# Help with battery to PV sizing

palausystem
Solar Expert Posts:

**122**✭✭✭
We have 3Kw of panels and need a new battery bank to match. Our current (first) bank is on its way out. It is 800Ah at 48v of Surrette S-530's which is obviously way too much.

Our daily usage is about 5.3 Kwh/day in winter and I'm thinking we might have a way to get it closer to 4.1Kwh/day. So I'm thinking maybe 8 of the Rolls S6460 AGM for 415Ah, or 16 of the Rolls S6-275 AGM for 500Ah in 2 strings.

We are in Concord, CA, and get a good 6 hrs on the Panels in mid-winter.

According to the formula (Rolls): Absorption time = 0.42 X (C/I) - (for 50% DOD), with 415Ah battery gives us 4.35 hrs of absorb, (40A peak charge) (does this formula hold true for AGM?)

I looked at the Concorde SunXtender and their formula for the AGM is (DOD/100) X Ah/charger output +2, which would bring it up to 6.18 hrs, not sure why the big difference?

Comments on AGM would be welcome too; as I am away a lot and the mate is a bit tech-phobic, being able to avoid watering and EQ charges would be worth a lot, but what would I be giving up? Longevity? The Rolls AGM have a 5 yr warranty vs the Concordes at 1 year. It would feel weird to not be able to an SG test to see what's happening.

Thanks for your comments...

18 Sharp 170w panels; Sunny Boy 3000; SMA 5048U; 16 Rolls S-530 48v 800Ah; Kohler 8.5 RES backup Gen

Our daily usage is about 5.3 Kwh/day in winter and I'm thinking we might have a way to get it closer to 4.1Kwh/day. So I'm thinking maybe 8 of the Rolls S6460 AGM for 415Ah, or 16 of the Rolls S6-275 AGM for 500Ah in 2 strings.

We are in Concord, CA, and get a good 6 hrs on the Panels in mid-winter.

According to the formula (Rolls): Absorption time = 0.42 X (C/I) - (for 50% DOD), with 415Ah battery gives us 4.35 hrs of absorb, (40A peak charge) (does this formula hold true for AGM?)

I looked at the Concorde SunXtender and their formula for the AGM is (DOD/100) X Ah/charger output +2, which would bring it up to 6.18 hrs, not sure why the big difference?

Comments on AGM would be welcome too; as I am away a lot and the mate is a bit tech-phobic, being able to avoid watering and EQ charges would be worth a lot, but what would I be giving up? Longevity? The Rolls AGM have a 5 yr warranty vs the Concordes at 1 year. It would feel weird to not be able to an SG test to see what's happening.

Thanks for your comments...

18 Sharp 170w panels; Sunny Boy 3000; SMA 5048U; 16 Rolls S-530 48v 800Ah; Kohler 8.5 RES backup Gen

Off-grid 48v, 3 kw PV, Sunny Island 5048U, SunnyBoy, US Battery RE L16XC 6v, 800ah, Genset: Kohler 8.5 RES propane.

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## Comments

17,615✭✭Well this is the way I do it and it seems to work:

3000 Watts * 0.77 = 2310 average Watts output.

2310 / nominal 48 Volts = 48 Amps peak current.

48 is 10% of 480 Amp hours.

No wonder the 800 Amp hours of Surrettes died.

Absorb time is a matter of DOD and can't be calculated in advance due to daily fluctuations.

480 Amp hours discharged 25% gives you approximately (120 * 48 ) 5.7 kW hours of power.

3kw array over 4 hours equivalent good sun and 52% over-all system efficiency: 6.2 kW hours AC so that works with the battery capacity (this number should always be slightly higher than the battery number).

Differences between AGM's and flooded cells: AGM's cost more per Watt hour, can take/give higher current, charge at lower Voltage, have lower self-discharge rate, are lighter, do not count as hazardous for shipping, can't be monitored by SG, and generally do not last as long.

But I would not buy the Rolls-Surrettes as their reputation is no longer that good. Too many reported problems including dead from new. See if you can find a local source for Crown or Deka (aka East Penn or US Battery).

364✭✭✭The LIfeline AGMs are also Concorde batteries and have a 5 year pro-rated warranty. Is the Rolls also pro-rated? Not sure why the warranty difference between the Concordes, maybe someone else knows details.

