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20 second UPS?

Discussion in 'Tesla Energy' started by Cosmacelf, Jul 25, 2015.

  1. Cosmacelf

    Cosmacelf Active Member

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    If you use the Powerwall as a power backup solution, you have to contend with the fact that it does indeed deliver backup power ... Starting about 5-10 seconds after the power goes out. So for certain things that you'd rather not have any power interruption for (like your computer, maybe your network gear), you need a UPS.

    The problem with existing UPS is that their lead acid batteries last about 4 years. At which point you've got an expensive and messy upgrade procedure. And the UPS is designed to bridge far more than 20 seconds of downtime. Since lead acid batteries don't deal with rapid deep cycling week, you have to get a reasonably sized one.

    So, my thought was, I wonder if there is a market for a super capacitor UPS? IF you could build it out of components that would last 20 years, you could build a fairly small and light unit since it would only need to bridge a 20 seconds power gap in a Powerwall (or any generator) household. Thoughts?
     
  2. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    A lot depends on the particular UPS. The 6 KVA Powerware I have is not particularly hard to replace batteries in, and not particularly messy either (pull out a carrier, insert the new cells, replace the carrier). It's only expensive if you also purchase the carrier rather than just the cells. Of course, the unit itself wasn't cheap but it's been going strong for about ten years and provides about three hours of power.

    There have been many posts about super capacitors replacing batteries for the last fifteen years in the various forums I follow, yet they never have. I classify them with the "battery of the week" that will replace all existing batteries in a year because they are so much better. Now if someone actually comes up with a real product--that's another matter, but until then...
     
  3. miimura

    miimura Active Member

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    A normal standby UPS has runtime of about 5 to 8 minutes at full load. That is 7C-12C discharge. If you could get a different battery chemistry that could handle a higher discharge rate, you could make the battery and its runtime proportionally smaller. The basic question is, are you willing to pay up front for the cost of 4 to 6 Lead Acid battery replacements in order to get a 20 year battery. I think most people would only be willing to pay for 2 battery replacements in advance to get 5X life.
     
  4. deonb

    deonb Active Member

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    Is it for sure that the Powerwall isn't line-interactive?
     
  5. miimura

    miimura Active Member

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    The Powerwall itself is just a battery. The inverter and its connection scheme will determine how interactive or how uninterruptible it will be. The basic problem is that being able to feed back into the grid and being uninterruptible are likely mutually exclusive. Think about it this way - when the grid goes down your inverter will not be able to hold up the grid voltage until it can be cut by an automatic transfer switch. The best you could do is like a standby UPS where you bring it back up within a limited number of milliseconds. I think the larger the power rating, the harder this is to do quickly. The only cost effective way to do this is to support a "critical loads panel" in this manner.
     
  6. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Once you have a decent UPS, it's easy, and generally low cost, to add a panel to the existing wiring so that a number of receptacles are UPS powered.

    200 amp service:
    Main panel --> 100 amp subpanel --> 2 x 50 amp 240V circuit to UPS --> sub panel --> 120V receptacles (minimum one in each room)
    Main panel --> 100 amp subpanel --> 2 x 20 amp 240V washer/dryer circuits
    Main panel --> 100 amp subpanel --> 50 amp 240V circuit for future HPWC
    Main panel --> 100 amp subpanel --> 4 x 15 amp circuit for local 120V receptacles
    Main panel --> 50 amp 240V a/c circuit
    Main panel --> 50 amp 240V 14-50 circuit
    Main panel --> 50 amp 240V oven circuit
    Main panel --> 15 amp 110V receptacles (refrigerator, microwave, etc.)
    Main panel --> 15 amp 110V receptacle isolated ground
     
  7. Cosmacelf

    Cosmacelf Active Member

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    Somewhere there was a solaredge talk/presentation that quoted a 2-10 second gap before it would deliver power. IIRC, their spec was 10 seconds, but that it usually worked at 2 seconds.
     
  8. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Unfortunately, two seconds is a lifetime in electronics, but you could do with a much lower capacity UPS.
     
  9. nwdiver

    nwdiver Active Member

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    I haven't seen ANYTHING definitive in regards to how the power wall operates without the grid... if I've missed that I apologize... please re-post.
     
  10. Cosmacelf

    Cosmacelf Active Member

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    It is kind of buried. Here's the link: Webinar: StorEdge: Optimized Inverter Solutions for Home Energy StorageGreentech Media

    It is a webinar and you have to register (for free) to access the video. IIRC it is about 20-30 minutes long, and there are stretches of stuff you'd already know. But stick with it, the presenter does give some good info about Powerwall and SolarEdge integration.
     
  11. wdolson

    wdolson Active Member

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    Replacing batteries in a UPS isn't all that bad. They are heavy, but not messy unless one splits open, which I've never seen. I did have one bulge out once and I had to disassemble the UPS to get the battery out.

    One of my UPSs had the batteries go bad last week and the new batteries arrived today. 6 pounds a battery, which makes for a bit to horse around. It is annoying to replace them, but a good UPS like APC makes it pretty painless. They even claim you can hot swap the batteries, but I don't try it.

    I'm a little wary of large capacitors. I have seen capacitors explode and it isn't pleasant. I had a relatively large one explode in my face when I was in school. The instructor freaked out but fortunately the caustic fluids only splashed a little on my arm which were quickly rinsed in the emergency sink. The wood of my work bench were etched by the stuff. A single lead acid battery isn't going to kill you unless you get hit by it. It can't produce enough current fast enough or a high enough voltage to cause any damage. Even an ICE car battery can't.

