Powerwall 2 calendar ageing in Backup-only mode

[m3-pw2]

I wanted to find out how to extend lifetime of Powerwall 2 in Backup-only mode. The current Tesla app does not provide a way to set maximum SOC lower than 100%. I realize that 100% is probably a lower % of real capacity.

I have read in numerous places that lithium-ion batteries stored at high SOC result in increased chemical reactions and faster reduction in capacity. Temperature is another major factor, but the batteries are installed in basement and are not exposed to extreme temperatures.

Even though Tesla does not provide a setting to limit SOC for backup-only mode, I was able to tweak Time-based control mode to achieve that goal, and right now, the Powerwalls are running at 75% SOC.

I am fine with a lower SOC for a rare outage event. StormWatch will handle cases where the risk is known to increase. And in case an outage occurs, solar system will charge them to full capacity when weather permits.

If keeping SOC at 75% and the batteries lifetime extends several years, then it would be worth the trouble. Anyone knows of research that shows how battery life is affected at various SOC storage?

 

[avirtuos]

In general Lithium batteries dislike being at low or high SOC and also dislike extreme temperatures. However changing chemistry, charge and discharge rates, and having a 'buffer' of capacity that is not user accessible all augment this. Powerwalls use a different cell chemeetry than Tesla cars, they also charge and discharge at less than 30% the rate of the vehicle cells. Lastly, there is likely some buffer of capacity and the top and bottom range which is how they offer a daily cycle 10 year 70% retained capacity warrantee. Long story short... I wouldn't worry about it. If you really want to, set the backup limit to reduce how deeply you cycle them each day and try to install them in a shaddy area to reduce max max operating temperature. Low temps aren't as big if a deal , they just limit charge/discharge but high temps even for an idle cell can trigger reactions that decay the cathode.

 

[NuShrike]

Check the Prius forums. Although most of the history is NiCd, they ran batteries at 80%-60% or 80%-40% SoC for very long lifetimes of more than 200K miles.

Personally, I'm over 220K miles, 14 years and barely noticeable diminished battery capacity with the factory battery.

On the consumer lithium side, I've ran a Sony VAIO lithium based laptop for 8 years with the SoC charge limiter set to 80% max, and shutdown at 40% SoC. With the laptop fully charged (to 80%) most of the time, at the end of 8 years, the battery capacity diminished from about 6 hours, to about 2 hours, ~40% reduced capacity.

 

[m3-pw2]

Found this research on modeling lithium-ion battery degradation which could give some insight.
https://www.researchgate.net/public..._Battery_Degradation_for_Cell_Life_Assessment

Figure 3b shows calendar aging over time at 25 deg Celcius. After 10 years, the difference between 100% SOC and 60% SOC is about 7%. 100% SOC vs 80% SOC is about 4%.

 

[cwied]

When interpreting that graph, remember that 100% on the Powerwalls is not actually a 100% SoC for the batteries. They intentionally limit the charging to preserve battery life. Also, different chemistries have different curves. I personally choose to trust Tesla's algorithms since they have more experience with batteries than most other companies.

 

[m3-pw2]

When interpreting that graph, remember that 100% on the Powerwalls is not actually a 100% SoC for the batteries. They intentionally limit the charging to preserve battery life. Also, different chemistries have different curves. I personally choose to trust Tesla's algorithms since they have more experience with batteries than most other companies.

Tesla limited warranty is at least 70% capacity after 10 years. So if after 10 years it ends up with capacity remaining of 75% or 82%, it is inline with the warranty.

From user perspective, my incentive is to achieve a better outcome if possible and maximize the benefit of my investment.

 

[jtjat1]

Tesla limited warranty is at least 70% capacity after 10 years. So if after 10 years it ends up with capacity remaining of 75% or 82%, it is inline with the warranty.

From user perspective, my incentive is to achieve a better outcome if possible and maximize the benefit of my investment.

I agree there’s a rationale for reducing the charge level in backup-only mode, especially with summer temperatures arriving and long days likely providing good solar production in the event of an outage.


I have 2 PWs + Gateway added to a pre-existing 10kW PV array. I got the system only for its backup function, and like the OP I found it’s possible to set the PW maximal charge level. My recipe:


In the app, Customize >Advanced >Balanced>Reserve for Power Outages> 75% (or as desired) >Edit price schedule. I set peak hours to midnight to 11:30 PM for both weekdays and weekends. The PW charge level dropped to 73% -- I don’t know why there’s a 2% discrepancy -- over a day or two and has stayed there with negligible cycling. I presume Storm Watch will bump it up to 100% when necessary, or I can do that myself. In any event I plan to increase the charge level closer to 100% by November.


I haven't fully tested this, but so far so good. One caveat: in the US the Powerwall when added to a PV system is configured to accept charge from solar only, which is a requirement for the 30% Federal Investment Tax Credit. I don’t know the situation outside the US, and the workaround above may not work everywhere.

 

[m3-pw2]

My recipe:


In the app, Customize >Advanced >Balanced>Reserve for Power Outages> 75% (or as desired) >Edit price schedule. I set peak hours to midnight to 11:30 PM for both weekdays and weekends. The PW charge level dropped to 73% -- I don’t know why there’s a 2% discrepancy -- over a day or two and has stayed there with negligible cycling. I presume Storm Watch will bump it up to 100% when necessary, or I can do that myself. In any event I plan to increase the charge level closer to 100% by November.

