Friends, three months ago in December 2022, I wrote about my technique for estimating the degradation of the capacity of my Tesla Powerwall 2 battery (Link).
The idea was to consider data from days in which the battery is charged to 100% capacity at night, and then discharges fully to 0% during the day. This only happens in winter when household demand is high and solar PV generation is low. After correcting for any solar generation one can make a reasonable estimate for the practical working capacity of the battery.
Click on diagram for a larger version. Illustration of day in which the Powerwall fully discharged.
We have now had another 60 days during which the battery discharged fully and using the same technique I described previously, I have re-evaluated the degradation of battery capacity. The results are shown below in two graphs: it’s the same data in both graphs they are just plotted on different times scales.
Taking all the data into account, the trend line suggests that battery capacity is degrading at roughly 2.6% per year. Since the battery has a nominal capacity of 13.5 kWh, this corresponds to a loss of capacity of 0.35 kWh/year.
Click on image for a larger version. Full discharge capacity plotted versus date. The blue dots show all the data and the dots surrounded with a pink circle show data with solar contribution during the day. The trend line suggests capacity is being lost at 2.6%/year.
The Powerwall was installed in March 2021 and if the degradation were to continue at 2.6%/year in future years then the battery would lose 10% of its capacity by the winter of 2024, and 20% of its capacity by the winter of 2028.
Click on image for a larger version. Full discharge capacity plotted versus date. The blue dots show all the data and the dots surrounded with a pink circle show data with solar contribution during the day. The trend line suggests capacity is being lost at 2.6%/year.
What to do?
Nobody wants their £10,000 battery to be losing capacity. But there is very little I can do about it! It’s inherent in the nature of the batteries and of the charge/discharge cycles they experience.
One option would be to prevent the battery from fully discharging by forcing it to retain, say, 1 kWh in reserve. However, while this might reduce the rate of degradation, it would deprive me of the battery capacity that I was hoping to preserve!
So my plan is to do nothing. If the degradation continues at this rate I will still have a 10 kWh battery in 2031 – and that’s still a very useful size of battery.
If I feel the need for more storage, my thought is that it would probably eventually make sense to upgrade one of my solar PV inverters to be a hybrid inverter, and then add extra battery capacity in the loft. These batteries will have the Lithium Iron Phosphate chemistry that is supposed have very low degradation. This option would likely cost much less than buying a replacement – or additional – Powerwall.
Of course, by the time this becomes important Powerwalls may have fallen in price and be readily available (!). Or the use of batteries in vehicles for domestic storage may have become commonplace.
In short, this is tomorrow’s problem.
Protons for Breakfast 
March 6, 2023 at 12:21 am |
But if limiting the battery to 12 kWh now means it will still have 12 kWh capacity in 2031… Surely that’s worthwhile?
March 6, 2023 at 9:53 am |
Dan,
Good Morning. If reserving 1 kWh were guaranteed to produce longevity then it might make a sensible option. But to have any reasonable chance of stopping degradation I think I would need to limit capacity as well – and I don’t know how to do that.
Also, although a linear trend is the most likely, I am hopeful that the rate of degradation will slow down.
And in the worst case, my experience can act as a warning to others!
Best wishes
Michael
March 6, 2023 at 10:16 am
True, while limiting capacity to 12 kWh (by setting a 1 kWh reserve) would reduce degradation, it wouldn’t eliminate it.
And your points about 2031 likely being very different to 2021 for consumer energy storage choices are very well made. I think V2G is surely inevitable once available with CCS as it makes so much sense.
I should resist commenting after midnight 😄
March 12, 2023 at 11:50 am |
Have you attempted to control for temperature more than just comparing winter to winter?
March 12, 2023 at 12:47 pm |
Ben, good afternoon.
No. The Tesla battery has it’s own temperature control. They can be placed on any outside wall but my battery is located in the porch of the house so is not exposed to wind – a relatively benign environment.
All the best
Michael