    A large capacitor can produce enough current to fry you pretty quickly. With old TVs there were people who were killed trying to work on them when they touched a large capacitor in there. A friend of mine said he took delivery of some capacitors used in a substation once. They came in on a flatbed truck. One of them had the shorting bar fall off and he went to put it back on. The static electricity from the road had charged up the capacitor and he didn't know it. When the shorting bar touched the contacts, it vaporized. He wasn't in the circuit, so he wasn't hurt, but he had a bar of steel vaporize a few inches from his hand. He was a little shaken and very happy he only had one hand on the bar. If he had a hand on each end of the bar, he wouldn't have been there to tell the tale.
     
  12. Ampster

    Ampster Member

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    Other inverters have a shorter response time. There may be a workaround with the Solar Edge if they have a constant inverter on mode that can be set to only draw 100 watts from the inverter. Then there is no lag if the grid goes down . For those of us who are load shifting we may have our inverter on for a good portion of the peak hours anyway. I will have to drop the grid sometime to see how my inverter responds when that happens.
     
  13. wk057

    wk057 Senior Tinkerer

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    The inverters I used in my solar project (see signature) are 8kVA continuous capable and stackable up to 80kVA. I have them running from salvage Model S cells, but they'll run from any ~40-60VDC battery source.

    They have a UPS mode that keeps the inverter output in sync with incoming AC power and loads never even see so much as one cycle of missing power when the AC input is lost or reconnected. Pretty awesome way to do it.

    UPS mode has some overhead vs. the 'Backup' mode, however. Backup mode functions more like a traditional UPS with a transfer relay and a minimal delay (100 miliseconds or so) when AC input power is lost. Most equipment can handle this though. UPS mode keeps the inverters online and active constantly at some power cost (something like 35W per inverter).

    So, it's definitely possible to have a whole house UPS with the right equipment.

    I'm actually not sure where the 20 second number for the PowerWall comes from, honestly. I couldn't see a battery system taking quite that long to engage and properly setup should be instantaneous.
     
  14. deonb

    deonb Active Member

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    I agree that a car battery can't kill you unless you get hit by it.

    However, the ability of a battery vs. capacitor to produce current doesn't make any difference. You need voltage to kill you, the current supply ability of the source makes no difference:
    https://www.youtube.com/watch?v=XDf2nhfxVzg
    and/or:
    https://www.youtube.com/watch?v=ZxBF7WC0TQk

    Neither a 12V battery or 2V supercapacitor has enough voltage to overcome human body resistance.

    TV capacitors are dangerous because they hold 160V+, and if you're on the wrong side of the flyback transformer, you can get zapped with thousands of volts. You're going to have an similarly bad time if you open up a Tesla battery and touch its 400V+.
     
  15. Ampster

    Ampster Member

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    It was a remark in the SolarEdge webinar, but I thought it was more like 10 seconds. You are correct, a battery should be able to respond much faster so I think it was an inverter limitation.
     
  16. Ampster

    Ampster Member

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    #17 Ampster, Jul 29, 2015
    Last edited: Jul 29, 2015
    Voltage is the enabler that allows current to kill you. You are correct, it is not the capacity to produce current that is dangerous. It is the current flow through your body that kills you. You can run several hundred volts through your body at a milliamp and it will tingle. The videos were interesting, and helped me remember the connection between the two. That is what makes a 400v pack dangerous.
     
  17. deonb

    deonb Active Member

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    You can't run several hundred volts through your body at a milliamp. Ohms law is not negotiable.

    If you touch several hundred volts (let's say 400V), the amount of current going through your body is going to be ~ 400/10kOhm = 40mA, which is more than enough to kill you.

    If you touch something that's supply-side current-limited, it's really also voltage limited. E.g. let's say a supply gives out 400V at 1mA. That means the supply you touch has an internal resistance of 400kOhm (has to be). Add a 10kOhm human body resistance to it and you form a voltage divider across a 400kOhm and a 10kOhm resistor. That forms a 400/410 voltage drop over the supply, and a 10/410 voltage drop over your body. That means you're only really dropping ~9.75V of voltage, and thus, drawing 0.975mA of current.

    There is something else that is of course survivable. High voltage over short period. E.g. static electricity. If you touch a doorknob and you get 10'000V from it, you actually really do end up with 1A going through your body. The reason you're not instantly fried is because the period is too small (in the order of nanoseconds or microseconds).

    You don't just need current to kill you, you need both current & time. (Coulombs, to be precise).
     
  18. Ampster

    Ampster Member

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    #19 Ampster, Jul 29, 2015
    Last edited: Jul 29, 2015
    Thank you, that is a much better explanation than I was able to give. The two examples of high voltage that I first encountered as a kid were an electric animal fence and static electricity.
    NB: A few years later I learned Ohms law and later that amps kill. This little diversion of the thread has been helpful to clarify the importance of respecting the power of these systems.
     
  19. wdolson

    wdolson Active Member

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    Yup, it's the current that kills and voltage that enables it. A 12 V ICE battery can produce a lot of current, but with only 12V across your body's resistance, the battery won't produce enough current in that instance to cause damage.

    There are many ways to produce very high voltages with negligible currents. Electric animal fences work that way. Static electricity is also very high voltage, but negligible current.

    Large electrolytic capacitors are usually 50V or more and with enough capacity to have a long time constant. It takes about 5X R*C time to discharge a capacitor. A large capacity capacitor can keep pumping out current for a significant amount of time. For the sort of applications we're talking about in this thread, the capacitors would likely be high voltage and high capacity.
     

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