I also took advantage of the Advanced (Time-based) and set the peak hours to match solar production hours. It worked great except for one thing.
- In backup mode, the battery is charged when its SOC drops below 97%, and then it is charged up to 99%, and there is no charging throughout the day.
- In Advanced mode, the battery was continuously charged to stay within 1% of the reserved capacity. So there is a trickle of electricity going into the battery every time it drops below that threshold.

I am concerned about this continuous charging, and not sure if this is good for the longevity of the battery. I assumed that the backup-only behavior was designed with longevity in mind, and that the way I setup the Time-based mode is not a typical configuration and that in practice it doesn't reach or stay in that state for long time.

I called Tesla and unfortunately I didn't get anywhere. They had canned answers such as the batteries have a 10-year warranty and I should just use backup-only mode. I asked them what would be the difference in degradation after 10 years of a battery in backup-only mode that is charged at 100% vs one that is kept at 75%. Obviously they don't have the answer to this, but just wanted to make a point. In the end, I put a request to add a target max charge to backup-only mode. Perhaps if more people request this feature it will one day be implemented.

 

[jtjat1]

I also took advantage of the Advanced (Time-based) and set the peak hours to match solar production hours. It worked great except for one thing.
- In backup mode, the battery is charged when its SOC drops below 97%, and then it is charged up to 99%, and there is no charging throughout the day.
- In Advanced mode, the battery was continuously charged to stay within 1% of the reserved capacity. So there is a trickle of electricity going into the battery every time it drops below that threshold.

I am concerned about this continuous charging, and not sure if this is good for the longevity of the battery. I assumed that the backup-only behavior was designed with longevity in mind, and that the way I setup the Time-based mode is not a typical configuration and that in practice it doesn't reach or stay in that state for long time.

I called Tesla and unfortunately I didn't get anywhere. They had canned answers such as the batteries have a 10-year warranty and I should just use backup-only mode. I asked them what would be the difference in degradation after 10 years of a battery in backup-only mode that is charged at 100% vs one that is kept at 75%. Obviously they don't have the answer to this, but just wanted to make a point. In the end, I put a request to add a target max charge to backup-only mode. Perhaps if more people request this feature it will one day be implemented.

In my setup, charging and discharging of the 2 Powerwalls is minuscule. For example, yesterday they put out 0.1 kWh and took in 0.7 kWh. Most of this time they just sit there with SOC 72-73%. The green lights must use a few watts and perhaps there's cell balancing or other maintenance activities going on. But I would think any impact on degradation rate is negligible.

 

[jtjat1]

In my setup, charging and discharging of the 2 Powerwalls is minuscule. For example, yesterday they put out 0.1 kWh and took in 0.7 kWh. Most of this time they just sit there with SOC 72-73%. The green lights must use a few watts and perhaps there's cell balancing or other maintenance activities going on. But I would think any impact on degradation rate is negligible.

As a follow-up to this, we had very high winds in central NJ earlier this week. Storm Watch was onto that and the Powerwalls charged up to 99%. About 12 hours later we lost power and the Powerwalls started to power the house,while the solar inverters shut down because the Powerwalls were full. Having substituted Eaton UPSs for the previous APC and Cyberpower UPSs there was no frequency issue and everything worked fine – thanks forum members who figured out that Eatons are the way to go! The Eaton attached to my desktop kicked in for a second or so before the PWs took over and the sound alerted me to the power outage, which I verified with the app. After power came back on after a couple of hours, the Powerwalls continued to discharge until they were back to 73%. So that was all very satisfactory.

 

[gpez]

As a follow-up to this, we had very high winds in central NJ earlier this week. Storm Watch was onto that and the Powerwalls charged up to 99%. About 12 hours later we lost power and the Powerwalls started to power the house,while the solar inverters shut down because the Powerwalls were full. Having substituted Eaton UPSs for the previous APC and Cyberpower UPSs there was no frequency issue and everything worked fine – thanks forum members who figured out that Eatons are the way to go! The Eaton attached to my desktop kicked in for a second or so before the PWs took over and the sound alerted me to the power outage, which I verified with the app. After power came back on after a couple of hours, the Powerwalls continued to discharge until they were back to 73%. So that was all very satisfactory.

There is a lot of misinformation about the off-grid/grid-off frequency issue that has been perpetuated. Glad to hear the Eatons are working out but they're simply not needed if the Powerwall is properly configured assuming your solar inverters are UL 1741 SA compliant.

If you inverters comply with UL 1741 SA, the certification that governs inverter behavior in response to out of spec power (such as frequency), then Tesla can update your Powerwall configuration to limit the maximum frequency applied to 62hz, which is down from the default frequency of 66hz.

For UL 1741 SA compliant inverters they'll behave the exact same with 62hz as 66hz and they'll turn off within 0.16s. What won't behave are LED lights, anything with a motor (microwave, vacuums, garage door openers, etc), induction cooktops, AC clocks, and all sorts of other stuff. APC and Cyberpower UPSs are fine with 62hz power but complain with 66hz. I see too many people throw out their UPSs in favor of the more tolerant Eatons rather than simply asking Tesla to update their Powerwall configs. Not sure if this is you, @jtjat1, but hopefully this can help someone else!