4Springs' Home Battery Storage System

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4Springs
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4Springs' Home Battery Storage System

Post by 4Springs » Wed, 22 Aug 2018, 19:41

Some ramblings on my intentions for my Brighsun cells.
I have 32 of the 220AH units - I missed out on the 23kWh batteries, settled for two of the 11kWh units. Managed this because we have two AEVA members in the household. So slightly less power for twice the price!

We have a 10kW solar array with a SolarEdge inverter. The inverter is "battery ready", but only for one of their compatible batteries. Voltage would need to be something over 300VDC to be compatible, plus it would have to talk the same language. So I'm walking away from DC coupled for the moment and going with an AC coupled system. My battery won't even be in the same building as the solar inverter and panels...

I'll be charging from AC, so I can put the battery anywhere on the property. I just need to charge when the sun is up. This is exceedingly easy to manage with a Smappee Switch. The Smappee Energy Monitor is a 3rd party device which monitors your solar generation and your household usage. You can also buy switches, which can be programmed to turn on when certain conditions are met. The condition that applies in my case is that I'd like the 240V supply to the battery charger to turn on when my property is exporting more than X amount of electricity.

I'll need a Battery Management System to look after my cells. I'll be building more of the one that I've built before. I have another thread on this (viewtopic.php?f=64&t=5577), ask me if you'd like to buy any modules (I'd supply the circuit boards and components, you'd need to build them yourself). The main attraction of using my BMS is that I can modify it to do whatever I like. But also it is really cheap, and I have 32 cells to monitor.

I'll also need a battery charger and an inverter. To start with I've purchased three UPSs from ebay. A UPS is a combined battery charger and inverter, so everything is already done for me! Secondhand from ebay without batteries these three 3kVA units cost me $68 each. These particular UPSs are designed for a 48V battery, so they should charge up my LFP cells as long as I tell the BMS to turn the chargers off when they reach a certain voltage. The UPSs are not likely to charge the pack fast (although three of them should be three times as quick as one of them), but they will be a good way to get me going relatively quickly. In the future I could add another battery charger.

The UPS idea means that I will not be connecting my (inverted) battery output to the grid, or even into the house circuits. This means that I don't have to get an electrician, I can do this all myself. It also means that I can only use the output for loads close to the battery. What I'd really like to power is the house (stove, heatpump etc.), but in this configuration I'll have to settle for the cars. Most days we use 10kW+ in the cars, so this is not a bad solution. Plenty of space for the battery in the garage too. Probably in a secondhand metal cabinet.

The Smappee can control when the batteries are charged. Simplest way is to switch on/off the AC supply to the UPS.
The BMS can turn off the chargers when the battery is full. Simplest control would be a relay to turn off the AC supply (i.e. in series with the Smappee).
The BMS will also need to stop the system from discharging the batteries too far. Possibly by supplying AC back to the UPS again, so that the load is still supplied un-interrupted.

The UPSs are fairly sophisticated, they can accept RS-232 commands. The BMS can output RS-232, so the control may get more sophisticated than just turning relays on and off.

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Re: 4Springs' Home Battery Storage System

Post by jonescg » Wed, 22 Aug 2018, 20:34

I think the AC coupled (or should that be de-coupled?) system is a winner for most EV drivers. There's enough stored in a battery to charge an EV that's travelled an average of 50 km a day. The trick is to ensure it's charged on the cheapest power generated at the right time. Sounds like the gizmo you have will do the trick!
My gizmo is a socket timer... Hey, it's WA and we can count on sunshine 300 days a year!
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Re: 4Springs' Home Battery Storage System

Post by TasJack » Thu, 23 Aug 2018, 10:08

This is also my preferred approach. I still have an active enquiry with Tas Networks on local grid connected requirements, but not expecting anything constructive.

I plan (so far) to run 2 independent 24 volt systems off grid, each of around 11kwh. With lower sunshine energy in Tasmania, particularly in winter, I expect to permit one system to fully charge in off peak hours (tariff 93) supported by available solar. Trying to fully charge 23kwh from winter sunshine is not feasible. The other charged system will be available for discharge at this time. Rather than complex integration with the solar system I plan to use a simple timer to ensure operation between 9am and 4pm. Discharge will be used for EV charging (12.8kwh max with the Outlander). A "night fill" option might be nice to have, using off peak energy.

