Thoughts on Battery Bank Design

kelperis · September 2018

I am setting up a solar power system in my house. I am starting with 6 200ah 12 volt batteries. I have split the system into two parallel strings with three batteries each. I have two each of high capacity inverter (8000/32000) and two each of medium capacity (3000/5000). I plan to have a battery minder managing both strings, and also connect all of the inverters to each string, and also have a dedicated charge/controller for each string. I have set this up so that I can spread the load across the banks and devices, to keep utilization low, but also to allow me to double the batteries, creating two strings of six batteries. This lets me upgrade and keep the two strings matched as far as age/size. I don’t want to mix the strings. This lets me add load to the system in a controlled manner, and take batteries offline to complete maintenance individually. My question is...

What feedback would you all give me about the design? Any glaring problems, or weak spots.

Ric

Estragon · September 2018

A couple of questions/clarifications:

- Two parallel strings of 3x12v batteries in series suggests a 36v system voltage, which is unusual. Is there a reason for this voltage?

- 400ah for the parallel strings is on the small side for an inverter capable (if I'm understanding correctly) of 32kw peak output (is that each, or for two combined?). That's nearly 900a at 36v, which most types of battery will have trouble with, and which would need some very heavy wire to handle. A system of that size would normally use higher capacity (eg 1500ah) 2v batteries at 48v.

Thom · September 2018

I think you need to rethink . Why 12v battery ? Guessing it’s 12v system

thom

Estragon · September 2018

A 32kw peak 12v inverter? Hard to believe such a thing exists.

littleharbor2 · September 2018

Estragon said:

A 32kw peak 12v inverter? Hard to believe such a thing exists.

Here's a discussion of one of these fire traps I brought up a while back. PowerJack is famous for them.

http://forum.solar-electric.com/discussion/351368/latest-offering-from-power-jack-ugh/p1

Here's the latest offering. Just as scary. http://vi.vipr.ebaydesc.com/ws/eBayISAPI.dll?ViewItemDescV4&item=323300670797&t=1531543281000&tid=10&category=177706&seller=wakamamakilakila&excSoj=1&excTrk=1&lsite=100&ittenable=false&domain=ebay.com&descgauge=1&cspheader=1&oneClk=1&secureDesc=0

kelperis · September 2018

> @Estragon said:
> A couple of questions/clarifications:
>
> - Two parallel strings of 3x12v batteries in series suggests a 36v system voltage, which is unusual. Is there a reason for this voltage?
>
> - 400ah for the parallel strings is on the small side for an inverter capable (if I'm understanding correctly) of 32kw peak output (is that each, or for two combined?). That's nearly 900a at 36v, which most types of battery will have trouble with, and which would need some very heavy wire to handle. A system of that size would normally use higher capacity (eg 1500ah) 2v batteries at 48v.

> @Estragon said:
> A couple of questions/clarifications:
>
> - Two parallel strings of 3x12v batteries in series suggests a 36v system voltage, which is unusual. Is there a reason for this voltage?
>
> - 400ah for the parallel strings is on the small side for an inverter capable (if I'm understanding correctly) of 32kw peak output (is that each, or for two combined?). That's nearly 900a at 36v, which most types of battery will have trouble with, and which would need some very heavy wire to handle. A system of that size would normally use higher capacity (eg 1500ah) 2v batteries at 48v.

All of the batteries are 12volt. They are in parallel, I intended to add another string of six as I increase usage. I am under the impression that I can connect to each bank. I am looking for redundancy in the system. The higher capacity inverters have mixed reviews, although most people agree that if you keep utilization low, they do okay, but if you overwork them, you get unpredictable results. All of the batteries are new, and are 12 volt 200ah Renogy gel batteries. Six in two parallel banks, then later I will add six more, all new and then I will have two banks of six 12 volt 200ah, for total of 12.

kelperis · September 2018

> @Thom said:
> I think you need to rethink . Why 12v battery ? Guessing it’s 12v system
>
> thom

It is a 12 volt system.

mike95490 · September 2018

Sorry you wasted money on dangerous gear.

