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ChiefPompey

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Apr 14, 2021
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Hello All -

After playing around with drones and RC cars and learning a bit about batteries, curiosity got the best of me and the thought of building a larger sized battery kept rattling around in my head. Leave it to the internet to connect a bunch of like minded people and have ideas and information flowing freely! Video after video on Youtube made me think "Hey, this isn't just a pipe dream!" It's really exciting to find this forum and I look forward to interacting with you all!

I'm currently in the process of planning and learning about building a battery to power my home.. I live in the US and I currently have a grid-tied 12.06kw solar system on Enphase micro inverters. I would like to have a battery for backup power and the ability to go off-grid. Though I have a familiarity with many of the concepts here, a few things leave me with questions - specifically around putting all the pieces together for a home battery bank.

My thought is that a 14s100p battery would work for my needs, possibly two of them if I find that I'm capable of making this all work. From what I've read so far, I would require an AC coupled system to make this all work, and I think I need a hybrid inverter for the battery to the "energy pool." The SMA Sunny Island initially caught my eye, but this is rather expensive. I'm having trouble searching the internet for more information because I suppose that I do not know the relevant questions to ask.

What alternatives are there in terms of inverters for grid-tie systems to allow off-grid use? Is this simply called a hybrid inverter?

Thank you all very much!

--ChiefPompey
 

floydR

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Joined
Aug 23, 2017
Messages
1,148
Welcome to lithium Anonymous support : Right now almost everyone is going to large format LiFePO4 200Ah and above. I am piecing together a mixed chemistry off grid system. Also helps if you make up your mind on thre voltage battery system you will be using (12v not realistic, 24v, 36v, 48v, 60v) I have a mix a 24v Xantrex Prosine 1000 watt 120v, a 3000 watt Reliable/WZRELB inverter120v, a 48v Growatt split phase inverter 240/120. Currently None are online. :( too many choices.
Later floyd
ps https://www.acsolarwarehouse.com/news/enphase-iq8-microinverter-to-operate-without-the-grid/
https://diysolarforum.com/threads/b...phase-inverters-without-grid-connection.8028/
 
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Roland W

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Oct 9, 2017
Messages
125
What alternatives are there in terms of inverters for grid-tie systems to allow off-grid use? Is this simply called a hybrid inverter?
Hi, welcome!

"Hybrid Inverter", can be a preety broad term. Everything, what is not a pure Grid-tied inverter or a pure Off-grid inverter, is called a Hybrid.
But there are many nuances into this. Some inverters are All in One (PV-Array and Load), some focus on coupled discharge into the grid when you are telling them to do so, most are kind of Off-Grid inverters connecting you back to Grid if battery is low or demand to high, etc.

So, when you are planning to use an existing Grid-tied System to repurpose for DIY energy storage as well, you would first need to know, what exactly you really want.
1. If it is mainly, to be protected from Grid failures, then you would have to look into those inverters, which are actually the Off-Grid type, with alternative Grid connection. But in this case, you of course need to choose a device, which can cover your typical demand, as in case you are overloading the unit, it will simply switch over to Grid.
2. If you primarily just want to use your power behind the meter, before you are giving it away for pennies to the utility, then you would buy an AC coupled inverter, which will take excess into your battery, and discreetly push it back, when you else would import energy. If you would in this case as well want to access battery power for emergency use, you might have to use an extra cheap small Off-Grid inverter to run some essential loads in those times.

Its your choice. Many things are possible in DIY as well. Prior research is the best way to prevent stepping from one rabbit hole into the next.

Have Fun!
 

ChiefPompey

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Apr 14, 2021
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Thanks everyone for the replies.

I was planning on using 18650 cells - which would put me in the 48v range for the battery.


Thank you for these links Floyd. This gives me something to chew on. Unfortunately, I do not have IQ8 micro inverters, I believe they are IQ7+. Nonetheless, I think this will help me in my journey.

1. If it is mainly, to be protected from Grid failures, then you would have to look into those inverters, which are actually the Off-Grid type, with alternative Grid connection. But in this case, you of course need to choose a device, which can cover your typical demand, as in case you are overloading the unit, it will simply switch over to Grid.
2. If you primarily just want to use your power behind the meter, before you are giving it away for pennies to the utility, then you would buy an AC coupled inverter, which will take excess into your battery, and discreetly push it back, when you else would import energy. If you would in this case as well want to access battery power for emergency use, you might have to use an extra cheap small Off-Grid inverter to run some essential loads in those times
This does give me something to think about. My solar system qualifies for net metering, so it was sized for total yearly use instead of peak use (my energy production is insufficient for typical use in the summer months but more than adequate in the spring fall and winter). Can you please give me an example a model of each? I think this will give me some things to further research to understand the nuances between the two cases you outlined.
 