My metal cabinet is nearing completion, shelf to be fitted and cladding to fit. Then set up the cells. Still watching with interest the functionality of the Batruim BMS.

Nice to see the same cells being implemented in a variety of setups.

KISS - keep it simple (stupid me) :) is my approach.

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Re: 4Springs' Home Battery Storage System

Post by jonescg » Thu, 23 Aug 2018, 11:58

TasJack wrote:
Thu, 23 Aug 2018, 10:08
I plan (so far) to run 2 independent 24 volt systems off grid, each of around 11kwh. With lower sunshine energy in Tasmania, particularly in winter, I expect to permit one system to fully charge in off peak hours (tariff 93) supported by available solar. Trying to fully charge 23kwh from winter sunshine is not feasible. The other charged system will be available for discharge at this time.
Still not sure why you would do this - wouldn't it be better to have the full capacity available? It might not completely charge on day one, but you won't use it all so by the end of day two it's full. If you can't fill all 23 kWh with winter sun, you probably shouldn't be taking too much from it at night either. And at a pinch you can always charge it from off-peak power if you really needed to keep it off the bottom.
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Re: 4Springs' Home Battery Storage System

Post by ray.johnston » Thu, 23 Aug 2018, 17:12

I'm certainly keen to use the cell monitors (and a UPS). Add me to the number of boards needed.....

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Re: 4Springs' Home Battery Storage System

Post by TasJack » Thu, 23 Aug 2018, 17:31

But half the charge cycles per 12kwh unit? Still uncommitted and watching the tech talk with interest.

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Re: 4Springs' Home Battery Storage System

Post by antiscab » Thu, 23 Aug 2018, 22:27

Cycle count does a bad job of telling the story of service life. Either extreme of soc wears cells. High charge or discharge rate wears cells. High temp or extremely low temp wears cells.

Splitting the pack in two likely won't help your service life
Matt
2017 Renault zoe - 25'000km
2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells

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Re: 4Springs' Home Battery Storage System

Post by 4Springs » Sat, 25 Aug 2018, 06:20

TasJack wrote:
Thu, 23 Aug 2018, 10:08
With lower sunshine energy in Tasmania, particularly in winter,
My house, also in Tasmania in winter, seems to be a poster child for a 20kWh battery. We have a 10kW solar array, a 7kW inverter, two people and two EVs. Here are our figures for the past week:
Sunday: consumed 19kWh, produced 32kWh, self consumption 33%
Mon: 27, 17, 39%
Tue: 33, 26, 16%
Wed: 25, 31, 11%
Thurs: 33, 26, 19%
Fri: 29, 31, 10%

Here is a lovely graph of Friday's production and consumption, produced by the Smappee software:
Screenshot_2018-08-25_07-53-08.png
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The green is solar generation, the grey is consumption. The graph goes for 24 hours, from midnight to midnight. The vertical axis has some lines on it, they are each 1kW, so my maximum consumption hit 6.6kW, and my maximum generation peaked at 5.2kW.
Consumption in the morning is the heatpump and one of the cars (warm it up before I drive to work). Consumption in the evening is the heatpump, stove, then the other car after 9pm (cheaper tariff after 9pm, so the cars are on a timer).

With a home storage battery, on this day shown, I'd like to start charging about 9am, when my generation is about 2kW. Generation falls to 2kW by about 4pm. To store 20kWh in this 7 hour period means charging the battery at 2.8kW or higher. 2.8kW/48V = 58A. Maximum continuous charging current is 100A for these cells, so this would be well within specification. I'll be putting mine in parallel, so I could go up to 200A.
Note that for people with the larger cells, the limit to charge them is 100A, or 4.8kW.
Ideally I'd like to charge the battery faster when there is solar generation available - for cases where there is some sun and then some cloud. The three UPSs will give me some flexibility, but probably not a lot of capacity. It will be interesting to see how fast they do charge. In the future I could use something like a TC Charger, which can vary the charging current.

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Re: 4Springs' Home Battery Storage System

Post by 4Springs » Sat, 25 Aug 2018, 06:56

TasJack wrote:
Thu, 23 Aug 2018, 17:31
But half the charge cycles per 12kwh unit? Still uncommitted and watching the tech talk with interest.
Indeed, half the charge cycles, but the cycles are deeper and the charge/discharge rate is higher.

