Maximizing Powerwall battery life: backup-only or self-powered?

[Doom Man]

First post! I’m deciding on the settings of my new (December 2019) system. I have 6.6kw of PV and 2 Powerwalls that charge from solar only.

My utility has old-school net metering, with a 1-to-1 credit for excess production. Utility power costs the same all day and night. So there’s no electric-bill incentive to use the battery instead of the grid until one of those factors changes.

My primary purpose for the Powerwalls is as a backup – I live in a storm-prone area with frequent short outages from thunderstorms and multi-day outages every few years from tropical systems. (FWIW, a generator wasn’t an option for various reasons.)

So what is my best setting? I’m concerned that backup-only mode will keep the PW’s at 100%, shortening their useful lifespans. I’m also concerned that using self-powered mode will mean losing 10-15% with each charge and discharge thus wasting money. Is there any benefit to switching between self-powered and backup-only, so the battery is getting charged and discharged? Or do charge/discharge cycles shorten battery life?

 

[jjrandorin]

First post! I’m deciding on the settings of my new (December 2019) system. I have 6.6kw of PV and 2 Powerwalls that charge from solar only.

My utility has old-school net metering, with a 1-to-1 credit for excess production. Utility power costs the same all day and night. So there’s no electric-bill incentive to use the battery instead of the grid until one of those factors changes.

My primary purpose for the Powerwalls is as a backup – I live in a storm-prone area with frequent short outages from thunderstorms and multi-day outages every few years from tropical systems. (FWIW, a generator wasn’t an option for various reasons.)

So what is my best setting? I’m concerned that backup-only mode will keep the PW’s at 100%, shortening their useful lifespans. I’m also concerned that using self-powered mode will mean losing 10-15% with each charge and discharge thus wasting money. Is there any benefit to switching between self-powered and backup-only, so the battery is getting charged and discharged? Or do charge/discharge cycles shorten battery life?

Its my understanding that, batteries staying at a high state of charge for long periods of time is not the healthiest thing for them, in general. Now, Powerwall batteries are supposed to be different than tesla car batteries, but I still think staying at a high state of charge permanently would not be the best thing.

If I were in your specific shoes, I think I would use Self Powered, and set the reserve at like 80%. This way you are doing shallow charge / discharge cycles, but still have plenty in case you have an issue.

 

[GenSao]

Welcome to the TMC!

As Battery University states, "Environmental conditions, not cycling alone, govern the longevity of lithium-ion batteries. The worst situation is keeping a fully charged battery at elevated temperatures."

In your case, leave it on back up mode most of the time is fine. As there is no memory effect, there is no need to discharge the battery all the way. During the summer (high temps) allow the battery to discharge to say 60-70% with time based control (balanced) with the peak during the hotter part of the day. As there is no need to discharge all the way to 60-70%, you can set the peak to end when temperatures are moderate.

This happens to match a typical TOU rate schedule (which you are not on).

 

[Doom Man]

If I were in your specific shoes, I think I would use Self Powered, and set the reserve at like 80%. This way you are doing shallow charge / discharge cycles, but still have plenty in case you have an issue.

Interesting, thanks. Setting aside the emergency power issue, to clarify "shallow charge/discharge cycles": is the benefit of setting an 80% reserve that the battery is not going below 80%, or that the net loss is only 20%? I.e., would it be the same (or more) benefit if there were a way to have it charge to 70% and discharge only to 50%, in case they add more options later?

 

[jjrandorin]

Im not versed enough to provide that much advice on that, lol. I have done a fair amount of reading on batteries since becoming a tesla car owner (sort of comes with the territory I guess) and one thing I feel comfortable saying is that its "better" for the battery to have more frequent, shallower charge /discharge cycles than to have larger 0-100 charge / discharge cycles.

