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Tesla PowerWall DIY battery expansion capacity

pgrovetom1

Member
Aug 27, 2019
59
7
Sonoma, CA
I'm having a 3 PW2 system installed but my summer solar generation ( about 16KW) capacity greatly exceeds the 3 x 14.5 KW = 43.5 KWHr storage capacity of the 3 PW2s. Before jumping into the 3 PW2 cost, I reviewed the possibility of doing a DIY system

Building my own pseudo(Powerwall) based on L-P-Gs
Building my own pseudo(Powerwall) posts. He did a nice job that seems quite feasible but I decided against it because of the difficulty of it being illegal and not possible to get a permit. I suspect the PW2 done legally will add value to my home thats in the PG&E fire power shutoff area and an illegal install would be a headache. So I'm asking another question on the forum.

It seems much of the battery and inverter work L-P-G did could be reused with a different coordination controller.

Has anyone considered or seen anyone who has successfully installed expansion capacity for their PW2 installation to increase the capacity to match their solar production? What I'm thinking is that I can install a battery system that is built from 4 or so 5.3KWHr ( 21 WHr) Tesla used batteries that could be in my garage and simply add additional backup that the Tesla system could use ( without knowing it).

It seems like a simpler problem that the full PW2 and all the PG&E, Permit, Automatic disconnect and inverter issues. By charging these batteries while solar power is being generated, the PW2 system would "think" its simply a load in the house it needs to produce. It could also fully charge itself while not in backup from PG&E again looking like a home load.

Then when in backup mode with no PG&E AC, it would need to fool the PW2's into believing it was "less home load" but still be responsive should the PW2's ask it to back off via UL1741 frequency control. The PW2's would then not use their own power charge and the expansion would power the home till it was exhausted and then the PW2's would take over. I haven't looked deeply into the coordination issues with the PW2s but its seems doable and could compensate for people who mistakenly installed only one or 2 PW2's and have discovered they lose Solar power during backup...

It would seem the best strategy would be to find a way to try and generate the actual home power use less some margin and do that until exhausted. Then during the next day, a power monitor on the solar would send a wireless signal to the expansion battery system with the power being generated and that would restart charging. If the solar power dropped due to clouds or whatever, it would stop charging.

It also might make sense to try and put large loads such as air conditioners or EV battery chargers behind an AC disconnect during backup mode to reduce the inverter demands.

Thoughts.

Here is L-P-G's basic design for a full PW2 look alike:

LPG PW2 diagram.jpg
 

miimura

Well-Known Member
Aug 21, 2013
6,365
6,069
Los Altos, CA
Since you're in PG&E territory, you have net metering available. It is unlikely to be cost effective to expand your storage beyond what the 3 Powerwalls are already doing. Just let the Summer surplus go to the grid so you can use those credits during the Winter.

Have you looked at my thread Powerwall 2.0 Backup Runtime Extender ?? It discusses ways to supplement the Powerwalls when the grid is down.
 

pgrovetom1

Member
Aug 27, 2019
59
7
Sonoma, CA
My interest is based on backup capacity. I'm aware even the PW2s won't be cost effective for non backup use.

I'm more concerned about riding out cloudy weather while the power is off. My tradeoff is versus a generator such as propane. But any fuel will run out and needs maintenance.

I had 2 4 day power outages last fall and I expect it to get worse. The fire risk is typically before its cloudy in November as rain ends the risk. But I want to be able to ride out the outage on my solar without worrying. I want to charge an EV without draining my PW2s. I would like to run my irrigation pump for fire control when power is out. So I would like to have more than 40kwhr during outages. I would like to dig build a backup if feasible.
 

miimura

Well-Known Member
Aug 21, 2013
6,365
6,069
Los Altos, CA
My interest is based on backup capacity. I'm aware even the PW2s won't be cost effective for non backup use.
Actually, I find that Powerwalls ARE cost effective for time shifting, at least up to the kWh of your daily Peak period usage. This requires large differentials between Off-Peak and Peak utility prices, like the PG&E EV rate schedules.

