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Powerwall 2.0 Backup Runtime Extender

miimura

Well-Known Member
Aug 21, 2013
6,495
6,260
Los Altos, CA
I just completed testing my proof of concept Backup Runtime Extender for my Powerwall 2.0 system. It worked pretty much as expected. I was able to inject about 500W into a normal 120V household outlet and the Tesla App registered reduced household usage and reduced Powerwall output in Self Powered Mode with no grid draw. The energy came from the 12V system of my RAV4 EV that uses a Tesla DC-DC converter similar to the Model S to maintain the 12V battery from the traction battery.

After several discussions here on TMC about generator support (and the lack thereof) and the difficulty restarting Powerwalls after low SOC shutdown, I decided that I wanted a way to make sure that the Powerwalls would not shut down during a prolonged power outage during the winter when my solar production is very low.

In order extend the Powerwall's backup runtime, you must use an inverter like a solar grid tied inverter that synchronizes to the existing power waveform. Most mobile power inverters don't do this - they generate their own free running waveform, much like a generator does. So, I started looking for an inverter that I could use. I came across this line of inverters that is listed for use with 12V solar panels or 24V battery. My first thought was to use a 24V DC power supply running from a mobile power inverter, into this grid tie inverter, then into a 240V transformer, then into an extension cord, to the Tesla Backup Gateway and through the solar CTs. I originally wanted this power injection to look like solar generation. However, I concluded after a short time planning the system that most of that was unnecessary. I found a DC/DC step up converter with a wattage higher than the grid tied inverter, so that eliminated a DC-AC-DC round trip. The bonus is that I don’t need the 240V transformer and I can use the existing outlet that’s next to the car’s normal parking space in the garage.

Power Export Connection.jpg


Powerwall Runtime Extender Test.jpg


Strengths:
  • DC/DC Step Up Converter is high efficiency and runs cool
  • Powerwalls can handle significant split in charge/load on opposite phases (needs to be verified with grid disconnected)
Weaknesses:
  • This grid tie inverter gets hot within a few minutes and seems to reduce output with temperature until the internal fan turns on. It is only about 78% efficient (AC out / DC in)
  • Power output of the chosen components is barely strong enough to be useful (higher power versions are available or can duplicated on the opposite phase for better balance)
  • Significant conversion losses stack up from the EV traction battery to the Powerwall battery.

Key Parts List

600W Grid Tie Inverter
https://www.amazon.com/gp/product/B071F5XJMD/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1

UXCell DC/DC 720W Step Up Converter 12VDC to 24VDC
https://www.amazon.com/gp/product/B01M0IEYZJ/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

DC Power and Energy Meter with Current Shunt
https://www.amazon.com/gp/product/B013PKYILS/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

AC Power and Energy Meter with CT
https://www.amazon.com/gp/product/B00YY1KOHA/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1

4 Pin Waterproof Connectors
https://www.amazon.com/gp/product/B01A6M8CD8/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

100 Amp 12VDC Circuit Breaker
https://www.amazon.com/gp/product/B076FYCRJ5/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

Hyclat 2-4 Gage Battery Disconnect (Anderson type)
https://www.amazon.com/gp/product/B01KHQR0K4/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
 

strangely

Member
Mar 29, 2017
121
73
SoCal
I was thinking of doing something like this using our Nissan Leaf and a couple of enphase micro inverters, since they can output spilt phase 240V. I went as far as buying some spare M215s for this purpose off of eBay and had them running off of some 36V Lithium Ion battery packs a few times as a test, however I ended up tripping the inverters and putting them a faulted state and gave up for the time being.

So that said, curious why you haven't considered some enphase inverters yourself? All you would need to do is choose 12-36V DC DC converter instead and you should be good.

Those cheap 120V inverters don't seem to last too long.
 

miimura

Well-Known Member
Aug 21, 2013
6,495
6,260
Los Altos, CA
I was thinking of doing something like this using our Nissan Leaf and a couple of enphase micro inverters, since they can output spilt phase 240V. I went as far as buying some spare M215s for this purpose off of eBay and had them running off of some 36V Lithium Ion battery packs a few times as a test, however I ended up tripping the inverters and putting them a faulted state and gave up for the time being.

So that said, curious why you haven't considered some enphase inverters yourself? All you would need to do is choose 12-36V DC DC converter instead and you should be good.

Those cheap 120V inverters don't seem to last too long.
Since I have Enphase on my roof, Enphase was my first thought for this project. However, on a $/W basis, it just wasn't going to happen. I even looked at used ones on eBay but I came to the conclusion that if they were taken down they were likely bad and I didn't want to do component level SMD repair on it too. Since this is an emergency backup kind of situation, I wasn't going to invest the big money in something that would last 15 years 24/7.

Also, part of the reason for this particular test was to show that you don't have to put the power in at 240VAC, the Powerwalls can easily handle 500W of imbalance on one side of the neutral. In any case, there are other units available that are larger, output 240V, and have adjustable battery parameters like adjustable Watt output.
 

strangely

Member
Mar 29, 2017
121
73
SoCal
Ok well if you do want to give this a try, then there is a company called renvu near you that I bought most of my PV system system components from, and they sell the M215s for $57 which is pretty respectable price wise.
 
