Assessing Powerwall battery degradation

Click on image for a larger version. Three screenshots from my phone showing the performance of the battery on 9th and 17th January 2022, and 11th December 2022. The key data concerns the total amount of energy discharged from the Powerwall. See text for details.

Friends, the Tesla Powerwall2 battery that we installed in March 2021 has transformed the way we use electricity and allowed us to go off-grid for prolonged periods each year. I have no regrets.

But lurking at the back of my mind, is the question of battery degradation.

This phenomena arises due to parasitic chemical reactions that occur as the battery approaches either full charge or full discharge. These reactions ‘capture’ some lithium and remove its ability to be used to store charge. Hence one expects the capacity of a battery to decline with extended use, particular near the extremes of battery capacity.

This particularly affects batteries used for domestic applications as they are often charged fully and then discharged fully – particularly in the winter.

The extent of the degradation depends on the specific chemistry of the battery. More modern battery chemistries labelled as ‘LiFePO4: Lithium Iron Phosphate” perform better than the previous best in class so-called “NMC: Nickel Manganese Cobalt”. Unfortunately, the Powerwall2 uses NMC batteries. This article has a comparison of the properties of different lithium-ion battery chemistries.

Battery degradation is a real phenomenon, but unsurprisingly, battery manufacturers do not make it straightforward to spot. I first looked at this about a year ago, but I don’t think my analysis was very sensible.

I now think I have a better method to spot degradation, and 20 months after installation, initial degradation is apparent.

Method. 

The new method looks at data from winter days during which the battery is discharged from full to empty, with little or no solar ‘top up’.

In winter our strategy is to charge up the battery with cheap electricity (currently 7.5p/kWh) between 00:30 and 04:30 and to run the house from this until the battery is empty. When it’s cold and the heat pump is working hard we can use up to 30 kWh/day and so the nominal 13.5 kWh of stored electricity is not enough to run through the day. So we run out of battery typically in the early evening and then run off full-price electricity until we can top up again.

The run-time can be extended by a top-up from the solar PV system, which can be anything from 0 kWh in overcast conditions, up to around 7 kWh in full December sun.

My idea is to measure the Powerwall’s total discharge and to compensate for any solar top up. By restricting measurements to days when the battery goes from full to empty, I don’t have to rely on estimates of battery remaining capacity. These days mainly occur in December and January.

For example, today (12 December 2022), the battery was charged to 100% at 04:30 and discharged 12.8 kWh to give 0% just after midday. I the estimate battery capacity as 12.8 kWh.

But on 7 December 2022, the battery was charged to 100% at 04:30 and discharged 15.5 kWh to give 0% just 22:00. This was a sunny day and the battery was topped up by 3.0 kWh of solar. I thus estimate battery capacity as 15.5-3.0 = 12.5 kWh.

In this latter case the way to compensate for the 3.0 kWh of charging is not clear. Why? Because the 3 kW of solar is used to charge the battery and so this may be done with say 95% efficiency (say) in which case only 2.85 kWh of solar energy would be stored. So there is some ambiguity in data which is solar compensated, but for this analysis I am ignoring this difficulty.

The data are shown below:

Click on image for a larger version. Graph showing total Powerwall discharge after compensating for any solar top-up. See text for details.

Discussion. 

The nominal capacity of the Powerwall2 is 13.5 kWh. This is – presumably – the stored electrical energy of the batteries when they are fully charged. To be useful, this energy must be discharged and converted to AC power, and this cannot be done with 100% efficiency.

Considering the data from the winter of 2021/22, the average Full-to-Empty discharge was 13.1 kWh, and so it looks like the discharge losses were around 3%. I think this is probably a fair estimate for the performance of a new battery.

The data show a considerable amount of scatter: the standard deviation is around 0.2 kWh. I am not sure why this is. Last winter, the battery would sometimes only charge to 99% rather than 100% and I corrected for this. That is why the capacity data do not lie entirely on exact tenths of a kWh.

Considering the data from the winter of 2022/23, the average Full-to-Empty discharge is currently 12.8 kWh. This represents a reduction in capacity of 2.3% (0.3 kWh) compared with last winter. However, there is a whole winter ahead with another 50 or so full discharges before spring and that average could well fall.

If the trend continued then battery capacity would fall to 10 kWh in around 2030. That would still be a useful size battery, and by that time hopefully a newer (and cheaper!) model will be available.

I‘ll be keeping an eye on this and will write an update at the end of the winter season. But I thought it was worth publishing this now in case fellow battery owners wanted to monitor their own batteries in a similar way.

