How to measure battery degradation

[thimel]

I'd like to find the best way to measure the gradual degradation of battery capacity that is expected to happen over the years. My thought is to find a reasonably accurate and reproducible measurement method and then measure it every six months or so to see how it degrades.

Here is the best method I can think of but I am hoping others can improve on it.

Charge the car to 100%.
Reset the trip odometer.
Reset the average mi/kwh meter.
Drive without climate control or lights until a desired discharge level (could be to the first low battery warning or to a transition from n to n-1 battery gauge bars.)
Calculate (total kwh used) = (miles on trip odometer)/(average mi/kwh).

In principle, this measurement will not depend on how you drive. In fact, it probably will because of internal resistance in the battery, so it will be best to try to drive in a reproducible fashion (perhaps 55 mph on the freeway).
Depending on how the mi/kwh is calculate, regenerative braking may screw up the measurement.
As the battery capacity depends on its temperature, it would be best to always do this at about the same ambient temperature.

Does anyone have a better idea? Of course, discharging the pack into a resistor while measuring its voltage would be far more accurate, but this takes equipment most owners don't have.

 

[turbo2ltr]

Hmm, watch the capacity meter on the dash?

 

[thimel]

That capacity meter has quite poor granularity (8% per bar) and a completely unknown algorithm behind it.

 

[AndyH]

Just take the car to the shop and have them pull the cell internal resistance number out of the computer. The car's already using voltage, internal resistance, and capacity changes to calculate the degradation number it uses to adjust the bars. Or just want for Turbo to finish sleuthing the codes.

Make the test runs - absolutely - it'll be fun! But on those days we're feeling like couch potatoes, we can just ask the car.

 

[Electric4Me]

Just take the car to the shop and have them pull the cell internal resistance number out of the computer. The car's already using voltage, internal resistance, and capacity changes to calculate the degradation number it uses to adjust the bars.

Are you saying that the car will adjust what "full" means over time as the battery degrades, always showing all the bars when it's holding as much energy as it's currently able to?

 

[AndyH]

There are two sets of bars in the 'fuel gauge.' The inner (and wider) bars show how much fuel we have remaining from a 'full tank', while the outer shows the capacity loss over the life of the pack.

Think of capacity loss like a gas tank that shrinks over time. Half a tank is still half a tank (the inner bars) but the tank goes from 10 gallon capacity when new to 8 gallons after 10 years.

The service manual clearly describes the way the Leaf tracks pack degradation - and it's more accurate than running a range test.

That being said, we'll still need to drive the car and explore the summer range and winter range and hilly route range, etc. so we know what the capacity loss means to us.

I hope that makes sense.

edit...spelling...

 

[thimel]

AndyH, any chance you could post the information from the service manual that explains how they calculate degradation?

Turbo, maybe you could add a digital version of this degradation number to the product you are developing.

Thanks,

 

[AndyH]

The easiest thing to do is pull the internal resistance numbers for the cells. It's probably stored in the battery control module inside the pack. The manual explains that the numbers have to be reset if a cell or battery module is replaced.

Here's an example from one of my LiFePO4 packs. Note internal resistance (AC Ri Charged mOhm) and capacity. See how Ri shows the good and bad cells? (Ignore 11 - it's a typo - should be 0.80.) Also note that there's plenty of capacity variation in these el-cheapo Thunder Sky cells.

The car can cross-check the Ri number by checking voltage drop under load (higher Ri means more sag).

In addition, if he can pull it out of the traffic, Turbo may be able to get the 'final' degradation number the car sends to the instrument panel.

Andy

 

[DaveinOlyWA]

why dont u take Nissan's offer for a battery degradation check provided free of charge every 6 months?

or did i dream that in a "what i wish would happen?" scenario?

 

[AndyH]

why dont u take Nissan's offer for a battery degradation check provided free of charge every 6 months?

or did i dream that in a "what i wish would happen?" scenario?

You got it - it's a required warranty visit.

 

[thimel]

A while back someone posted what I took to be the Nissan battery report. It just had a bunch check marks under the "OK" column IIRC. I don't recall seeing a number for battery degradation.

 

[AndyH]

If one wants to track pack degradation, they need to remove all other variables. The absolute worst way to test anything about a vehicle is to take it on the road because it's impossible to drive the same way twice in a row. Temperature differences, feeling in the right foot, attitude, mood, expectations, wind, etc. etc. all enter into a road test and introduce waaay too much noise for the results to be useful.

That's why lubricants are tested in a test cell with a freshly rebuilt engine running standardized fuel with conditions adjusted to a standard atmosphere according to a standardized test procedure and conducted by a trained tech. That's also why the LA4 and US06 test cycles are performed on a dyno.