122✭✭✭So it sounds like a 500Ah AGM battery is in the ballpark.

East Penn/Deka website seems only to show Gel battery, no AGM. Crown shows a "sealed" battery with no additional info. US battery has a 260 Ah AGM - thus 2 string 520Ah - would that be pushing it?

Rolls has a 250Ah model, though your caution is noted. (When I type "AGM" into the forum search bar I get nothing back?).

Seems there's few choices in the 480Ah-520Ah range. I checked the Concorde and Lifeline and I could get 440Ah or 600 Ah with no inbetween.

So with my current PV setup so far it looks like the US or the Rolls. I'm wary of going down to 440Ah with the Lifeline...

The LIfeline AGMs are also Concorde batteries and have a 5 year pro-rated warranty. Is the Rolls also pro-rated? Not sure why the warranty difference between the Concordes, maybe someone else knows details.I read that the Lifeline has a better warranty than the concorde - more expensive. Rolls has 2 year free, 3 yrs pro-rated.

17,615✭✭Forum quirk: the search doesn't work with short 'words' like AGM.

Workaround: use Google and type site:solar-electric.com AGM

48 Amps on a 520 Amp hour bank would be (48 * 100 / 520) 9% rate so that would still work. You wouldn't want to go larger than 685 Amp hours (7% rate) so you have a fair latitude.

I see a Sun Xtender with a 224 Amp hour rating (at 24hr; 20hr rating will be slightly lower) and another at 256. I'd say either of those would work.

5,729✭✭✭✭✭Should be slightly LOWER... I think Sun Xtender did the 24hr rate just to look better/more AH when compared to batteries done in the industry standard 20 hr rate.

- Assorted other systems, pieces and to many panels in the closet to not do more projects.

17,615✭✭Yep. Changed it. That's what happens when you edit style mid-thought; it was going to be "at the 24hr rate which is slightly higher than the 20hr rate we usually use" but that was too wordy.

I think they use the 24hr rate because the standard application isn't RE but something that would require power for a full day before charging.

As always every battery is a compromise for RE.

122✭✭✭"3kw array over 4 hours equivalent good sun and 52% over-all system efficiency: 6.2 kW hours AC so that works with the battery capacity (this number should always be slightly higher than the battery number)".

Which battery number? Not sure I get that one

17,615✭✭The theoretical 480 Amp hours. 25% of that capacity is 5.7 kW hours. The panels should always be able to produce more than that end-to-end.

1,218✭✭✭✭Remember that bank size is as much about your loads as it is about PV. The formula used here is, i think, Bank Wh= daily Wh x 4.

Of course you dont want to undercharge, but the charge controller has the ability to limit the upper end, should your setup end up needing a higher pv to bank ratio.

AGMs are good for higher charge currents for sure, but the "closed" nature of them, makes it difficult to ever know how well your charge regimen is doing. At least i find it frustrating, mainly because the orginal manfacturers documention was poor. LiFePo4 is another option i guess, if you have the stomach for new technology.

http://zoneblue.org/cms/page.php?view=off-grid-solar

122✭✭✭I few years ago I posted with some similar questions and someone responded that my 3.1kW of PV should be considered as 40A of peak output. This time it is suggested that it be 48A peak. Are there different views as to how to reach that number?

Thanks

2,385✭✭✭✭It depends on the voltage you used to divide the watts. For a 3.1 X .077 / 60 volts , 40 amps looks right

If you use the nominal 48 Volts then the system comes in over 48 amps, technically not correct for off grid charging.

17,615✭✭It's an estimate at best.

3100 Watts @ 77% efficiency (there's one variable) is 2387 Watts.

Divide by system nominal 48 Volts (also useful as the minimum battery Voltage set point) you get 49 Amps peak current. Divide by system Absorb Voltage of 58.4 you get 40 Amps. The second number may not occur because it would depend on load draw as well as current to maintain Absorb Voltage. (The 1 Amp difference is a matter of calculating from 3100 Watts or 3000 Watts.)

I normally use the nominal because of calculating for charging a battery bank drawn down to 50% SOC. In essence that's maximum current at minimum Voltage. You may never see any of these current numbers due to the numerous factors involved.

17,615✭✭No technically that is correct for off-grid charging: you will see the greatest current demand at the lowest battery Voltage. By the time you reach Absorb Voltage the current demand should be considerably less.