Posts Tagged ‘Battery Degradation’

Assessing Powerwall battery degradation

December 12, 2022

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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