You asked about the service manual info on capacity loss. It's all over the place.

The "Li-Ion Battery Gradual Capacity Loss Signal" is circulated on the CAN bus. It's likely the message used to set the outer bar on the instrument display. (EVB-16)

"The Li-ion battery controller is the core of battery control. This Li-ion battery controller detects the voltage and current of the assembled battery, the temperature of each module, and the voltage of each cell to judge SOC (state of charge) and calculates possible input/output values, meter indication value, and chargeable value to send these data to VCM (vehicle control module). VCM controls the vehicle, according to the battery state."(EVB-12)

"The Li-ion battery controller performs control as follows:
• Monitors the battery state and transfers chargeable/dischargeable power to VCM (vehicle control module) to prevent an error, such as overvoltage, over discharge, overcurrent, and an excessive temperature rise in the battery.
• Detects an error (overvoltage, over discharge, overcurrent, or excessive temperature rise) immediately at the time of error occurrence and requests the system main relay shutoff to VCM to shut off the discharge/ charge line.
• Maintains the optimum battery state constantly with a cell capacity adjustment function to prevent a reduction in charging/discharging capacity caused by cell capacity variations.
• Detects the connector fit state with the function to detect the fit of the high voltage harness connector and transfers the detected state to VCM so that the vehicle does not start with an unsteady state.
• Detects the insulation resistance state with the function to detect the insulation resistance between high and low voltage and transfers the detected state to VCM so that the vehicle does not start with an unusual state.
• Estimates a battery charge state and low battery state, based on the data obtained with the battery state detection function, and reflects on the battery capacity meter."
(EVB-16, -17)

"Li-ion Battery Capacity Level Gauge (1)
VCM receives the Li-ion battery capacity signal from the Li-ion battery controller, and VCM transmits the signal to the combination meter to display the maximum capacity of the Li-ion battery. When the capacity of the Li-ion battery decreases with age and usage, the level of the gauge will also decrease." (EVC-21)

"Perform Li-ion Battery Gradual Capacity Loss Data Clear when the Li-ion battery pack assembly or Li-ion battery
controller is replaced with a new one. VCM saves the Li-ion battery deterioration information sent from the
Li-ion battery controller and manages the Li-ion battery replacement timing. When the Li-ion battery pack
assembly or Li-ion battery controller is replaced with a new one, there is a difference between the Li-ion battery deterioration data stored in VCM and the actual Li-ion battery deterioration level. In this case, VCM will incorrectly manage the Li-ion battery replacement timing. So perform Li-ion Battery Deterioration Data Clear to clear the Li-ion battery deterioration data stored in VCM." (EVC-103)

(EVB-116)

"If a malfunction such as abnormal voltage occurs with a cell of the Li-ion battery, replace the module containing the malfunctioning cell. When replacing the module, calculate the adjustment voltage value from the maximum cell voltage of other normal cells and adjust the replacement module to that adjustment voltage value."
(EVB-56)

 

[Herm]

You could also jack the front wheels up in the air and run the car with the cruise control set at 90mph.. should be good for 2-4 hours until the battery is fully discharged.. record the range. Maybe a good idea to jack it up and remove the front wheels.

It would be best if you did not do this under the full sun.. and record the ambient temperature so you can duplicate the test in a couple of years.

 

[DaveinOlyWA]

You could also jack the front wheels up in the air and run the car with the cruise control set at 90mph.. should be good for 2-4 hours until the battery is fully discharged.. record the range. Maybe a good idea to jack it up and remove the front wheels.

It would be best if you did not do this under the full sun.. and record the ambient temperature so you can duplicate the test in a couple of years.

lol...oh ya right.

no mass, no wind resistance... bring a book!!

 

[garygid]

At least one dealer has offered to do the "yearly" battery check more often, like initially and a 3-month intervals, and FREE for our little group of Owners.

Check what your dealer will charge for the required annual battery-health checks. I have not seen anything that suggests that these checks are normally "free", but I am not yet an "Owner".

 

[DaveinOlyWA]

i was under the impression that the battery checks were free and part of the warranty maintenance program...if not free, how much is it?

 

[Herm]

lol...oh ya right.

no mass, no wind resistance... bring a book!!

Put a fan on it, blow the fumes away!

 

[DaveinOlyWA]

lol...oh ya right.

no mass, no wind resistance... bring a book!!

Put a fan on it, blow the fumes away!

ummm what?

 

[mwalsh]

Put a fan on it, blow the fumes away!

Actually, not such a bad idea...gotta try and keep those coolant temps down, otherwise you'll be blowing hoses!

 

[DaveinOlyWA]

With wheels jacked up? U would get probaly a few thousand "miles" from a charge and I b willing to bet she would run pretty cool even at 90 mph