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reddy

Member
Jan 26, 2013
984
1,441
Amarillo, TX
Very interesting project. I have a Tesla S but the 12v battery is a tiny little thing. You’re saying the Dc to Dc converter in the S supplies a constant 12v with no problem ?

Also, my power wall has a cold start terminal on it. Have you ever tried that ?
 

Frankman60

Member
Jun 21, 2016
417
86
San Diego, CA
Thanks for that information. Has anyone on this forum tried this to cold start Powerwalls and does it actually work? How many amps are required at 12 volts for this to work? Lastly, why is this the first time that I have seen this mentioned on this forum when it is something everyone should know?

Applying 9-12 v DC current to these jumpers allows for cold start.

View attachment 414724
 

NuShrike

Member
Nov 13, 2017
461
193
SoCal
Thanks for that information. Has anyone on this forum tried this to cold start Powerwalls and does it actually work? How many amps are required at 12 volts for this to work? Lastly, why is this the first time that I have seen this mentioned on this forum when it is something everyone should know?
Nobody has figured this out quite yet. I'd assume something that can jump-start a car may be enough for this.

Going by the size of the terminal contacts, can't be that many amps.
 

reddy

Member
Jan 26, 2013
984
1,441
Amarillo, TX
Nobody has figured this out quite yet. I'd assume something that can jump-start a car may be enough for this.

Going by the size of the terminal contacts, can't be that many amps.

My installer told me a 9v transistor radio battery would work. It’s just used to initiate a sine wave for the solar inverters to reference.

Unfortunately no manual in this, apparently you can call Tesla support and they can walk you through the cold start process.

There are completely off grid installations now, and this is how they initiate the system.
 
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Dan123

Member
Jun 19, 2018
451
298
Miami
I am going to try this PW extended with a Model 3 battery.

Do you think this step-up converter would work?
https://www.amazon.com/gp/product/B00O0EDMES/ref=ox_sc_act_title_1?smid=A2OCOGC9B25845&psc=1

Also, if I buy an Enphase M215 inverter, how do you turn it on and control it? I have an existing installation with Enphase IQ7 and IQ7+ inverters. Can I add the new M215 to the envoy system? Do I need some sort of installer account? I

https://www.renvu.com/Solar/ENPHASE/ENPHASE-M215-60-2LL-S22-60-Cell-215W-240-208-VAC-MC4

I presume I also need a 20amp fuse for this.

I presume that I need to first connect the inverter into the 120 outlet. Then, somehow, turn on the inverter? How do I turn it on?
 

miimura

Well-Known Member
Aug 21, 2013
6,495
6,260
Los Altos, CA
I am going to try this PW extended with a Model 3 battery.

Do you think this step-up converter would work?
https://www.amazon.com/gp/product/B00O0EDMES/ref=ox_sc_act_title_1?smid=A2OCOGC9B25845&psc=1

Also, if I buy an Enphase M215 inverter, how do you turn it on and control it? I have an existing installation with Enphase IQ7 and IQ7+ inverters. Can I add the new M215 to the envoy system? Do I need some sort of installer account? I

https://www.renvu.com/Solar/ENPHASE/ENPHASE-M215-60-2LL-S22-60-Cell-215W-240-208-VAC-MC4

I presume I also need a 20amp fuse for this.

I presume that I need to first connect the inverter into the 120 outlet. Then, somehow, turn on the inverter? How do I turn it on?
I would get a DC-DC step-up converter that is rated at least 50% over the inverter. For that 215W inverter, I would get a 24V 15A DC-DC step-up converter. This 20A one looks good.
https://smile.amazon.com/Cllena-Converter-Voltage-Regulator-Transformer/dp/B07PY1LVSW/ref=sr_1_4

I don't think you have to do anything special to start up the micro-inverter. However, it must be on a 240V circuit, not 120V. For a single inverter, I would use a double pole 15A or 20A breaker.

When I was originally planning the setup in the original post in this thread, I was going to install a L6-20 outlet below my Generation panel and run an extension cord around to the garage. That way I would be able to inject 240V power and also have 16A 240V EV charging during a utility outage. My two 50A 240V charging circuits are in the main panel and not backed up.
 

gpez

Member
Apr 25, 2019
724
592
USA
Anyone know if the Chevy Volt is capable of something like this? I'm trying to find out what the rating of the traction -> 12v battery DC-DC converter is and if it can spare even a few hundred watts.
 

miimura

Well-Known Member
Aug 21, 2013
6,495
6,260
Los Altos, CA
Anyone know if the Chevy Volt is capable of something like this? I'm trying to find out what the rating of the traction -> 12v battery DC-DC converter is and if it can spare even a few hundred watts.
Yes, a Volt can do this just as well as the RAV4 EV that I demonstrated. Just be aware that when you leave the car on to keep the 12V system powered from the High Voltage system that the engine can start at any time. Be sure to do it in a well ventilated area.
 