The batteries are rated for XXXX number of cycles but at a rudimentary level it would take 10 charges of 90-100 to = 1 charge of 0-100. For powerwall specifically, since there is no way to set the "top end" of the battery like there is on the car (and they hide some of the capacity from us anyway, because we get 13.5 kWh per powerwall even though they are 15 kWh batteries) i dont think, on a powerwall, "100%" is "100%". Its likely less from an absolute term.

Im just thinking about what I would do if I was in your shoes. I would likely set the charge to 80% or even 85% on self powered, and then once a month or so I would likely set the reserve to like 10-15% and let the system "exercise" the batteries... for my own well being, just to see the system charging and discharging properly.

I am in a place with pretty stable power, but I had 2 power outages last year. I purchased 2 powerwalls, recently installed, to go with the already existing 8.7 kW sized solar I have. I have only had my powerwall for a couple of weeks, but I already at this point cant imagine having solar again without powerwalls or SOME sort of battery storage system.

Anyway, You are correct that there is energy loss in the conversions back and forth, but to me its a minimal amount so I am not concerned about it.

Note, I am not saying that setting it to 80% self powered is "right".. its just what I would do if I was in your shoes, at least until someone with more knowledge than me corrected me

 

[jjrandorin]

@GenSao Thanks! See, there is someone more knowledge than me to provide better info

 

[cwied]

One thing I would point out is that no one really knows what 100% charge (reported by the Powerwalls) means. We do know that it isn't truly 100% of the capacity of the batteries because the batteries accept quite a bit of energy at 100% when on Stormwatch. I personally think that you'd probably be fine just leaving the Powerwalls in backup mode if you're concerned with the inefficiency of cycling the battery.

 

[KY-Lonewolf]

I've mentioned this before.............
I have my four (4) powerwalls set to Backup-only 100% mode.
The 4 powerwalls currently consume 0.9 kWhs per day as maintenance. (0.225 kWhs per powerwall)
They recharge to full every other day provided there is enough PV production available (they don't recharge every day).
My Powerwalls recharge when they reach 96% charge every other day.
The source of my data is the Powerwall iPhone app.

 

[nyprepper]

I've mentioned this before.............
I have my four (4) powerwalls set to Backup-only 100% mode.
The 4 powerwalls currently consume 0.9 kWhs per day as maintenance. (0.225 kWhs per powerwall)
They recharge to full every other day provided there is enough PV production available (they don't recharge every day).
My Powerwalls recharge when they reach 96% charge every other day.
The source of my data is the Powerwall iPhone app.

I have 2 powerwalls set to backup only 100% mode too......and I have observed identical behavior.

 

[Doom Man]

As Battery University states,

Thanks, that's a great resource!

 

[Soda Popinski]

I can't recall where I read it, but from my understanding, PowerWalls have more than 13.5 kWh of cells, so 100% isn't really 100%. If it were, I'd expect Tesla to suggest only charging it to 90% like they do with the cars.

On a side note, I finally did a 24 hour "grid-off" test to make sure the battery did indeed charge from solar when disconnected from the grid (someone here mentioned theirs didn't). Worked perfectly fine for me. Started with 100% capacity, and ended with 100% capacity after dipping down to ~65% or so. I'll have to try it again in the summer.

 

[jjrandorin]

I can't recall where I read it, but from my understanding, PowerWalls have more than 13.5 kWh of cells, so 100% isn't really 100%. If it were, I'd expect Tesla to suggest only charging it to 90% like they do with the cars.

On a side note, I finally did a 24 hour "grid-off" test to make sure the battery did indeed charge from solar when disconnected from the grid (someone here mentioned theirs didn't). Worked perfectly fine for me. Started with 100% capacity, and ended with 100% capacity after dipping down to ~65% or so. I'll have to try it again in the summer.

The powerwall datasheet lists the battery as 14k with 13.5k usable. I swear I remember reading somewhere it was 15k but when I went looking, thats what I found:

https://www.tesla.com/sites/default.../Powerwall 2_AC_Datasheet_en_northamerica.pdf