minimal battery bank?

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Megunticook
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My RE system: 6.4 kW solar array (21 QCell 305 panels in strings of 3)
GS8048A FLEXpower Radian (Radian 8048A, 2 FlexMax 80 cc's, Mate 3s, FlexNet DC, Hub)
8x6v Rolls-Surette 550 FLA batteries, 428AH
Location: Maine
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minimal battery bank?

Post by Megunticook » Fri Sep 29, 2017 4:44 am

I'm building a hybrid system around the 8048A but could use some advice on the battery bank. Since battery technology seems to be advancing rapidly and my budget at the moment is limited, I'd like to build a minimal interim bank just to get up and running with the Radian, essentially using the grid as my "backup" when demand exceeds PV output, at night, etc. I'm in the process of establishing a net metering agreement with Central Maine Power (part of the picture here is that the net metering terms are being downgraded starting January 1, which is why I'm trying to get up and running ASAP).

My initial plan was to install a minimal bank of flooded lead acid batteries--I asked the folks at Outback about this and they said no problem, just make sure you have at least 400AH. But as I research this online I'm seeing various discussions where people indicate a small battery bank can create issues. The array I have planned is 7 triple-module strings (915watts per string), for a total of 6.4kW. I'll have two FlexMax 80 charge controllers.

I was thinking the battery bank would stay fully charged 99% of the time, on float when the array is producing (I would prefer not to let the Radian use the grid to charge/maintain the batteries unless absolutely necessary). Our household loads (approx. 10kWh/day) would be powered by the array with any excess being sold to the grid. When the array is not producing (or not producing enough to cover the loads), the grid would provide the balance of power needed.

In the event of a grid outage, we'd use the batteries to power a very small subset of critical loads, knowing that this might only be for a day or possibly less. We'll have a generator also.

Again, this is just an "interim" setup that allows us to get up and running and establish a net metering agreement before the end of the year, a much larger battery bank will get installed in the future.

Can you see any potential issues caused by having such a small battery bank? (other than not having much backup power available)

Thanks much for any advice.
https://sunnypower.org

Mike Curran
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My RE system: Outback - Garage roof:
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- 2 grid-tied GVFX3524's classic stacked for 120/240VAC (2007)
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Re: minimal battery bank?

Post by Mike Curran » Fri Sep 29, 2017 11:58 am

A 400AH battery will give you 200Ah down to 50% SOC. You shouldn't cycle them below that. 200Ah × 48V = 9,600Wh, which seems a good match for your daily loads, IF your PV output is sufficient to recharge them

6.4 KW of PV × 3 hours of sun (plug in the right number for your location) = 19kWh which seems adequate to recharge your batteries and sell a bit, too.

Sound like your interim battery plan could also be your final plan... :smile:
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https://enlighten.enphaseenergy.com/public/systems/Hctc107221

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Megunticook
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Posts: 242
Joined: Fri Sep 29, 2017 4:08 am
My RE system: 6.4 kW solar array (21 QCell 305 panels in strings of 3)
GS8048A FLEXpower Radian (Radian 8048A, 2 FlexMax 80 cc's, Mate 3s, FlexNet DC, Hub)
8x6v Rolls-Surette 550 FLA batteries, 428AH
Location: Maine
Contact:

Re: minimal battery bank?

Post by Megunticook » Sun Oct 01, 2017 4:49 am

Thanks--that's reassuring. I'm pretty sure we're going to want more battery capacity, but that can wait until next year or the year after.
https://sunnypower.org

koalamotorsport
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Re: minimal battery bank?

Post by koalamotorsport » Wed Oct 18, 2017 8:09 pm

You can absolutely do a small battery bank.

I have 2 Radian 8048 inverters being fed by almost 12kw of panels. They are hooked to a 420Ah battery bank consisting of 8 Interstate LC16HC batteries that cost about $2k.

The batteries have proven to last about 6 hours of night use, and as little as 7 minutes of day use, but even that is long enough for the Automatic Generator Start command to get things up and running.

With your limited needs, I'd suggest you could even go a lot smaller than that. I'd be interested to know why Outback says 400Ah minimum. What would be the detriment to going below that, because if there is no detriment to the equipment, then for the reasons you stated, as long as you have 48V, at any Ah rating, your system would run.