You cannot save energy, with essentially 4 parallel systems , you gain redundancy & unique flexibility, but loose efficiency.

The best way to recover from your situation is to return or sell the electronics, buy 2 more identical batteries (for a total of 8), and build yourself a 48V 400ah bank. (4S2P) 4 batteries in series for 48v, paralleled with an identical bank.

The battery bank will need at least 40A delivered solar for recharge, which a 3Kw PV array might be able to deliver, but a 4kw array will save you a lot of generator fuel.

Use quality 48V gear. Double check your loads, they seem unreasonable with the wattage of inverters you planned on.

The 48V 400ah battery bank can support a 4,000w inverter ( for a short while, 85A load )

mcgivor · September 2018

Not the best plan for a number of reasons, as the number of parrallel batteries increase so dose the potential of imbalance, two perhaps three is manageable. Adding new batteries at a later date is not advisable due to difference in capacity between old and new, capacity is lost over time. Current at 12V nominal is high, needing expensive ballance of system components, 12V is ok up to around 1000W.

kelperis · September 2018

Thanks for your thoughts. I suppose I will see how it goes. Specifically where are the inefficient parts. I can make it one big bank of 6 12 volt batteries for a total of 1200ah just as easy. Are you saying that accessing two different banks adds overhead, and would be better off just having one big parallel bank. When the time comes to add additional capacity, wouldn’t that be a problem with the different age of batteries, even though they would be from the same vender. I was hoping to isolate the system from that situation.

kelperis · September 2018

> @mcgivor said:
> Not the best plan for a number of reasons, as the number of parrallel batteries increase so dose the potential of imbalance, two perhaps three is manageable. Adding new batteries at a later date is not advisable due to difference in capacity between old and new, capacity is lost over time. Current at 12V nominal is high, needing expensive ballance of system components, 12V is ok up to around 1000W.

So six batteries, 12v parallel, for a total of 1200ah can only support 1000W? At this point, unless the equipment fails to deliver as advertised, I guess I have to go with it. All of the batteries match now. If I buy six more, new and matching, and keeping them in separate banks, managed by battery minder, doesn’t that negate the age problem. I was hoping to split the load between the two, and provide redundancy for part of it.

Estragon · September 2018

Ignoring the 32kw peak, which it maybe does for a second or two before turning into a puddle of lead or whatever it's made of, even the 8kw claimed sustained current at 12v is approaching 700a. How much power does it use just being on at low utilization?

You'll need some really beefy wire for the interconnects and inverter connection.

With 3 batteries in parallel, each interconnect positive. should be fused, as well as an appriate sized breaker for the inverter wire. Keeping the battery states of charge balanced could be a problem, especially with the kind of currents you're dealing with at 12v system voltage. I'd highly recommend getting a clamp-on DC amp meter, and checking charge/discharge current balance regularly. With sealed batteries getting reliable state of charge can be an issue, so it's more difficult to catch imbalance problems in time to remediate.

Connecting all batteries to all inverters means you effectively have 6 in parallel (or 12 if you add another 6) with varying resistance, so I suspect balance problems are inevitable.

Are all the inverters supplying all AC loads, and if so, how are you planning to avoid backfeeding unsychronized inverters?

kelperis · September 2018

> @mike95490 said:
> Sorry you wasted money on dangerous gear.
>
>
>
>
> You cannot save energy, with essentially 4 parallel systems , you gain redundancy & unique flexibility, but loose efficiency.
>
>
>
>
> The best way to recover from your situation is to return or sell the electronics, buy 2 more identical batteries (for a total of 8), and build yourself a 48V 400ah bank. (4S2P) 4 batteries in series for 48v, paralleled with an identical bank.
>
>
>
>
> The battery bank will need at least 40A delivered solar for recharge, which a 3Kw PV array might be able to deliver, but a 4kw array will save you a lot of generator fuel.
>
>
>
>
>
> Use quality 48V gear. Double check your loads, they seem unreasonable with the wattage of inverters you planned on.
>
>
>
>
>
> The 48V 400ah battery bank can support a 4,000w inverter ( for a short while, 85A load )