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gpez

Member
Apr 25, 2019
724
592
USA
Yes, a Volt can do this just as well as the RAV4 EV that I demonstrated. Just be aware that when you leave the car on to keep the 12V system powered from the High Voltage system that the engine can start at any time. Be sure to do it in a well ventilated area.

That's actually really great to hear. Any idea what the wattage maxes are? Wondering what the 12v system DC-DC max is and what the ICE ability to charge is as well. My understanding was that the ICE does limited charging. References here and here (though they're from 2008 and I know the Volt has gone through some minor revisions since).
 

miimura

Well-Known Member
Aug 21, 2013
6,495
6,260
Los Altos, CA
That's actually really great to hear. Any idea what the wattage maxes are? Wondering what the 12v system DC-DC max is and what the ICE ability to charge is as well. My understanding was that the ICE does limited charging. References here and here (though they're from 2008 and I know the Volt has gone through some minor revisions since).
EVTV has some Volt DC-DC converters for sale. They say 2kW. They're air cooled, so I would not run anything continuous at more than 1kW. As long as the car is completely ON, it should start the ICE when the traction battery gets low. However, I've never done it myself, so take that with a grain of salt. It is proven to work with a Prius, so there's that.

EVTV Motor Verks Store: Volt 2kW Accessory Power Module DC-DC CONVERTER, DC-DC Converters, VoltAPM
 
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gpez

Member
Apr 25, 2019
724
592
USA
EVTV has some Volt DC-DC converters for sale. They say 2kW. They're air cooled, so I would not run anything continuous at more than 1kW. As long as the car is completely ON, it should start the ICE when the traction battery gets low. However, I've never done it myself, so take that with a grain of salt. It is proven to work with a Prius, so there's that.

EVTV Motor Verks Store: Volt 2kW Accessory Power Module DC-DC CONVERTER, DC-DC Converters, VoltAPM

You rock, thank you for this!

For context I'm seriously considering getting a Volt and if I do it'd be a great augmentation to my Powerwall system which is only for backup (we have full net metering with no TOU charges). The scenarios I'm considering are a prolonged winter outage which here in the pacific northwest can mean very limited sunlight. Winter outage could be from a storm but also considering the potential of an earthquake, which would make utility restoration times long and gas supplies low. Having a hybrid EV even at a few hundred watts as a supplement to my Powerwall backup would be great.
 

mckemie

Member
Mar 27, 2013
160
48
Dale, Texas
I am much taken with this idea.

I have a good supply of M215s and intend to do some testing in coming weeks. I hope to get 4-5 M215s working on a single power source. Of course, I will test one inverter, then go from there.

BTW, the M215s are being sold for less than $40. Sadly, it still takes about $15 to make the ac connection. So, less than $55 for each ~250 watts you add to your microgrid. Plus the the voltage booster, of course. In a protected environment, one might be tempted to just cut off the expensive weatherproff 240vac connectors.

This is an ideal use for a weak batteried Leaf or an imiev. It turns out, I have two imievs. One way to look at it: a 4-5 inverter, ~1kw DC-DC powered device would have about 1/5 the power of a PowerWall but at least as much energy. You have hope of extracting 8-14 kwh from your EV overnight. Or course, that would not make much of a dent in a Tesla battery.

I don't know how to give preference to the supplimental power but that seems desireable.

I've read that a Model 3 tear down has revealed that gridtie and microgrid inverter is included in the car's power electronics. Can anyone give me a source for that? That is, Tesla is prepared to spring V2H and V2G on us. Likely with only a software update.

I have one PowerWall on one of my three meters. Tesla has been unwilling to sell/install more PowerWalls for me. So, I am looking for alternative micro grid systems. So far, I have not found anythiing compelling. Suggestions solicited. The discussed device could give a lot of function to a minimal microgrid. 1kw not enough? Just add another EV. And another......
 
Last edited:

Dan123

Member
Jun 19, 2018
451
298
Miami
I am much taken with this idea.

I have a good supply of M215s and intend to do some testing in coming weeks. I hope to get 4-5 M215s working on a single power source. Of course, I will test one inverter, then go from there.

Where do you get the M215s?
 

JayClark

Member
Aug 6, 2019
224
183
Arizona
In order extend the Powerwall's backup runtime, you must use an inverter like a solar grid tied inverter that synchronizes to the existing power waveform.

Just curious if you've tested the scenario where the small runtime extended inverter (even if really small) provides more watts than the house load? Does the small inverter just ramp down in that scenario because it's aware of the house load?
 

Dan123

Member
Jun 19, 2018
451
298
Miami
Just curious if you've tested the scenario where the small runtime extended inverter (even if really small) provides more watts than the house load? Does the small inverter just ramp down in that scenario because it's aware of the house load?

I suspect the voltage would increase beyond the Powerwall's limit, and the Powerwall would turn itself off (turning the grid-tie inverter with it).
 
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