Powerwall 2 Degredation vs Charge capacity

[af88]

Hi, I'm about to purchase two powerwall 2 for my home, but before I sign the contract I'm trying to do my due diligence to get info on the cell chemistry. My goal with these two batteries is to use them 100% only for power outages, with Edison threatening to shut down power in high wind scenarios which is frequent but they have not shut power yet in the area i would like to have backup power since the generac generator did not work out due to city setback rules for permits. I already have a solar system, oversized so I'm never going to use the battery to offset costs. It will strictly be used for backup power only.

I would like information on charging and storing powerwall 2 battery charged at 100% with minimal cycles and degradation over time. There is a pretty good chart i pulled from a youtube video showing Telsa model 3 battery info but this depicts thousands of cycles. I'm familiar with Li chemistry to know charging at 100% is not ideal but the rate of degradation is very much dependent on the cell chemistry and software managing the battery. Not knowing much about the powerwall 2 I'm hoping someone here might have some with a similar chart. The dilemma is at 80% charge I'm giving up 20% capacity for those hot summer days when the AC is on so I would like to know what the actual rate is and make a determination on what to leave the battery charge level do daily until it is needed.

 

[power.saver]

You cannot set the max charge level, it is always 100%. In backup mode, they will stay at this level until you have an outage.

 

[af88]

I had no idea this is the way it operates. When I called Tesla tech support they did not explain this to me at all.

With that said is 100% truly 100% of the capacity or is there some correlation in terms of 100% of a recommended capacity?

In terms of degradation what is it at 100% full capacity for backup when Li chemistry does not like to be stored at 100% capacity?

 

[power.saver]

It is speculated that 100% is not actually 100%, because under certain StormWatch conditions, batteries at 100% accept a little more charge. But it is probably very close to 100%.

I can't comment on the long term degradation at 100% because I use mine for TOU load shifting, and they are not at 100% for more than a few hours each day.

 

[af88]

I would really like to see some data from Tesla on this. I contacted them and I'm waiting.

I would be shocked if the 100% is the real capacity. Not sure how Tesla can guarantee the 10yr/70% cap with the battery being fully charged at 100% all the time on standby. Something does not add up and the fact you mentioned under StomrWatch it accepts more charge leaves me wondering what is going on.

Is there any condition in which the end user dictates the percentage the battery charges to? If the user has no intervention of capacity setting all this is moot anyway but makes me wonder how much of the advertised capacity is truly available for use in blackout conditions.

thanks for your feedback.

 

[power.saver]

There is no end-user setting for max charge, it is always 100%.

 

[af88]

Thanks for the feedback. I guess I need to also ask Tesla what that 100% really means.

 

[Brovane]

I had no idea this is the way it operates. When I called Tesla tech support they did not explain this to me at all.

With that said is 100% truly 100% of the capacity or is there some correlation in terms of 100% of a recommended capacity?

In terms of degradation what is it at 100% full capacity for backup when Li chemistry does not like to be stored at 100% capacity?

Based on what I have seen with Stormwatch mode the normal full 100% for the PW2's is actually 90% capacity.

You might find this thread informative. Powerwall Stormwatch

 

[af88]

Very interesting thread, thanks for sharing, it would be nice if Tesla actually posted the details about the BMS and what the relationship is between the APP and API reporting to true capacity.

Reading through it i have another question because I saw a few posts about where the powerwall charges from PV or grid. Does the Grid not charge the powerwall at night for those who are on TOU and use the battery during the day and charge at night? just curious.

 

[Brovane]

Very interesting thread, thanks for sharing, it would be nice if Tesla actually posted the details about the BMS and what the relationship is between the APP and API reporting to true capacity.

Reading through it i have another question because I saw a few posts about where the powerwall charges from PV or grid. Does the Grid not charge the powerwall at night for those who are on TOU and use the battery during the day and charge at night? just curious.

No on TOU I do not charge from the grid during the night for the US. My solar panels charge the system during the day before 2pm. From 2pm-8pm I draw down the battery.

The only time my Powerwall's have charged from the grid is during stormwatch.

 

[miimura]

Very interesting thread, thanks for sharing, it would be nice if Tesla actually posted the details about the BMS and what the relationship is between the APP and API reporting to true capacity.

Reading through it i have another question because I saw a few posts about where the powerwall charges from PV or grid. Does the Grid not charge the powerwall at night for those who are on TOU and use the battery during the day and charge at night? just curious.

In the USA, if you have solar, it will only charge from solar except during a Storm Watch event. If you don't have solar, you can't get the tax credit and then the system has to charge from Off-Peak grid power and will discharge during Peak.

In other countries like Australia and the UK, it will charge from both grid and solar as it sees fit.

 

[af88]

No on TOU I do not charge from the grid during the night for the US. My solar panels charge the system during the day before 2pm. From 2pm-8pm I draw down the battery.