The array that I have so far is 2000w total of 14 panels, with plans to double that at some point. I plan to supplement with battery minder charging at night. Felt like I needed to let the system break in, and see how it performs, then modify it next year based on what I learned.

mcgivor · September 2018

kelperis said:

> @mcgivor said:
> Not the best plan for a number of reasons, as the number of parrallel batteries increase so dose the potential of imbalance, two perhaps three is manageable. Adding new batteries at a later date is not advisable due to difference in capacity between old and new, capacity is lost over time. Current at 12V nominal is high, needing expensive ballance of system components, 12V is ok up to around 1000W.

So six batteries, 12v parallel, for a total of 1200ah can only support 1000W? At this point, unless the equipment fails to deliver as advertised, I guess I have to go with it. All of the batteries match now. If I buy six more, new and matching, and keeping them in separate banks, managed by battery minder, doesn’t that negate the age problem. I was hoping to split the load between the two, and provide redundancy for part of it.

No, what I meant was when loads go above 1000W then it's time to think about a higher nominal voltage to keep current down to manageable levels and improve efficiency. Consider the maximum surge rating 32 000W that's 2667A, 667A at 8000W. You really have to reconsider this setup.

kelperis · September 2018

> @Estragon said:
> Ignoring the 32kw peak, which it maybe does for a second or two before turning into a puddle of lead or whatever it's made of, even the 8kw claimed sustained current at 12v is approaching 700a. How much power does it use just being on at low utilization?
>
> You'll need some really beefy wire for the interconnects and inverter connection.
>
> With 3 batteries in parallel, each interconnect positive. should be fused, as well as an appriate sized breaker for the inverter wire. Keeping the battery states of charge balanced could be a problem, especially with the kind of currents you're dealing with at 12v system voltage. I'd highly recommend getting a clamp-on DC amp meter, and checking charge/discharge current balance regularly. With sealed batteries getting reliable state of charge can be an issue, so it's more difficult to catch imbalance problems in time to remediate.
>
> Connecting all batteries to all inverters means you effectively have 6 in parallel (or 12 if you add another 6) with varying resistance, so I suspect balance problems are inevitable.
>
> Are all the inverters supplying all AC loads, and if so, how are you planning to avoid backfeeding unsychronized inverters?

> @Estragon said:
> I am setting up a solar power system in my house. I am starting with 6 200ah 12 volt batteries. I have split the system into two parallel strings with three batteries each. I have two each of high capacity inverter (8000/32000) and two each of medium capacity (3000/5000). I plan to have a battery minder managing both strings, and also connect all of the inverters to each string, and also have a dedicated charge/controller for each string. I have set this up so that I can spread the load across the banks and devices, to keep utilization low, but also to allow me to double the batteries, creating two strings of six batteries. This lets me upgrade and keep the two strings matched as far as age/size. I don’t want to mix the strings. This lets me add load to the system in a controlled manner, and take batteries offline to complete maintenance individually. My question is...

What feedback would you all give me about the design? Any glaring problems, or weak spots.

Ricarly. With sealed batteries getting reliable state of charge can be an issue, so it's more difficult to catch imbalance problems in time to remediate.
>
> Connecting all batteries to all inverters means you effectively have 6 in parallel (or 12 if you add another 6) with varying resistance, so I suspect balance problems are inevitable.
>
> Are all the inverters supplying all AC loads, and if so, how are you planning to avoid backfeeding unsychronized inverters?