 

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11 Responses to “Assessing Powerwall battery degradation”

  1. Bruce MacNeil Says:

    Fascinating – there is useful information here.

    I assume some of the same logic applies in keeping an EV battery charged between 20% and 80% and avoiding the extreme ends.

    Cheers – appreciate your always interesting analysis.

    Bruce

  2. David cawkwell Says:

    I am currently saving my pennies for an electric EV like the MG that has vehicle to load/vehicle to house ability. That batteries in an EV mean it is like buying a battery and getting the car for free.

    • protonsforbreakfast Says:

      David: I think that is a very smart strategy.

      I am puzzled that Tesla do not enable this, but it is great that MG do.

      All the best: Michael

  3. cclambie Says:

    I had a look on the blog, and wonder if you have done some calcs for Battery without Solar? Like this guy did: https://youtu.be/iWSTgfvlWhU?t=1264
    I wonder if we (your readers) can all write to their Local Authorities and get them to fund battery installation in most/ all homes on a similar interest free loan as the “urban renewal” loans available – £25k, 5 years, interest free.
    This would reduce peak demand massively, and therefore reduce grid reliance on gas peakers, reducing average cost, and reducing the cost.
    I was thinking of creating a Crowd Funding option to buy say 10,000 15 kWh home storage batteries from CATL or BYD or even a British manufacturer like the Oxford team that has worked out Sodium Ion batteries… safer than Li-ion for home storage – stick them in the basement, no fire risk.
    Thoughts?

    • protonsforbreakfast Says:

      Craig,

      Good Morning from the UK.

      Yes! Battery without solar makes sense. It’s what I have in the winter months and it saves me loads of money, and avoids shifts demand away from peak hours.

      So if you have the money, I recommend it. The degradation on LiFePO4 (lithium iron phosphate) batteries is apparently lower than on standard Li batteries – especially after full discharges. Also these batteries do not catch fire. So there’s no need to wait for sodium ion batteries.

      I doubt you would get a local authority to fund individuals purchase of multiple 10 -20 kWh batteries because at their scale it would make more sense to fund a single big battery e.g. 1 MWh. This would not need 100 installations with their gory wiring problems!

      On my tariff there is a factor 6 in difference between cheap rate and peak electricity. This makes it possible to save a lot of money – it would probably justify borrowing money at commercial rates!

      All the best

      Michael

      • cclambie Says:

        Sounds like a plan!
        I guess the problem with a “big battery” is you are then dealing with National Grid, as opposed to simple installation and VPP opportunities.
        This also makes getting solar more likely in the future, which helps with distributed generation, that is helpful in the UK Summer – might even see cooling requirements in summer in the UK soon!
        So households asking for loans individually and arranging to do it is easier than getting a council to do a community battery maybe?
        These guys are working on that:
        https://optimistic.kartra.com/portal/POWERfreemembership/index
        I think it might be easier to do a VPP.
        But worth doing both!

      • Dan Grey Says:

        I profoundly disagree with this; batteries charged from anything other than solar waste energy. Why? The round-trip lost is 10%. The difference between the least efficient and most efficient CCGTs is about 5%.

        Actual peaker plants – OCGTs and reciprocating gas engines – rarely run and provide a vanishingly small amount of power over a year.

        I totally understand that using something like Octopus Go makes a battery without solar cost-effective but it doesn’t reduce emissions, which personally I think is paramount.

      • Bruce MacNeil Says:

        Battery storage – without household solar or small wind – makes huge sense.

        Shifts the demand from peak to off peak and with broad adoption may forestall additional generation construction.

      • cclambie Says:

        I agree. Dan, you are forgetting that overnight rates are low as Wind is blowing a lot at night!
        And otherwise negative energy incentives go to waste – the power supply is there if you want it or not.
        More storage on the grid is better for all.
        Even if it is small scale.

      • protonsforbreakfast Says:

        Craig,

        > More storage on the grid is better for all.

        Yes, I think I agree with that.

        All the best

        Michael

      • protonsforbreakfast Says:

        Dan, I agree that batteries do not directly reduce emissions. Aside from energy lost charging and discharging, they also consume non-trivial amounts of energy 24/7 through their BMS and environmental controls.

        But.

        They enable individuals to install heat pumps and solar PV systems – large investments – whose value is increased by the use of a battery.

        I ran though the hour by hour CI for a few years and it looked like time shifting consumption with a battery was broadly 15% lower CI than the average. So on balance, I think its operation is broadly carbon neutral.

        M

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