BMS programmed logic such as reduce supercharging rate and turn on battery chiller > 80% SOC on older age batteries (just assume worn and higher resistance thus more heat with higher current and SOC)
===
Regarding my pack and why it shows module all 6 bricks in module 9 and 11 (near front hump and behind passenger tire) having highest imbalance ( post #60 ). I'm guessing could be corroded or failing BMBs. Corrosion could be from the higher moisture ingress from front of the pack (just from air breather and equalization) or failed umbrella valves + caps (protective ski rails over these valves+caps usually packed with wet mud between A-B pillar from front tire spray in wet road regions, post #52). Another alternative could be BMB's onboard voltage sensor circuit becoming miscalibrated (have analog to digital converters etc). Don't really know unless pack is opened up and measure brick voltages directly.
===
Anyway, just some guesses based on some known hardware capabilities I'm aware of and battery architecture. Food for thought.
First on my "successful" rebalance to reduce imbalance from 21mV to 17mV @ 70 SOC ( post #71 ). It went back to 21mV in a few weeks. So net nothing was gained afterwards.
@Zuikkis post #77 may be showing a problematic BMB (board on each module) Each BMB has 6 voltage sensors on the 6 bricks in the module, have bleed resistors for balancing and 2 temp sensors. BMBs can have corrosion simply from the moist air (through air breather and equalizer) flowing into the pack and condensating. @mr_hyde BMB corrosions ( post link has detailed failure mechanisms ) are towards front of the pack where air breathing vent and air equalization valve is mounted. @Zuikkis car in Finland maybe have these issues. It could also be some seals in the rear of the pack (umbrella valves and hole seal caps) got compromised causing corrosion since Module 15 (near front of rear passenger tire) shows the imbalanced brick.
===
Regarding CAC. I think this is just a calculated number and probably similar to SOH (State of Health) in other EVs. I think of the pack in the following way
New pack has new cells and lowest resistance. Overtime, worn cells have growing resistance and reduced capacity. How does BMS tell what capacity is lost? A couple of papers below shows how to calculate battery resistance and calculate SOH
How to Measure Internal Resistance of a Battery - Cell Saviors
Looking to measure the internal resistance of a battery or cell? We have a step by step process to help get it done!
Energy (Wh): A Parameter for State of Health (SoH) Estimation
Kapil Faliya, Battery Algorithm Engineer at LION Smart explains how their algorithm predicts the exact remaining useful life of EV batteries.
www.e-motec.net
I would guess Tesla BMS calculates these with a load. Basically, take voltage reading before load, apply load (say a complete charge / drive discharge cycle ), take voltage reading after load. calculate. This is probably why we hear people getting a reman/rebuilt battery needing to fully cycle it a few times to improve maximum range.
Beyond just cell capacity wear, BMS also need to account for imbalance. Can't drive the car beyond lowest voltage bricks emptying its capacity first.
Therefore, I'd imagine improving range or capacity requires more balanced pack and less worn pack.
===
How to trigger rebalance in BMS is quite the mystery. I'd imagine Tesla's high conservatism to avoid battery fires would like BMS to collect a full spectrum of statistics (at minimum all bricks voltages on all SOC % over large temperature range window) and maybe some windows of high charge and discharge rates. BMS probably want to see the cells can behave safely over a wide range before trigging rebalance logic. As I understand, each module's BMB has the tiny bleeding resistors and BMS tell each BMB each brick's bleeding to voltage target. So BMS is very much the decision maker even though BMBs do all the work.
There are probably also other BMS programmed logic such as reduce supercharging rate and turn on battery chiller > 80% SOC on older age batteries (just assume worn and higher resistance thus more heat with higher current and SOC)
===
Regarding my pack and why it shows module all 6 bricks in module 9 and 11 (near front hump and behind passenger tire) having highest imbalance ( post #60 ). I'm guessing could be corroded or failing BMBs. Corrosion could be from the higher moisture ingress from front of the pack (just from air breather and equalization) or failed umbrella valves + caps (protective ski rails over these valves+caps usually packed with wet mud between A-B pillar from front tire spray in wet road regions, post #52). Another alternative could be BMB's onboard voltage sensor circuit becoming miscalibrated (have analog to digital converters etc). Don't really know unless pack is opened up and measure brick voltages directly.
===
Anyway, just some guesses based on some known hardware capabilities I'm aware of and battery architecture. Food for thought.
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It would be very interesting to pull back the curtain and learn what the Wizard is doing pulling all the ropes and levers inside the BMS/BMB logic. In every instance, Tesla climbs on the desk and shouts the S word as loudly as they can (Safety!) to justify limiting capacity, charge speeds and tightening up tolerances for things like the imbalances that will trigger BMS_u029. At the end of the day, they decide it is better (or less bad) for the brand to have reduced performance or additional battery replacements needed than push the envelope and have even one preventable battery fire.
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