tinoale wrote:About that, what is the exact purpose of this battery age value in the BMU ?
My guess is that when you replace the entire traction battery with a "new" one, you enter it's age so that the initial Ah capacity estimate is not that of a brand new battery but that of one that has sat in a warehouse for that amount of time.
Some have noticed that they could reach 0% of SoC while the lowest cell voltage is still relatively high (3.2V for instance). Thus, not really using the whole battery capacity that they have available.
I'm not sure that I'd call 3.2 volts relatively high. My car - at least back when I tested a full discharge when it had about 37Ah capacity, shut down completely at 3.63 volts/cell. Keep in mind that voltage falls much more quickly below about 20% SoC, so there is actually very little capacity between 3.63 volts and 3.2 volts - maybe 5% at most.
Also you have to allow for voltage drop under load. For example if you never want your cells to go below 3 volts at any time, discharging to 3.2 volts unloaded would give a voltage below 3 volts when loaded. So a higher cutoff voltage is used to allow for voltage drop under load to prevent the cells going under voltage when you accelerate.
They suggest that resetting the age value will force the BMU to re-learn the actual capacity of the battery based solely on cells voltage.
This will make the car overestimate the available capacity for a little while, as long as the car learns the actual capacity.
What do you think ?
BTW I have apparently (by mistake) done just that. I went in BMU menus (Diagbox) looking for that option. I did not go into all the screens so I did not think I had actually done anything but this morning I had 45.7Ah in CaniOn, so I guess I went one click too far
I had 33.7Ah right before.
Anyhow, it is done so... I guess I'll see how it goes from now.
kiev wrote:That's amazing--we have to figure out what you did to get those results.
Resetting the BMU's Ah estimate is easy - I've just done it on my car to run the same experiment. (Because, why not ?
I took screenshots but since it's a PITA to attach images to this forum I'll just describe it.
In the repair section you go into the BMU, in the maintenance procedures there is one "Traction Battery Replacement Procedure". Running this just takes seconds and at the end of it the Ah capacity reported in Canion jumped up from 34.0 to 45.8Ah.
So I guess that answers the age old question of what is the capacity of a new battery in these - according to the BMS when reset it's 45.8Ah !!! This is very close to the 46Ah I had estimated via other means so I'm chuffed with that... Interestingly in the first few minutes the figure dropped to 45.6 and then 45.3 - this with the car just sitting on the driveway and the key being turned on and off a few times, but it seems to be steady at 45.3 now - until I drive it, when it will no doubt make a large downwards correction. (it's currently charging to 100% and will be used again later today)
Of course the car is now greatly over estimating the usable capacity of the battery which means I need to be careful with my range as "unexpected" appearance of the tortoise is very likely near it's true range when the BMS gets a wake up call... so I'm going to monitor every driving session for the next few days with Canion to see what happens with the Ah figure and how often and when it makes corrections.
Interestingly after running the battery swap procedure it tells you:
"If the new traction battery was manufactured more than 3 months ago, carry out the traction battery maintenance procedure the date of production of the battery is indicated at the rear of the traction battery and is accessible through the rear LH wheel arch".
I've run the traction battery maintenance procedure once before and I don't remember it asking for a manufacture date of the battery.
The maintenance procedure is very simple and it basically just instructs you to turn the car on and set the heating to maximum and wait until "a" cell voltage is between 3.75v and 3.775 volts, (which is approx 20% SoC) then gets you to plug the charger in to do a 100% charge - at the end of that a correction is made to the Ah figure.
Back when I ran this I was already seeing more rapid degradation of my battery, and the figure went down from 37.6Ah before the test to 36.1Ah after the test
although it did rebound by itself to 37.1Ah about 400 miles later...
Interestingly today I noticed a page in the BMU called "BMU capacity after maintenance". This has many rows of "Maximum capacity of the cells after the last traction battery maintenance" and "Minimum capacity of the cells after the last traction battery maintenance".
For the first pair it gives 41.0Ah for maximum capacity and 38.1Ah for minimum capacity - this will have been recorded during the same test where it adjusted the usable Ah capacity to 36.1Ah and then rebounded to 37.1Ah. The remaining rows are all zero which presumably means the test has never been run before in the life of the car.
So my interpretation of this is that running the battery maintenance routine measures the spread of cell capacity from the worst to best cell, and at that time (more than a year ago) there was already a 3Ah spread. I'd love to know what the spread is now, so I might try running the test again next weekend. (As it takes nearly a whole day to run counting discharging and charging, and I need to use the car this weekend...)
One thing we've learnt from this is that it's unfortunately very easy to temporarily fudge the Ah capacity to a high value by using a diagnostic tool, just like you can on a Leaf, which means it's something unscrupulous sellers could do...