> @Estragon said:
> Ignoring the 32kw peak, which it maybe does for a second or two before turning into a puddle of lead or whatever it's made of, even the 8kw claimed sustained current at 12v is approaching 700a. How much power does it use just being on at low utilization?
>
> You'll need some really beefy wire for the interconnects and inverter connection.
>
> With 3 batteries in parallel, each interconnect positive. should be fused, as well as an appriate sized breaker for the inverter wire. Keeping the battery states of charge balanced could be a problem, especially with the kind of currents you're dealing with at 12v system voltage. I'd highly recommend getting a clamp-on DC amp meter, and checking charge/discharge current balance regularly. With sealed batteries getting reliable state of charge can be an issue, so it's more difficult to catch imbalance problems in time to remediate.
>
> Connecting all batteries to all inverters means you effectively have 6 in parallel (or 12 if you add another 6) with varying resistance, so I suspect balance problems are inevitable.
>
> Are all the inverters supplying all AC loads, and if so, how are you planning to avoid backfeeding unsychronized inverters?

> @Estragon said:
> Ignoring the 32kw peak, which it maybe does for a second or two before turning into a puddle of lead or whatever it's made of, even the 8kw claimed sustained current at 12v is approaching 700a. How much power does it use just being on at low utilization?
>
> You'll need some really beefy wire for the interconnects and inverter connection.
>
> With 3 batteries in parallel, each interconnect positive. should be fused, as well as an appriate sized

I am using 1/0 cable for all of the battery connects. The inverters vender indicate the have the appropriate fuse or breaker to prevent backfeeding. So are you saying I need add another breaker. I have the batteries connected through busbars, and switched, but was relying on the vendors. I have two venders, two identical inverters from each.

Estragon · September 2018

> @kelperis said:
>
> The array that I have so far is 2000w total of 14 panels, with plans to double that at some point. I plan to supplement with battery minder charging at night. Felt like I needed to let the system break in, and see how it performs, then modify it next year based on what I learned.
>

In most climates, 2000w STC rated array will produce ~1500w max in full sun. 1500÷12v=125a, about right for your 6x200ah batteries.

kelperis · September 2018

Someone asked about all inverters providing AC. They do, but they don’t overlap. The transfer switches are used to provide AC to my circuit panel, circuit by circuit, not just powering the whole house. I use the smaller inverters to power the smaller (lights, tv, one so forth). I also use the smaller inverters for some of the 20amp loads, the bigger inverters are wired to provide for the higher draw (microwave, fridge and freezers,) all of these items have switched to control where they get the AC from.

kelperis · September 2018

> @Estragon said:
> > @kelperis said:
> >
> > The array that I have so far is 2000w total of 14 panels, with plans to double that at some point. I plan to supplement with battery minder charging at night. Felt like I needed to let the system break in, and see how it performs, then modify it next year based on what I learned.
> >
>
> In most climates, 2000w STC rated array will produce ~1500w max in full sun. 1500÷12v=125a, about right for your 6x200ah batteries.

Thanks, I was worried I screwed the math up. I also have battery minder to top them off over night. I am hoping not to need it, and I will add batteries and/or panels as I see how it works..thanks

Estragon · September 2018

1/0 is good for ~150a depending on type.

kelperis · September 2018

I can’t get heavier battery cable, it’s thicker than my car batteries.

Estragon · September 2018

Your car battery isn't potentially supplying 700a sustained. That's one of the reasons not to use big inverters on 12v DC systems, the wire size gets unreasonable.

What size DC breaker is on the inverters?

mcgivor · September 2018

kelperis said:

I can’t get heavier battery cable, it’s thicker than my car batteries.

To increase capacity with 1/0 simply parallel the run, so for 300a 3×1/0 for both negative and positive, depending on distance it may need to be more, to offset voltage drop, a huge factor with 12V. The breakers to protect the conductors will in all likelihood cost more than the inverters combined, one needed for each positive.

mike95490 · September 2018

Estragon said:

.....

You'll need some really beefy wire for the interconnects and inverter connection........