I'm more concerned about riding out cloudy weather while the power is off. My tradeoff is versus a generator such as propane. But any fuel will run out and needs maintenance.

I had 2 4 day power outages last fall and I expect it to get worse. The fire risk is typically before its cloudy in November as rain ends the risk. But I want to be able to ride out the outage on my solar without worrying. I want to charge an EV without draining my PW2s. I would like to run my irrigation pump for fire control when power is out. So I would like to have more than 40kwhr during outages. I would like to dig build a backup if feasible.
The backup runtime extender model that is discussed in the thread I linked is exactly for the scenario you describe - riding out cloudy weather when the grid is down. My point is that it doesn't make sense to use a new stationary battery for this purpose, but rather to use an EV or hybrid that is otherwise used for transportation to make it through these emergent situations.

Dedicating a battery system to your irrigation pump that is basically a giant UPS also has merit and can be justified if you can use it to save property from fire in a rural setting. Although, comparison to a ICE powered emergency water pump would be warranted.
 

GenSao

Member
Aug 3, 2017
550
938
Pleasant Hill, CA
Tesla states in their Powerwall FAQ, "To ensure reliable operation during power outages, at least one Powerwall is required for each 7.6 kW AC of solar included in the backup circuit". As your 16 KW DC solar system is within this requirement (16 kW < 7.6 kW x 3), you should be fine.

Assuming your solar inverters are recent UL compliant, the Tesla Gateway will monitor solar production and can limit/stop production as needed to protect the storage capacity and charging of the Powerwalls.
 

aesculus

Still Trying to Figure This All Out
May 31, 2015
4,472
2,539
Northern California
But I want to be able to ride out the outage on my solar without worrying. I want to charge an EV without draining my PW2s. I would like to run my irrigation pump for fire control when power is out.
I have the same scenario but with one BIG difference. I am at the edge of the PSPS so if needed I just need about 10% to get to an urban SC that is downhill. Now it's probably true that there will be many others like me doing the same but.

I basically sized my system to run without the car and probably much of my AC too as long as it remained sunny. I just did not think it was worth doing more as those two components are about 80% of my daily load when used.
 

gpez

Member
Apr 25, 2019
689
567
USA
My interest is based on backup capacity. I'm aware even the PW2s won't be cost effective for non backup use.

I'm more concerned about riding out cloudy weather while the power is off. My tradeoff is versus a generator such as propane. But any fuel will run out and needs maintenance.

I had 2 4 day power outages last fall and I expect it to get worse. The fire risk is typically before its cloudy in November as rain ends the risk. But I want to be able to ride out the outage on my solar without worrying. I want to charge an EV without draining my PW2s. I would like to run my irrigation pump for fire control when power is out. So I would like to have more than 40kwhr during outages. I would like to dig build a backup if feasible.

@pgrovetom1 out of curiosity what EV? I took the learnings from @miimura's Powerwall 2.0 Backup Runtime Extender thread and use my Chevy Volt for extending my Powerwall runtime. If there is an outage during the summer I can charge it from the PV.

I have 1 Powerwall so having the Volt as a backup backup is really nice.
 

pgrovetom1

Member
Aug 27, 2019
59
7
Sonoma, CA
I sold my Leaf and am thinking about another. I have not decided. My girlfriend has a Volt which charges on my system. I don't like the idea of using an EV unless the manufacturer provides the capability via the charger system. How do you do it? How much power do you utilize from a Volt? My interest is using stand alone batteries that supplement the PW2 and not using an EV which might impact its warranty and could need to be disconnected and used just when the power is needed. A used 5.3KWhr Tesla battery can be bought for about $1000.
 
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gpez

Member
Apr 25, 2019
689
567
USA
I sold my Leaf and am thinking about another. I have not decided. My girlfriend has a Volt which charges on my system. I don't like the idea of using an EV unless the manufacturer provides the capability via the charger system. How do you do it? How much power do you utilize from a Volt? My interest is using stand alone batteries that supplement the PW2 and not using an EV which might impact its warranty and could need to be disconnected and used just when the power is needed. A used 5.3KWhr Tesla battery can be bought for about $1000.