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Roland W

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Oct 9, 2017
Messages
125
This does give me something to think about. My solar system qualifies for net metering, so it was sized for total yearly use instead of peak use (my energy production is insufficient for typical use in the summer months but more than adequate in the spring fall and winter). Can you please give me an example a model of each? I think this will give me some things to further research to understand the nuances between the two cases you outlined.

There are tons of devices from all sorts of manufacturers. Brand, chinese, expensive, inexpensive. A good place to look is inside the inverter threads of this forum.
Off-grid (with Grid backup): MPP Solar and clones, Growatt, many brand companies
Grid tied: Sofar ME3000SP, Sun2000GTIL, etc. In this type, Brands typically are bundling with proprietary battery tech (communication protocols) and are not as DIY friendly. So it will be mostly Chinese equipment you can use easily

In my system I am doing it with a Sun2000 and AC chargers
 

ChiefPompey

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Apr 14, 2021
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Thank again to everyone for the suggestions and links. This has led me down the right path in gathering more information.

One thing that has been bugging me is trying to figure out which cells to choose for the battery. As I understand, there are high drain 18650 cells which are capable of up to 30A discharge. There are other cells which are capable of much less (around 7-10A). When putting these together into a 14s100p battery, the math gets a little fuzzy for me in terms of total discharge at 48v.

How can I calculate the safe discharge from this battery pack using the discharge rating for the individual cells (assuming all cells are the same brand and model)? I understand (or at least think) that the inverter will limit the discharge in practice - but I'd like to get a good handle on the math behind it.
 

Roland W

Active member
Joined
Oct 9, 2017
Messages
125
Thank again to everyone for the suggestions and links. This has led me down the right path in gathering more information.

One thing that has been bugging me is trying to figure out which cells to choose for the battery. As I understand, there are high drain 18650 cells which are capable of up to 30A discharge. There are other cells which are capable of much less (around 7-10A). When putting these together into a 14s100p battery, the math gets a little fuzzy for me in terms of total discharge at 48v.

How can I calculate the safe discharge from this battery pack using the discharge rating for the individual cells (assuming all cells are the same brand and model)? I understand (or at least think) that the inverter will limit the discharge in practice - but I'd like to get a good handle on the math behind it.

Similar to a chain, where the whole chain is as strong as its weakest link, this rule applies to your pack as well.

If you are mixing cells with different C-rates, the cell with the lowest C-rate is determining the max discharge rate for the whole other cells in parallel. Lets say you have a 100p pack made out of 3000mAh cells. 50 cells are max 2C, 50 cells max 10C. Your max discharge rate is 2C for the whole pack:
3A*100p*2C=600A peak current.
For a 48V nominal system that would be a 30kW inverter on max load required to pull that current :)
 

ChiefPompey

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Apr 14, 2021
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Similar to a chain, where the whole chain is as strong as its weakest link, this rule applies to your pack as well.

If you are mixing cells with different C-rates, the cell with the lowest C-rate is determining the max discharge rate for the whole other cells in parallel. Lets say you have a 100p pack made out of 3000mAh cells. 50 cells are max 2C, 50 cells max 10C. Your max discharge rate is 2C for the whole pack:
3A*100p*2C=600A peak current.
For a 48V nominal system that would be a 30kW inverter on max load required to pull that current :)

This makes a lot of sense, but let me repeat to make sure I understand.

The total draw of the battery scales with the number of cells in parallel. So for a 1s2p pack, where each cell can draw 2A, I would be able to safely draw 4A off the pack. A 2s2p pack, I would also be able to draw 4A from the pack, but at a higher voltage. Is this correct?
 

Roland W

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Oct 9, 2017
Messages
125
This makes a lot of sense, but let me repeat to make sure I understand.

The total draw of the battery scales with the number of cells in parallel. So for a 1s2p pack, where each cell can draw 2A, I would be able to safely draw 4A off the pack. A 2s2p pack, I would also be able to draw 4A from the pack, but at a higher voltage. Is this correct?

Yes, correct. P is adding up Amps, S is adding up Voltage
 

Korishan

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Jan 7, 2017
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Welcome ChiefPompey, to the community. You may find some other quick info over in the FAQ section to answer some questions you may not know you even had.

 
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