Depth of Discharge:
If you have your cells arranged in a 23kWh battery, and take out 10kWh, you have taken each cell down to 57% capacity. In this example you do this every day.
If you have your cells arranged in two batteries, each 11.5kWh, and take out 10kWh from one of them, then you have taken each cell in the one pack down to 13% capacity. You alternate days, so each cell is discharged to this level every second day.

My feeling is that discharging to 13%, half as often, would be worse for cell lifetime than discharging to 57% every day. These are made-up numbers of course, but in general, as Matt said, going close to the lower (and upper) end of capacity will reduce cell life.

Charge & Discharge Current:
Assuming all else is equal, halving your pack will halve your allowable charge and discharge rates per cell. Your cells have a maximum charge rate of 100A. At 48V, this means 4.8kW is the maximum rate at which you can charge. The discharge rate is higher, at 230A (11kW). If you halve your pack, you halve these numbers (because you'd halve the voltage).
Now you might not be thinking of charging and discharging at these rates anyway, but I think that cell life can be somewhat tied to charge and discharge rates. I'm not sure how much this matters though, if you are not approaching the 'maximum' rates.

Dividing into two packs might have other advantages. I'm tempted to divide mine into 12V modules (4 cells). Each would weigh 20kg (plus a bit for a box), and would have 2.8kWh of capacity. With a portable inverter, this would give me 240VAC at 1.2kW - a nice substitute for a little generator!

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Re: 4Springs' Home Battery Storage System

Post by 4Springs » Tue, 04 Sep 2018, 19:16

I found a very secondhand cupboard in the shed. Now relocated to the garage:
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What's inside?
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I've strapped these up with the strapper I've only ever used once before. There are pieces of wood at the ends of the rows. Some of the cells are a bit pudgy, hopefully they are ok.
I have 32 cells, I'll arrange them in 16s2p (did I get that right?) - a series string of 16 pairs of cells, with each pair in parallel. 10 cells fit in a row, so there are two cells not shown. Guess I'll put them on the top somewhere. Annoying to strap two cells though, I might have to restrict them some other way.

In the last photo you can see the beginnings of the cell connections. I had a plumber here the other day, and he gave me some off-cut copper pipe. I've measured up where I want the flat bits, then flatten them using my hydraulic cable crimper and some flat steel plates. Then carefully measure, drill the holes, file them into slots because I didn't get it right. The one down the bottom is on the negative end of the pack, and is ready to have a flexible cable crimped onto it. The other one is in-progress, it still needs to be trimmed a bit, and some heatshrink added. Each bar will be more-or-less a custom fit to its own four cells, since the cells have minor variations to do with where exactly they are on the not-quite-flat shelves.
Cheap but labour-intensive way of connecting cells. I wouldn't want to use solid connections like this in a car, but this pack is going to be quite stationary.

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Re: 4Springs' Home Battery Storage System

Post by 4Springs » Fri, 04 Oct 2019, 18:20

A year later, and a fair bit has happened.
The most important thing is that I found an electrician who is willing to work with my 'home made' battery. This means that I can use the battery in a grid-connect system. I had previously thought that I'd be limited to a UPS style system since I'm not an electrician, and I thought I'd not find one who would be willing to sign off on something this home made.
The electrician has worked with the Selectronic SP Pro inverter/chargers, and said that he could install a system based on one. After looking at what they can do, I can see that this is a very good fit for a home made battery. All sorts of settings can be adjusted, including battery voltages, charging/discharging strategies and state of charge levels. I'll be able to interface my BMS to the SP Pro inputs. The electrician said something to the effect that this will not work to start with - you'll have to adjust things until you get it right. Music to my ears!

The idea is that the SP Pro will charge the battery when I have excess solar output. The output that it sees is AC from the solar inverter. It will not be connected to the DC solar panels, as this is too far away in my setup. You can set up priorities. So when there is solar power being generated the SP Pro will supply the following (in order of priority):
1. Feed the appliances in the house.
2. Feed the battery.
3. Feed the grid.

We've applied for permission, and received a letter back from the electricity transmission utility. They said that I have permission to connect the battery, but must not export more than 9.6 kW to the grid. This is important because the solar inverter is 7 kW, and the SP Pro is 7 kW. So they could theoretically export a combined 14 kW to the grid. I don't intend on exporting any of my battery storage to the grid at the moment, although that might change if the electricity tariffs change at a later date.

