jray3 wrote:Coulomb, thank you for your ongoing efforts. I'll throw in another data point. I'm planning to replace MR BEAN's battery with a junkyard salvage unit shortly after 100k miles, currently at 99,220 on the odometer.
Current stats according to my MUT3 clone:
Battery current capacity: 27.3 Ah
Battery maximum input power: 45.75 kW
Battery maximum output power: 63.50 kW
Voltage spread between high and low cells is now as high as 0.19 V.
Just so you know, the maximum input power and maximum output power figures are not a measure of the health of the battery with respect to degradation, so you won't see any difference here between an old battery and a new one.
They're actually real-time figures used by the BMS to inform the MCU and other peripherals what the maximum amount of power the battery can supply or receive at any given time so that charging/regen/discharging are limited to remain within these safe limits. These limits are based on the current SoC and cell temperatures.
For maximum input power (charging, regeneration) this is severely limited at 100% SoC (only about 8kW) and gradually increases to 45.75kW by the time you get down to (approx) 80% SoC. It is also affected dramatically by temperature. Below 12C it is limited to no more than about 24kW, and below 0C it is down to around 12kW. (approximate figures off the top of my head)
This limit affects both regeneration and rapid charging, although I have noticed that at high SoC it still doesn't charge as fast as this maximum input power limit, however that may be due to me having a couple of cells with higher than normal internal resistance. (Which limits rapid charging speed)
High cell temperatures would also limit maximum input power however I think the cells have to be well past 40C before it will start to limit charge rate and it's rare to get the cells on these cars that hot. (At least in the UK!)
Maximum output power limits maximum acceleration, however this figure doesn't vary nearly as much. It does drop a little when you get down to about 10% SoC, (which is why turtle mode can limit acceleration) and it may also drop slightly at below freezing temperatures or cell temperatures above 40C, but only a little.
So looking at these two figures at a random SoC or cell temperature doesn't really tell you much of anything.
At cell temperatures between 12C and 40C and SoC between about 20% and 80% I would expect these two figures to be at their maximum.
DCFC sessions are running significantly longer these days with a degraded battery plus temperatures near freezing. The pack warms up nicely, perhaps too warm at 39 Celsius (102 F) on module 3 and ten degrees C cooler elsewhere in the pack.
If it was only the cell capacity that has degraded then rapid charge times would actually get faster. The thing that slows down rapid charging time is increased cell internal resistance, which is usually a sign of cell(s) that are starting to fail prematurely.
I know this first hand as in the last year two of my cells have dropped significantly in capacity relative to the others and their internal resistance is higher as well. This high resistance causes them to reach the peak cell voltage of 4.105 volts too early during a rapid charge (at a lower than usual SoC) and as a result the BMS is forced to throttle the rapid charge rate back sooner than it should have to, even though all the other cells could take a higher charging rate.
This can clearly be seen in the Canion voltage graph - the two high resistance cells get to 4.105v very quickly during the rapid charge while the rest are around 4.075v at the same charge rate - those other cells could take a much higher rate before hitting 4.105v.
When i first got the car it would maintain the full 43kW up to about 55% SoC and stay above 22kW up to about 70% in warm weather. (Cells 20-30C)
Now with two high resistance cells even if I start at 20% SoC in warm weather it will only remain at the full 43kW until about 30% SoC, and is down to about 22kW by 50% SoC, 11kW by about 70% SoC and 7kW by 80% SoC.
Charge time from 20% to 80% used to be about 18 minutes, now it's about 28 minutes with the last 10% being really, really slow! (And sometimes it shuts itself off at 70% ish instead of waiting until 82% like it used to, so I have to restart it to get to 82%)
If you have Canion it's fairly easy to test for individual cells that have high internal resistance - first you have to get the battery quite warm - all cells between about 20-35C, ideally 25-30C as that gives minimum cell internal resistance and maximum rapid charging speeds. Not easy in winter but possible if you do a couple of speedy motorway sessions with a rapid charge in between.
Before charging, take a note on Canion of any cells that have unusually low voltage compared to the rest at 20% SoC. These cells are ones with lower capacity and potentially (but not necessarily) have high internal resistance as well.
Put it on a rapid charge and watch the cell voltage graph - if you have a weak cell that also has high internal resistance what you'll see is that before the charge it had a lower voltage than most other cells but during the charge it's voltage will almost immediately go higher than most other cells - if it goes from lower than the others to significantly higher than the others quite quickly on a rapid charge this is a sure sign that the cell has higher internal resistance as it will cause the voltage to be excessive under charge.
If you continue to monitor it until the charge rate starts to drop below 43kW you would see the high resistance cells sitting pegged at 4.105 volts, while other cells may be 20-30mV lower. That's what I see on mine anyway.
If I could get my hands on a couple of good cells I'd be tempted to replace them, because while I can put up with some capacity loss (now down to 35.2Ah after a series of large drops from the 39.9Ah I had two years ago) the much slower rapid charging speed is a real pain in the behind when I do actually want to rapid charge.