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Charge settings for deep cycle batteries in series?

blizzow

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Sep 20, 2021
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I have a growatt SPF 6000 DVM inverter and 9 panels set up in 3s3p. I have 8 size 31 deep cycle Dura Start batteries I got a good deal on. I have the batteries set up in two series of 4 for 48V.
I set my inverter up for FLA and am wondering what I should set the voltage charge settings to in order to get the most out of this battery set up. I feel like I can run about 100W for about 8 hours but my gut tells me I should get a lot more energy out of this battery bank.

Can anyone here give a suggestion on what I should set my max charge to? Or any other settings I should set on my inverter to tune my setup?
 
Have you tried the growatt SPF 6000 DVM inverter lead acid charge profile?
Have you looked at your battery specific recommendations from the manufacturer? This should trump most other recommendations.
 
I would be at 4x 14.6 or 58.4 volts. Standard FLA should be OK. Depends on absorption time and depth of discharge.
 
Have you tried the growatt SPF 6000 DVM inverter lead acid charge profile?
Have you looked at your battery specific recommendations from the manufacturer? This should trump most other recommendations.
I already have the inverter set on the lead acid charge profile (which is set to 58.4).

I haven't found any manufacturer charge recommendations for Dura Start Deep Cycle size 31 batteries. I definitely haven't found manufacturer recommendations on charge settings for when the batteries are connected with two parallel series of four batteries connected in serial.

I'm wondering about the low voltage cutoff, the high (58.4), as well as the optimum charging amperage.
 
Four in series is exactly 4x the single battery spec. Parallel has no effect on voltage.
 
Despite the words "Deep Cycle", those are still starting batteries, and won't last very long at all if cycled deeply.
They should be great for providing surge to start motor loads. They will carry the system through brief times without sun, and small load overnight.

100W x 8 hours = 800 Wh, 17 Ah at 48V.
From the "Reserve Capacity" minutes rating and current draw, you can see what claimed capacity they have.
Perhaps 17 Ah might be 25% DoD for a 68 Ah battery.

Don't know which is your battery, but here is one that says 140 minutes (2.33 hours) reserve capacity at 25A (I calculate 58 Ah), says 80 Ah.



Can you set your inverter for low-voltage disconnect so it doesn't go below about 85% SoC?
There should also be a time delay, allowing say 2 minutes or so below that voltage before shutting down. That would allow brief high draw, like for a microwave.

I favor powering loads pretty much direct from PV, and having excess PV capacity. PV is cheap and long lasting. Batteries are expensive and wear out.
If you do need to operate significant loads overnight or keep operating for days without sun, you need to calculate loads and size a true deep-cycle battery bank.
 
Don't know which is your battery, but here is one that says 140 minutes (2.33 hours) reserve capacity at 25A (I calculate 58 Ah), says 80 Ah.
The Ah rating is probably the more standard 20 hour rate (4 amps) and will yield more than the 2.33 hour rate (25 amps) due to the Peukert effect.
 
Don't know which is your battery, but here is one that says 140 minutes (2.33 hours) reserve capacity at 25A (I calculate 58 Ah), says 80 Ah.
Yes. Re-read that ad. Labels lie but reveal truth.
A Group 24 is a small battery and I don’t really see it supporting 25A for two hours, but maybe
Can you set your inverter for low-voltage disconnect so it doesn't go below about 85% SoC?
So you think 12.04 is too low on this one you provided as an example?
 
So you think 12.04 is too low on this one you provided as an example?

Don't know the specs for that battery.
For my SunXtender, 1 hour discharge rate 12.07V is 30% DoD, 24 hour rate 12.02V is 60% DoD, 120 hour rate 12.05V is 60% DoD.

I'm just suggesting a starting/deep cycle battery could give long life if not actually cycled, only used for "starting" heavy loads that are then carried by PV, or only lightly cycled.

Using the specs for my (true deep cycle) battery, at 120 hour rate after 12 hours drain (10% DoD), 12.69V or 24 hours for 20% DoD 12.58V. That's the sort of use I think starting/deep cycle could take and still last a long time. But just a guess.
 
That's the sort of use I think starting/deep cycle could take and still last a long time
Got it. I was missing your point.
I was thinking of a static “~50%” limit (12.04) not incorporating discharge rate in my thinking.
In my situation I’m seldom 12.1 usually 12.4-12.5 by AM. I was 12.9 at 9:30 last night (several hours after sunset) and 12.4 after this morning’s coffeemaker run; the sun is coming up. Def exceeded many folks’ idea these batteries only run a year or two, time will tell how far they’ll go.
 
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