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Re: 4Springs' Home Battery Storage System

Post by 4Springs » Fri, 04 Oct 2019, 18:49

If I'm doing a grid-connect system then I need a good battery box.
I decided that I don't have enough time to make one myself, and do have enough money to have one made. So this is what I came up with:
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Made by a local sheet metal fabricator. Cost about $2500 once it was painted and installed.

I've been putting the cells in.
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The inverter/charger is not brilliantly weather proof, so I'll install it in the box with the batteries. This means I'll need plenty of vents to let out the heat. Especially in the height of summer, as the cabinet is dark will catch the late afternoon sun. I've bought a 12V fan, which I can control with a thermostat, or possibly with the BMS. I might also put up a sunshade of some kind.

The cell interconnects are flattened copper pipe. A friend offered to flatten them for me with a machine, which is much neater than what I was doing originally.

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Re: 4Springs' Home Battery Storage System

Post by jonescg » Fri, 04 Oct 2019, 20:19

That is an epic setup. Awesome.
Is there room for more cells in that space or will you settle for the 22 kWh?

SP Pro is the ducks nuts of off grid inverters so it's also a safe bet as far as the network is concerned.
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Re: 4Springs' Home Battery Storage System

Post by 4Springs » Sat, 05 Oct 2019, 06:10

jonescg wrote:
Fri, 04 Oct 2019, 20:19
Is there room for more cells in that space or will you settle for the 22 kWh?
Ah, you noticed that space, did you?
The first design was just enough space for these cells. But a big cupboard shouldn't be much more expensive than a little one when it is being built locally, so I increased the size to fill the space available. My calculations indicate that a 20 kWh battery (the nominal capacity of this one) should be just about perfect for our current usage. In summer we don't use much more than that, and in winter we don't generate much more than that.
If experimentation shows that we could use a larger battery, there will be a 20 kWh one coming out of the Brumby shortly. I could potentially put some of those cells into this cupboard as well.

When planning the layout of these cells I could have fit up to 12 per shelf. This would have made three rows of tightly packed cells, with a spare shelf. But I'd already had the interconnects pressed, this configuration will give better airflow around the cells, and I am really not expecting to need that extra shelf for anything...

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Re: 4Springs' Home Battery Storage System

Post by 4Springs » Wed, 13 Nov 2019, 18:16

SP Pro inverter/charger is in!
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Installation took two electricians about 4 hours.
We set it up with a Wizard which had an option for 16 LiFePO cells. This set quite intelligent values into the unit, which gives me a good place to start.
The BMS is monitoring the cells, but since it was not hooked into the inverter/charger I was too scared to let it charge them up much. By the end of the day I'd wired in the "AAAAAHHHHH - SOMETHING IS WRONG DISCONNECT THE BATTERY NOW!!!!!" output of the BMS into the "Stop" input on the SP Pro. This works, if I turn off the BMS then the SP Pro alarms and stops both charger and inverter, just letting the grid connect solar system run like it did before.
The SP Pro has a programme to charge the battery. It goes from Initial to Bulk to Absorb to Float, and has an optional balance mode. Each of these modes is configurable with voltage and current setpoints, as well as time (how long once it's reached the voltage setpoint before it goes to the next mode). There are inputs I can use to force it into any of these modes.
Now I have to figure out exactly what I'm going to do with each of these modes, and how the BMS should interact. I'd like the BMS to tell the SP Pro to dial down the current when it starts to balance the cells.

For the moment it is sitting in the cabinet not doing anything. I need to do some more wiring and programming before I let it charge to full or discharge to empty. Then I need to be there to watch it to figure out any bugs. Bugs could cost me a pretty penny if I'm not there to catch them.

Speaking of money, here are my major costs:
Battery: $2000 (32 of the smaller AVASS cells)
Cabinet: $2751 (installed)
Inverter/Charger: $8411 (installed)
BMS: $200 (or so)
Total $13,262 installed, if I don't count my labour. This is for a 21 kWh system. Theoretically 21 kWh, we should figure this out shortly!

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Re: 4Springs' Home Battery Storage System

Post by jonescg » Wed, 13 Nov 2019, 20:06

I was just checking in on this thread today wondering how it was going!
How does it feel to know that your cabinet costs more than the cells it contains? :lol:

The SP Pro is one of the most highly regarded inverters in the field. Should give plenty of reliable service.
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