I have chosen to do this 430Ah bank for exactly the same reason you state. By the time I have enough money for the 4000Ah I technically need, I'll be able to buy 8000Ah thanks to technology improvements. For now, my system functions perfectly for my needs. I'm sure you'll find the same.
Thanks!
Brett

2 x GS8048
5 x MX60
32 x 280w Chaori modules
10 x 285w GCL modules
Puny 430ah FLA battery bank

2 x Fronius IG Plus Advanced 6.0
48 x 285w GCL modules

10kW propane generator
10kW diesel generator

blackswan555
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Re: minimal battery bank?

Post by blackswan555 » Wed Oct 18, 2017 8:56 pm

One thing to take into consideration is maximum charge current allowed into the batteries, Rolls for example say 5 to 20% of AH on FLA, It would be a possibility they end up getting all of your 6.4 kw about 130 amps (650 ish ah bat required) eg grid go-s down at 11am ? ,,,,,Outside possibility also your Inv at same time ?
You may want to look at different technologies that do allow high charge currents,
Tim
My comments are based on my experience and research, They are not endorsed or checked by Outback.I am an independent British electrician living in Spain, So please take this into account when reading /acting on my post`s.

koalamotorsport
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Re: minimal battery bank?

Post by koalamotorsport » Wed Oct 18, 2017 9:01 pm

Not sure if charging amps is as critical when there is a load on the batteries at the same time, i.e, selling. I'll leave that up to an Elec Engineer to comment.

But, when the grid goes down, the system will simply shut down the panels as needed to prevent the batteries from being overcharged.
Thanks!
Brett

2 x GS8048
5 x MX60
32 x 280w Chaori modules
10 x 285w GCL modules
Puny 430ah FLA battery bank

2 x Fronius IG Plus Advanced 6.0
48 x 285w GCL modules

10kW propane generator
10kW diesel generator

blackswan555
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Posts: 2637
Joined: Sat Aug 30, 2008 5:50 am
My RE system: Other peoples, VFX "E" versions, FLA`s, Generators.
Location: Ibiza Spain,

Re: minimal battery bank?

Post by blackswan555 » Wed Oct 18, 2017 9:39 pm

Not sure if charging amps is as critical when there is a load on the batteries at the same time, i.e, selling. I'll leave that up to an Elec Engineer to comment
No it is not as long as the load is there, I am talking about the time the grid falls over and the charge controllers are putting out 160 ish amps,,,,,, into partially charged batteries,
But, when the grid goes down, the system will simply shut down the panels as needed to prevent the batteries from being overcharged.
I am not that familiar with GT, What setting is that ?

Tim
My comments are based on my experience and research, They are not endorsed or checked by Outback.I am an independent British electrician living in Spain, So please take this into account when reading /acting on my post`s.

koalamotorsport
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Joined: Sun Nov 04, 2007 4:29 pm
Location: Cleveland, Ohio

Re: minimal battery bank?

Post by koalamotorsport » Thu Oct 19, 2017 8:30 am

blackswan555 wrote:
Not sure if charging amps is as critical when there is a load on the batteries at the same time, i.e, selling. I'll leave that up to an Elec Engineer to comment
No it is not as long as the load is there, I am talking about the time the grid falls over and the charge controllers are putting out 160 ish amps,,,,,, into partially charged batteries,
But, when the grid goes down, the system will simply shut down the panels as needed to prevent the batteries from being overcharged.
I am not that familiar with GT, What setting is that ?

Tim
The time span that the large amperage will be going to the batteries will be measurable in less than minutes. The charge controllers will disconnect the panels as needed to prevent overcharging the batteries. The MATE3, possibly earlier ones too, knows the Ah rating of the battery bank, so I'm sure it calculates that into the decision on when to reduce the solar input. I'm sure there will be a small transition period while the system reacts, especially if the batteries are already in a low state of charge for some reason. My guess is the primary math is based on battery voltage, which means the system could pump high amps in until that voltage target is met, but my experience is that the panels get disconnected pretty darned quickly when needed.
Thanks!
Brett

2 x GS8048
5 x MX60
32 x 280w Chaori modules
10 x 285w GCL modules
Puny 430ah FLA battery bank

2 x Fronius IG Plus Advanced 6.0
48 x 285w GCL modules

10kW propane generator
10kW diesel generator

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