False. You need impossibly beefy wire for the interconnects. And most of the cheap inverters will internally melt their 4 ga wires.

8,000 watts at 12V is 670A. That requires a copper rod over an inch in diameter to have losses low enough the inverter won't fault out from low voltage.

Buying the copper cables and attachment lugs, will cost you more, than scrapping what you have and buying new 48V gear.

Thom · September 2018

I would only use a max of 1000w inverter on a 12 v system. My off grid system has worked great , batteries I replaced at 8 years . I copied another’s posters system. This is a great place .

kelperis · September 2018

Thank you everyone for your input and advice. I have decided to move up my operational testing so that I can put the inverters through more thourogh testing under load. I have decided that a fire extinguisher and detectors would be a good addition to my system. At this point, it seems like the inverters are the greatest point of failure, and are the only voltage dependent equipment. I had thought that my plans to only use a small percentage of their capacity would keep the system safe and functional. I have the largest cables they make short of a solid copper rod. My battery minder can be used regardless of the voltage, the batteries can be hooked in series to get to 48volt, which is also true of the panels I believe. I only have $1700 invested in the four inverters, so if I need to switch the system out, it will only add a year or two to the financials. A big part of the rationale is to be partially independent of the county power, at least the critical stuff. I will keep everyone informed on this thread as I run the tests. Thanks again everyone.

petertearai · September 2018

Make it easy on yourself . sell those inverters as almost new , and go straight to 48 volts and a good inverter . you'll be pleased you did in the long run . Promise .

oil pan 4 · September 2018

It looks like you are trying to do the job of a 48v system with a 12v system.
Do you really want to find out the hard way why no one actually uses big 12v systems exactly like what you have?

If you want an inverter to run kitchen stuff off that pretty much completely rules out anything 12v.
For example I have a 24v, 255ah 560lb warehouse tug battery in my garage. I'm going to get a samlex 2,000w inverter and use it for backup power only, an extension cord will be ran to the kitchen, if someone wants to use an kitchen appliance, then that appliance gets plugged in and used.
I'm not even going to attempt to run my house off even a 24v forklift battery and 24v inverter, because that's the job of a 48v system.

The biggest inverter I would use on a 12v system is a 2,000w samlex.
Even then the heaviest load it sees is powering my welder, about 1,700w intermittently.
The biggest continuous load I would run on a 12v inverter would probably be a 120v refrigerator.

kelperis · September 2018

Ok, you convinced me...sort of. If I ditch the large inverters, and create two systems. One 48v system to handle the higher need stuff like refers, microwaves and so. Then a small 12v system to handle small stuff, lights, tv and such. The 48v would have two strings of PV panels using two combiner boxes, and that should make the long run to the CC a smaller size wire, and the 12v PV panels can be set up maybe 15 feet from the CC. That would give me 800ah battery bank for the 48v side, and a 400ah battery bank for the 12v side. All the rest of my stuff should still be usable, with a few spare parts in the closet to use if needed. Then when I increase capacity I can grow the 48v side. Anyone have a suggestion on a good quality 48v inverter that has a simple interface to the AC transfer switch? I am thinking in the 5-6000W that can handle kitchen stuff, and window unit ac and so on?

Estragon · September 2018

I use a pair of stacked 3.5kw Outback inverters. The advantage to a stack of smaller inverters include; slave(s) spend most of their lives sleeping with lower self-consumption, and when an inverter goes dark you can get by on the surviving one(s).

IIRC, each 3.5kw inverter takes ~40w lit but idle, vs ~8w sleeping. 32w x 24hr is ~770wh (about 1/2 a fridge worth). Most of my larger loads are short (toaster, microwave, pumps, tools), so the slave sleeps most of the time.

There's an internal transfer switch. If the system sees qualifying AC input, it's passed through for loads and to charge batteries. Some have the ability to supplement generator power with battery power to handle a sudden load (like a pump coming on), which might be handy.

Thoughts on Battery Bank Design

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