Powerwall: Assessment of degradation of storage capacity after 8 months

Friends, it is now 8 months (235 days) since we installed the Tesla Powerwall domestic battery (link).

And one of the questions I am most commonly asked concerns its likely lifetime.

All batteries degrade over time, even Tesla’s, but the practical question is “How much degradation occurs and over what period?”

By chance last night (8th/9th November) I had the opportunity to assess this degradation and, in case you are short of time and need to do something more important than read this blog, my estimate is that the battery degradation so far is immeasurably small.

For those of you still with me, allow me explain how I made the measurement and what the results suggest regarding the battery lifetime.

Powerwall Control

The Powerwall battery is apparently controlled by an ‘App’ on my phone.

The ‘Tesla App’ has pleasing – almost compulsively enticing – graphics showing power flowing to and fro from the grid; the battery; our house; and the solar panels. It’s a really engaging interface. It also allows detailed data to be downloaded for analysis.

Click image for a larger version. Screenshots from the Tesla ‘App’ showing the charging and discharging of the battery, its state of charge, and the overall operation of the battery system.

However, I say the battery is ‘apparently’ controlled by the App because in reality the battery is controlled and monitored 24/7 over the internet by Tesla. And Tesla give me only limited control via the App.

Currently the battery is set to ‘Time-Based Control‘ which charges the battery from solar PV when available or cheap rate electricity if required.

And Tesla make the choice about how much charge to take overnight based on it’s estimate for how much solar power will be available the following day.

I don’t have the algorithm it uses, but it seems to do a fair job.

The reason I consent to this egregious interference with my liberty is that in return for ceding control, Tesla promised that the battery would retain 80% of its specified 13.5 kWh capacity (i.e. 10.8 kWh) in 10 years time i.e. after a nominal 3650 partial charge cycles.

I think this is a guarantee worth having and so I submit to the Tesla-Brain.

In fact since I bought the Powerwall I believe this guarantee has been degraded to 70% after 10 years – which suggests it really is quite a tough specification.

Expected Battery Degradation

Various reports on the web, and Tesla’s 2020 environmental impact report, indicate that Tesla car batteries seem to retain around 90% of their range after 200,000 miles (320,000 km).

Click image to see a larger version. Excerpt from Tesla’s 2020 Environmental Impact Report showing roughly 10% reduction in EV range after 200,000 miles.

It’s hard to know how that colossal range would translate into the 3650 partial charge and discharge cycles of a domestic battery over 10 years.

In part it depends strongly on the range of the charge-discharge cycles. Charging and discharging over the middle of a battery’s range – between say 10% and 90% – is relatively benign. But rapidly charging and discharging from 0% to 100% degrades battery capacity. This is why Tesla want to have control over the battery.

My thought when I bought the battery, was that domestic service would be generally less stressful than service in a motor car. Why?

• The Powerwall has it’s own re-circulating fluid temperature control and does not need to operate in the climate extremes of a car battery.
• A Tesla car battery is about 4 times larger than a Powerwall’s 13.5 kWh, but the maximum EV discharging rates – which can affect battery life – are up to 25 times higher than the Powerwall’s transient maximum of 7 kW.

Other reports (link) suggest that Powerwall degradation might be considerably faster than for EV’s.

However, the general pattern of battery degradation (reflected in the figure above) is that the greatest rate of degradation is at the start of the service life of the battery.

If my Powerwall were to degrade linearly to 80% capacity over 10 years (120 months) then after 8 months I might expect to see 0.18 kWh decrease in capacity. Small, but possibly detectable.

What did I measure?

By chance last night the battery ran out just before midnight, so I knew it had zero ‘state of charge’.

Additionally the Tesla-Brain decided to fully charge the battery over the four hours of cheap rate (5p/kWh) electricity starting at 00:30. So I was able to observe a full charge from empty.

The graphs below (using data downloaded via ‘the App’) show what happened. Note the data only have a time-resolution of 5 minutes.

Click image for a larger version. Graph showing the charging of the battery from 00:30 at approximately 3.6 kW and the discharging of the battery after 04:30 at approximately 300 W to meet domestic demand.

Using the charging rate and the time I can work out the ‘state of charge’ of the battery and compare this with the specified capacity.

Pleasingly, the maximum state of charge appeared to correspond closely with the initially specified capacity.

Click image for a larger version. Graph showing the calculate ‘state of charge’ of the battery during charging from 00:30 to 04:30. Within the (considerable) uncertainties of this measurement, the maximum state of charge is closely in line with its specified original capacity.

Interestingly, while the battery was charging, the Tesla control circuitry also used cheap-rate electricity to run the dishwasher and top up the domestic hot water using the heat pump.

Click image for a larger version. Graph showing the household demand from midnight to 06:00. The battery was charging in the background during these high power events.

Conclusions

First of all, some caveats:

• All these measurements are self-reported by the Powerwall, and so should rightly be subject to sceptical interpretation.
• The data have limited resolution both in power and time.

However, when I have been able to check the reported values against independent measurements – e.g. for estimates of the energy reaped from the solar panels each day – I have found them in close agreement at the level of 0.1 kWh.

So taking these measurements at face value, I find no detectable degradation in battery capacity after 8 months or 235 days.

• If the battery capacity were degrading linearly over time to 80% of initial capacity after 8 years I would have expected to see 0.18 kWh decline in capacity.
• If the battery capacity were degrading faster than linearly – as it plausibly might – then I would have expected to see perhaps 0.3 kWh or 0.4 kWh degradation.

Obviously I will re-visit this issue at some point in the future, but the fact that there is no detectable degradation so far suggests that the retained capacity after 10 years may indeed exceed 80%.

Which would be nice.

Tags: Batteries, Powerwall