Measuring Battery Capacity Loss

[Zoomit]

Using the data available from the car, or the OnStar app, how would one measure the total battery capacity and how much it degrades over time? I understand the battery capacity is 18.4 kWh, and all of that is available, off the showroom floor.

Based on some generic EV charts, I'm guessing that my usage will degrade the battery 10-15% over 30,000 miles and I'd like to keep track of that loss.

 

[Zoomit]

I should reference the previous discussion of this topic in this forum: What's the total, and usable, battery capacity?

In it, Nashco states, "The only way to track the usable battery capacity yourself is to run it full to empty in the same conditions and record how many kWh are used from full to empty. GM hasn't made any other mechanism available."

 

[Nashco]

Only way to accurately track pack capacity is to charge it completely full, then discharge it with a consistently recreated use case. For example, x degrees going y mph steady state, until the battery is empty or extremely close to it. If you monitor the kWh used display, you'll know how much battery capacity was available. You'd then have to recreate this exact same scenario from year to year.

I've lost roughly 5% capacity in 18 months and 13k miles. I don't baby the battery and I have the A123 (2014 model year) battery. If you've got a 2015, then it's a new ballgame due to an entirely different battery.

Bryce

 

[Zoomit]

I don't think you have to have a "consistently recreated use case." Isn't the only requirement to start from a fully charge car and use all the energy? I don't think it matters how far the car is driven or under what conditions. The only value that matters is how much energy is consumed.

In fact, I think you could take a fully charged car and, without driving anywhere, max out the cabin heater and let it drain the battery over a few hours. At the end of that, the car will display how much kWh of energy was used, which gives you the total battery capacity.

 

[Nashco]

I don't think you have to have a "consistently recreated use case." Isn't the only requirement to start from a fully charge car and use all the energy? I don't think it matters how far the car is driven or under what conditions. The only value that matters is how much energy is consumed.

In fact, I think you could take a fully charged car and, without driving anywhere, max out the cabin heater and let it drain the battery over a few hours. At the end of that, the car will display how much kWh of energy was used, which gives you the total battery capacity.

If you have different temps and discharge currents, you have different battery efficiency, which results in more or less output with the same stored energy to start with. This is why you need a specific use case that you can recreate consistently, otherwise your results can't be compared.

Bryce

 

[Zoomit]

Excellent point. Gross degradation could be shown without a repeatable test, but to precisely measure the loss, you'd need to match the battery conditions (temp and discharge current) from test to test.

 

[Zoomit]

Or you could average the results of many tests. Similar drives, such as a commute, reduces the variability and could yield a reasonable estimate to compare capacity loss over time. The challenge here is that a commute doesn't completely drain the battery (hopefully!).

What about using the battery charge level as reported by the OnStar app to back out the full battery capacity? By noting charge level percentage and the kWh used in the vehicle display both before and after a drive, you can calculate how much energy the car considers to be in the battery at 100% charge.

Using data I've collected from my car over 22 somewhat similar ~30mi trips, I calculated a battery capacity of 17.9 kWh with standard deviation of 0.4 kWh. The car's mileage was between 250 and 1000 miles.

There is at least one issue with this method. The battery level percentage occasionally increases after turning off the car and, presumably, the battery recovers slightly. A reading from the app could be 65%, for example, immediately after stopping, but it may increase to 66% within a hour or so. My data was taken immediately after stopping, which could explain the slightly low estimate for the new 18.4 kWh battery. The difference of 0.5 kWh is 3% low and in line with the variability seen after the battery recovers.

All this assumes the car is recalibrating the 100% charge level as the battery degrades. I started a separate topic about this particular question here: Do degraded batteries still show as 100% when full? Of course, if the car didn't recalibrate with the battery, we could just directly watch the degradation as the car reports subsequently lower percentages (98%, 95%, …92%) as "fully charged". But I don't think that is how the car works.

 

[ImTedBell]

Why don't we take the driving out of the test to make it easier to reproduce? Just run the heat at full blast in the driveway with the windows down on a cold day. Just run it till the battery dies and check the estimated kwhr in the consumption display

You may have to defeat the 90 min shutdown timer tho.

 

[Nashco]

I said consistently recreated use case. You could do the heater at full with the windows down and a specific outside temp, but that would require you leaving your car open and turned on for multiple hours each time. This is probably pretty hard on the heating system as well. I find driving a stretch of road in cruise control when there's no traffic to be a bit more enjoyable than heating the outdoors, personally.

Bryce