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Powerwall: Want control to better optimize battery longevity

Discussion in 'Tesla Energy' started by abasile, Mar 26, 2018.

  1. abasile

    abasile Conscientious investor

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    Our newly-installed dual Powerwall system seems to be working well, but I have one concern. I'd like to be able to keep the average charge in a more optimal range for battery longevity while still using the grid to charge our EVs at night. Yes, I know there's a ten year warranty, but I'd prefer to do what I reasonably can to achieve the longest possible lifetime for our Powerwalls.

    At present, I don't have access to the "time based control" functionality, so I've been manually putting the Powerwalls into Backup mode each night at 10 PM, just in time for "super off peak" hours during which time our electric vehicles charge up (we do too much driving to avoid using the grid). Every morning at 8 AM, I've been switching the Powerwalls back to Self Generation mode. Our cool, sunny days have been enabling great solar production, so the Powerwalls have been reaching 100% charge by mid to late morning or so. Our solar production drops greatly by mid-afternoon due to shade, and the Powerwalls end up at roughly 75-80% charge by 10 PM.

    This isn't terrible, as the Powerwalls are only spending a few hours per day at 100%, and there's undoubtedly a buffer so that "100%" isn't truly 100%. Still, it's not ideal for Lithium ion batteries to cycle between 75% and 95-100%; I'd prefer to cycle between, say, 55% and 80%.

    I'd like to see Tesla add a "maximum target charge percentage" that I could change seasonally. During the winter, particularly with storms coming, I'd leave it at 100%. The rest of the year, I'd probably set it to 70% or 80%.
     
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  2. liuping

    liuping Active Member

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    The Powerwall 2 is listed as 14kWh but our SGIP application says they provide 13.2kWh storage each, so 100% is probably 94.3%.

    But I agree, if it help them much longer, I would not mind only charging to 90% most days.
     
  3. Shygar

    Shygar Member

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    Isn't the whole charge to 100% only a problem when you try to keep it fully charged for a long time? If you are charging it to full then 8 hours later start draining it, I don't see that as being an issue per say. I figure in 10 years the batteries on the market will put the powerwall to shame so I'm not really worried about it.
     
  4. abasile

    abasile Conscientious investor

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    I agree that charging to 100% (or 94-95%) and maintaining that SOC (state of charge) for only eight hours a day is much better than keeping the batteries at full charge 24/7. Even so, the more overall time that lithium ion batteries spend at high SOC, the more rapid their long term degradation rate will tend to be.

    Yes, but I'd rather not be forced to upgrade after 10+ years because of degradation. Even if it only costs $1000 to change a PW2 battery pack in ten years, why spend that money if it's not needed?

    Now, it may be that Tesla is expecting many Powerwall batteries to spend the bulk of their time at relatively high SOCs, particularly those used in backup-only applications. It may be that what I'm requesting wouldn't make much overall difference. In the absence of hard data on the PW2 batteries, however, my preference is to be conservative. I habitually avoid keeping our EV, laptop, and smartphone batteries fully charged for very long, and this is easy to do.
     
  5. liuping

    liuping Active Member

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    It would be nice to control when the batteries are charged. Right now they charge with the morning sun. That mean they stay at 100% most of the day. If they charged with afternoon sun (up to the start of Peak rates), they would be at 100% for only a few hours max. With the current TOU options (when available), I'm not sure we con convince the Powerwalls to only charge in the afternoon.

    Even if there are better batteries in 10 year, I'd rather not have to buy new ones if I don't need to.
     
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  6. Shygar

    Shygar Member

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    Yea I definitely agree and I had thought about this too, especially since for EV's they recommend not fully charging. My one PW2 is usually fully charged around 11am and I start drawing from it around 6-7pm right now and I run it to about 20% normally, although with these warmer days it's only been getting to 50%.

    Think of the backup only option. It's basically sitting at 100% all the time just waiting for an event. So I would hope that there is some way they are managing this to reduce degradation. Plus like you said with the TOU options coming, if we can tell it to draw from the battery at peak hours in the afternoon, then it really should be a minimal amount of time that we will be sitting at 100% for the non-backup-only users.
     
  7. abasile

    abasile Conscientious investor

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    It would probably also be best for the grid if our Powerwalls were charged during the midday hours. Particularly this time of year, midday is when there's the greatest potential for over generation and solar curtailment, meaning that utility-scale solar plants have to shut down or reduce their output.
     
  8. CA-Gbm84

    CA-Gbm84 Member

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    The chemistry used in Powerwall is different to the vehicles. Maintaining a high SOC is fine and you want to have energy available for backup events or high power draw.

    The ratio of Power:Energy has a bigger effect - ie if you are pulling a 5kW load, with only 20% SOC - you will do more damage than at 90% SOC.
     
    • Informative x 1
  9. LanscolvaA

    LanscolvaA New Member

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    ToU, in its current state, does not give any manual control to the user. It's all run by an algorhythm. For me this is a huge whole in the ToU offering which is so disappointing after waiting so long. All I can suggest is that as many users as possible write to Tesla requesting greater control of the Powerwall and ToU. Now is probably a good time to do this.
     
  10. abasile

    abasile Conscientious investor

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    You may be right, as Tesla may have tweaked the chemistry for better longevity at high SOC and/or implemented a significant buffer at the "top" of the batteries. I'm not aware of any public, hard data on this, though.

    Temporarily, anyway, I "solved" the issue for us by charging our Model S during peak hours, with the PW system in Self Generation mode. ;) We had just returned from a long drive and needed to charge during the afternoon. As a result, our Powerwalls started today at 25% charge. Today, we're going to have to charge our 2011 LEAF during the day, thanks to its diminished battery capacity (currently at maybe 1.3 Powerwalls' worth of capacity). So I doubt the PWs will even hit "100%" charge today.
     

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