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California 48V, LiFeP04, AC Coupled Bachelor Pad
Ahoy there, friends. 

After all the help here and on the FB group, I'm well on my way to home storage implementation. I've digested a lot of information over the past few months, so I'm going to attempt to architect this thread with a lot of background information so that newcomers down the line will be able to use it as a stepping stone for their builds.

Currently installed:
8.1kW PV system with Enphase IQ6+ Microinverters, with generation peaking at ~50kWh/day during summer, and down to ~2kWh/day during the rainy rainy days, and ~10kWh during winter. Located in Livermore, California

  1. System aimed at peak shaving, and (when the grid inevitably undergoes shutdowns due to who knows what) home backup storage. Because my PV system has microinverters, in order to charge my battery system (or be able to use the PV when the grid is down) I'll need to AC Couple the Battery inverter to the PV inverter system. This will allow my PV system to export to the grid (as it already does), and be able to charge my batteries when the grid goes down and has no frequency to which the microiverters can lock and inject.
  2. 48V - higher voltage means lower current for the same power (P=IV), which allows for thinner, cheaper wiring due to the decreased resistive losses needing to be accommodated by the system.
  3. LFP Batteries - Lithium Iron Phosphate is a battery chemistry that offers good energy density, great cycle life, exemplary safety, and ok cost when compared to the usual suspects of Li Ion NMC chemistry that you'll find in most cells used today. Addressing the negatives: Density was a disregard for me because it's going to be stationary, and I ended up being able to purchase ~20kWh of tested LFP for ~100/kWh, which is on par with current offerings for Li Ion. Info on chemistry here.
  4. 120/240V Split phase - I want to be able to wire up my house as it currently stands with no additional breaker boxed or modifications for "critical loads."
  5. ~50%DoD for regular daily use, and 90%DoD for grid backup use  - This will allow me to regularly use a good portion of my bank while optimizing for a good battery lifetime for regular use, and then obviously allow me to use the storage system for backup when needed. Batteries aren't huge fans of being run to their extreme high or extreme low states, so you generally want to center your cycling. Example: 50% DoD should be 25-75%, 90%DoD should be 5%-95% Info on DoD vs lifetime here
  6. EV Charging - needs 240V for any reasonable rate/efficiency
  7. Grid Chargeable - wanted to be able to charge the storage system from grid during off peak TOU (.12 from 12a-7a) times to be able to use at Peak TOU (.53 3a-9a)
  1. TOU Rate arbitrage. This seems to be a real pain, but I want to try it out. Rate arbitrage is the act of charging from the grid when it's cheap, then *selling back to the grid* when it's at peak. I'm told this requires a number of extra steps, permitting, and potential headaches, but it's intriguing for me as a way to "fund" some of this equipment. Open to suggestions on how to make this happen.
Equpment Plans:

Inverters - SMA 6048-US-10. Current ebay listings have you around $2.5-3k for two. I bought from the DC Solar Liquidation sale so mine were quite a bit cheaper.
These inverters are fantastic for the following reasons:

  1. Cheap - DC Solar went bankrupt, so their huge inventory of these inverters was auctioned off to the general public in July 2019. MSRP of these are $5k each, but on eBay/local craigslist equivalent, you can find them for around $1500 each with some sleuthing.
  2. UL Listed - This covers your ass in a couple ways: you're not as worried about some chinesium inverter having a meltdown (within reason, please wire your systems safely); if there is a fire, you'll be able to report that your inverter was UL listed to the insurance company, which decreases your negligence liability. Also guarantees that the device will be safe for line workers when the grid goes down, ensuring that no power is being sent to the lines that could put them at risk while working.
  3. 120/240V split phase capable (via pairing) - 2x 6kW 120V inverters can be wired up to in tandem to drive the respective 120V live voltages 180* apart, thus giving a complete split phase system
  4. Frequency-Watt control: This allows the storage inverter to make sure that the PV system doesn't pump out too much energy and overload the batteries. As the batteries fill up (and the grid is disconnected, the storage inverter increases the 60Hz frequency, and the PV inverters linearly decrease the power output by 60%/Hz from 100%@61.5Hz to 0%@63.17Hz. Documentation found here, page 6. Please note that this power curve will likely be different for other microinverters, so consult your spec sheets.
Batteries - 4x Used 24V 5.6kWh (4.5-5kWh tested) BYD LFP, ~$2k
  • Batteryhookup had a sale on these for ~$100/kWh, which was the threshold I was waiting for, so I jumped on em. Each 24V battery is 8s, so I plan on stacking 2 for 48V, and then another string in parallel for a 16s2p configuration.
BMS - Chargery 16T-300A, $156
  • 1.2A balance current 
  • High/low temp/voltage cutoff. Low temp cutoff is a rare find for lower cost BMS units. The datasheet doesn't spec LTCO, but I at least got word from the distributor that they did a hands on test and it did cut off at 0C. I'm getting in contact with the developer to get more info on it and see if he can make that a configurable parameter.
  • 300A charge/discharge current. Do I trust the full 300A? No - but considering my inverter has a 140A charge limit, it's better to make sure I didn't get the 100A version.

Interested in hearing any and all feedback, particularly from you Californians!
Redpacket likes this post
Its interesting - thanks for sharing all the detailed info Smile

>TOU Rate arbitrage. This seems to be a real pain, ..... requires a number of extra steps, permitting, and potential headaches,
I have a 13kw array but even so, my home (with air conditioning in summer) always consumes more power than I can produce. So I went 100% off-grid to avoid power company interactions - I use ATS'ing to automatically switch between my own power and grid. Sounds like you have excess power at times which has lead you to a hybrid system.

Another difference (if I read you correctly) is that your battery bank is more of stand-by/backup role because you use PV power directly but only use battery when grid is down? Being off-grid, my battery bank is charged/discharged daily.

Its fun to hear what others are doing.
I won't explicitly have excess as I have an EV, BUT if I can arbitrage, I will explicitly be cycling the system to charge nightly and discharge during peak times. With EV2A rolling through California with its Peak window from 4-9p, its gonna be a more enticing and economical to inject as much as I can.

It CAN be used as a backup, but the primary aim will be peak shaving. Charging at night or from solar and then allowing me to zero my peak usage.
Hi kpeters000, system that you described is exactly that I want to build as well.
I'm just starting learning solar systems and how it works. Planning to deploy solar in about 6 month.

Please keep us posted. Thank you!
Found a little hiccup with my plans after toying around with the SMA 6048.

It is designed as an Off-Grid inverter with grid connection capabilities, which is juxtaposed to Hybrid or Grid-Interactive.

The 6048 has some interesting grid connection functionality: Basically, you can define 2 time ranges (think daytime and nighttime) and SOC dependent Low-SOC grid connect trigger + High-SOC grid disconnect trigger for each time range. This is not a trigger to start or stop battery charging during those times - is it literally a trigger for whether or not the inverter connects to the grid. When it connects to the grid, it seems to automatically start charging the batteries until they are at the top limit SOC, and then it disconnects.

This poses a bit of an issue because my intended application was to charge the batteries from the grid at night with off peak grid power and contribute to the grid with solar during the day. With this grid connectivity functionality, it doesn't appear that I'll ever be able to have the inverter grid-connected yet also have the solar backfeeding. In the AC coupled configuration with the PV behind the inverter, the PV power would charge the batteries, and only the small excess would backfeed. This is problematic because time when sun is out is always going to be worth more as park peak or peak time credits vs the off peak grid power to charge the batteries.

It seems the only way to maintain solar PV backfeeding and still have them available for when the grid goes down would be to have the PV output behind an ATS that defaults to grid connectivity, then switches to the load side of the 6048 when the grid goes down.

Can anyone else tell me if I'm absolutely hammered in interpreting this, or if they've found another way around it?

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