Powerwall 2.0 Backup Runtime Extender has the details of my setup.

Summary is that I bought a non-grid tie 1200w pure sine inverter, hooked up Anderson connectors to the 2000w traction battery step down 12v transformer in the trunk, and now can use the 18kWh traction battery + generator capability of the Volt's ICE to power some of the load independent from the Powerwall during longer outages. Anderson connectors hide neatly in the utility panel in the trunk when not in use - no trunk space is lost. Very simple, cost me less than $250, and from my understanding does not affect the Volt's warranty. Same setup that comes in prepackaged kits like EV Extend, just DIY. While my 14-50 outlet isn't on the critical loads there are plenty 5-15s in the garage that are which will allow me to charge at 120v @ 12a if the PV is producing more than I need during an outage.

Again since the inverter is non-grid tie there is no backfeed to the house - I simply used a heavy duty extension cord to reach the first floor appliances, effectively shedding those loads from the Powerwall.
 

pgrovetom1

Member
Aug 27, 2019
59
7
Sonoma, CA
I wanted to share a diagram of the concept example I was investigating to see if I could get ideas and feedback.

The main purpose is to add additional backup on top of the PW2 to help ride out cloudy days while in backup and perform time shifting of solar production. My understanding is the PW2 cannot perform time shifting of solar if the system used the Federal solar credits.I received a Federal Tax credit.

An example would be to utilize two 500W UL1741 120V AC Micro inverters, one per leg ( since they use different amounts of power) ( L1/L2) to inject power from the battery. I chose in my example a 5.2KWhr Tesla battery given its about the right minimum size. Additional Tesla batteries could be used up to the charger limit. It could supply 10 hours of 500W as one example. My home never uses less than about 800W at night baseline so one strategy would be to monitor the home L1/L2 power and adjust the current limiters to match that power up to 500W per leg. The charger would be enabled during solar production hours if there was adequate power being generated as measured at the solar inverters.

The charger would not be connected if the battery was fully charged or the temperature was low. Each UL1741 micro inverter would be fed by the 24V battery through a variable current limiter. The current would be varied to manage the power produced and injected by the micro inverters. This would make the micro inverters power vary as the MPPT adjusts to the maximum current at 24V. At 24.6V, the limiter would be adjusted up about 20 A to allow the production of the full 500W. This allows the injected power to be any amount from zero to the maximum of 500W in this example. Each AC leg would be using a different amount of power which could tracked. The current would be set to zero if the temperature was too high or the battery fully discharged.

The current monitors on the solar, PW2 and home would communicate the current power to the Raspberry Pi controller via a radio link using LoRa as one example. By utilizing a real time clock and monitoring the solar power, PW2 power, home power and injection power, the injected current would be adapted to the circumstances and time of day. So for example, charging would be restricted to when there was sufficient solar power. Power injection would be based on an effort to provide the current homes power use up to the 1000W in this example.In non-backup mode, it could be programmed to charge during maximum solar production and discharge during the evening highest tariff time. This would allow power arbitrage even if the PW2's won't do it due to the Federal tax credits.

One issue, that needs to be investigated is when the PW2's raise the 60Hz to shut off the solar inverters. Do they raise the 60Hz only when they are fully charged? If they are powering a home at night and not fully charged, do they lower the 60Hz allowing the solar inverters to come on in the morning. Do they only raise the 60Hz when they are fully charged and cannot accept charge or do they know the time of day and solar production hours etc.. The concern is these UL1741 micro inverters will comply with the raised 60Hz shutoff. If the PW2s only raise the 60Hz when they cannot accept charge irrespective of time of day, then this should work ok.Otherwise the micro inverters would not be allowed to discharge if the PW2's raised the 60Hz to shut off the solar inverters.

Thoughts?

Extender.JPG
 

Attachments

  • Extender Diagram.pdf
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