The only time my Powerwall's have charged from the grid is during stormwatch.

In the USA, if you have solar, it will only charge from solar except during a Storm Watch event. If you don't have solar, you can't get the tax credit and then the system has to charge from Off-Peak grid power and will discharge during Peak.

In other countries like Australia and the UK, it will charge from both grid and solar as it sees fit.

Thanks for this information, i had no idea they worked this way. I thought for the people who use them during the day and drain them the grid would charge them at night.

The way the Edison rep explained it to me for just solar in general is that without a battery you are always using Grid power regardless of the time of the day, but Edison calculated what you put back into the grid when the solar is producing and they calculate at the end of the day total usage/surplus. So you are never really using solar to power the home, which is why when the power goes out the solar shuts down too. This is when a battery comes in handy to be able to utilize your solar when the grid goes down. Is this correct?

The fact we can't set the charge threshold I guess makes this more simple than trying to determine if what I set would void the warranty or not.

Let me ask this is there any drawback from having "stormwatch" turned on all the time if the goal is again to maximize the charge in a high wind scenario where Edison can shut down the grid in advance? I'm wondering if I set it up to backup mode only and leave stormwatch on all the time is there any drawback to this?

 

[Odiemac]

It’s all about what Tesla has set max charge voltage cut-off to. iPhones very aggressively push the max voltage per cell to something like 4.25V. Tesla cars are something like 4.1V at 100%.

I’m hoping 100% on a Powerwall is something like 3.9V or less per cell. I’ve been waiting for someone to tear one down and measure max voltage but so far this hasn’t happened to my knowledge.

 

[power.saver]

The only drawback to stormwatch is that it could draw power during the peak period. But if your goal is just backup mode, then the benefit of a fully charged PW before a potential outage outweighs that.

 

[af88]

Is there any way to set it in backup mode to be just that so it only operates in the event of an outage and it does not draw during the peak period? I don't understand why if it is set to backup mode with stormwatch it would draw during peak ours, that seems to defeat the purpose of having it set to backup mode. If it is set to stormwatch and there is no alert does it still draw during peak period? If it is not set to stormwatch and backup only it does not draw from the battery at all correct?

How about what Edison told me:
The way the Edison rep explained it to me for just solar in general is that without a battery you are always using Grid power regardless of the time of the day, but Edison calculated what you put back into the grid when the solar is producing and they calculate at the end of the day total usage/surplus. So you are never really using solar to power the home, which is why when the power goes out the solar shuts down too. This is when a battery comes in handy to be able to utilize your solar when the grid goes down. Is this correct?

I would be surprised if they charged the cells even that high. Li does not like to be stored anywhere near that for a long time, that is the fastest way to degrade a cell. This is why I would really like know what they the 100% correlates to in terms of real capacity. The fact they guarantee 10year/70% cap tells me that the 100% is really 3.85v for which is far from a full charge but that is how the guarantee the long life.

 

[Odiemac]

How about what Edison told me:
The way the Edison rep explained it to me for just solar in general is that without a battery you are always using Grid power regardless of the time of the day, but Edison calculated what you put back into the grid when the solar is producing and they calculate at the end of the day total usage/surplus. So you are never really using solar to power the home, which is why when the power goes out the solar shuts down too. This is when a battery comes in handy to be able to utilize your solar when the grid goes down. Is this correct?

No, the Edison rep is not correct. You send power back to the grid net of whatever your house is using. The reason it doesn’t work during outages is some (somewhat questionable) rules around grid-tie inverters and anti-islanding, like CA Rule 21.

 

[af88]

Interesting, makes me wonder what else Edison mislead me about.

How about the stormwatch questions:

Is there any way to set it in backup mode to be just that so it only operates in the event of an outage and it does not draw during the peak period? I don't understand why if it is set to backup mode with stormwatch it would draw during peak ours, that seems to defeat the purpose of having it set to backup mode. If it is set to stormwatch and there is no alert does it still draw during peak period? If it is not set to stormwatch and backup only it does not draw from the battery at all correct?

 

[power.saver]

No. When you are in backup mode, it periodically charges a little to keep the batteries topped off. If storm watch activates, it charges even more. Storm watch even overrides TOU modes, and begins charging immediately. Would you really want to delay this extra charging if a storm is coming and an outage is immanent?

 

[af88]

What is want is to use stormwatch to charge more when an outage is immanant but i don't want the battery being used during peak hours at all. That is why i asked if there was a way to set backup mode to be just used as backup mode only. Are you saying if stormwatch is enabled the battery is drawn during peak hours or is there a way to avoid this if TOU mode is turned off and only used for backup mode?

 

[power.saver]

Backup mode has no concept of TOU. It will charge when it needs to.