Multiple Charge Controllers

kelperis · October 2018

I am connecting three charge controllers to a single battery bank. The batteries have a max charging rate of 30a. If I set the CC’s all the same, won’t they all try to send 30 amps to the battery bank? I know they will both have the same battery information, but how do they handle the charging amps. Should I give each one a 10a charging limit so as not to overwhelm the battery bank?

Estragon · October 2018

In general, you'd want to limit each to 10a, assuming their arrays are likely to provide peak power at the same time. If, for example, you had east, south, and west arrays which wouldn't peak at the same time though, you could up the individual limits. IMHO

BB. · October 2018

Each charge controller makes its own decision, based on the battery bus voltage and charging current it is outputting.

In general, the controllers will not be "synced" with each other. One will output a bit more current/higher charging voltage, and others will "fall back" to float charging.

The proof that all is working well is to monitor the specific gravity of all the cells of the Flooded Cell Lead Acid battery bank... Or if they are some sort of sealed cell batteries (AGM, VRLA, GEL, Li Ion, etc.), then you need to check the voltage of each cell/individual battery against the service manual (i.e., ~14.75 volts for FLA charging, ~13.6 volts for float, ~12.8+ volts for full charge no load after resting for 3+ hours, etc.).

Adjust the charging set points and charging current to achieve the proper final state of charge (>~90% state of charge at least once or twice a week, not fall below 50% SoC during normal operation, suggested for Lead Acid chemistry batteries). And for FLA batteries equalize to ~100% SoC once a month or as needed (depends on brand/model of batteries).

-Bill

kelperis · October 2018

Thanks, each of the arrays are identical, and in the clear, and all oriented the same, so they show pretty much produce identically, and can give each cc 1/3. After I get it set up and running, I will probably revisit some of the other comments, and will definitely need help.

mcgivor · October 2018

Limiting the current of each controller at 10A will also limit the total available for loads to 30A before drawing from the battery, if no loads are used then not a problem, just wasted potential, assuming the array is capable of more. Being no information is provided regarding array capacity, battery type/capacity and loads, it's impossible to make relevant suggestions specific to your system, arranging the array in three separate orientations could be benificial by extending charge time without sacrificing potential, all hypothetical of course.

kelperis · October 2018

My setup is:

12v, 3 array’s 900w, 3 combiners (60a run to ZHC Solar 80 CC), to a bank(parallel) of 6 Renogy 200ah batteries(30a max charge current). 3 inverters 2 WZRELB 3000w and 1 ATS 8000w inverters. Plan to use no more than 50% load on the inverters, and no 220AC at all. All of this is interconnected with 1/0 cables with 250a and 300a breakers in front of the inverters. as well as 60a fuses on the lines coming in from the combiner boxes outside.

kelperis · October 2018

As I think through what strategy to use a single CC at first, using all 30 charging amps, allowing the load to first come directly from the CC, using what left to the batteries. This should give me the data I need to properly configure the group of three.

Estragon · October 2018

I think the ampacity of 1/0 is ~150a depending on insulation type, so you likely need bigger wire and/or smaller inverter breakers (or consider a higher system voltage).

IMHO, 3kw is too big of an inverter for a 12v system. 8kw@12v is something I'm surprised is even made.

kelperis · October 2018

Renogy indicates 285a for 1/0 wire. The smaller inverters have smaller wires, I forget the exact size, still awaiting the manual. Keep in mind that I have spread the load across three inverters, no more than 50% utilization. So in reality, I will only be running 1500w through the two smaller ones, and 4000w on the bigger one. I have tested the smaller ones, and they handled it fine, including some fans. I also tested the ATS with fans and a space heater, up to between 3500w and 4000w, and it handled it fine for 10-15 minutes, no hot wires, sparks or thrown breakers. I will not be running any 220 load through this system, even though the ATS has it hardwired in the back. Only 110 loads, and the microwave is the one I am most worried about. The fridge and freezers tested fine. With just my wife and I, we can adjust schedules to stay within the parameters. I only need to save 40% of my electric bill to pay for it in 10 years. I also avoid having to upgrade my old backup generator, as I will already be off the mains for critical things. I was able to get a much smaller generator to back up what move to solar, and that only when the county AND the solar are down at the same time. The solar loads are on transfer panels, so I can run off mains with a single switch change, or like I said, I can switch to the generator with another switch change. I suppose time will tell. Right now I am working on the charging side of the system. I have to get my panels set by the end of the month. Thanks for your input, it really is appreciated.

littleharbor2 · October 2018

kelperis said:

My setup is:

12v, 3 array’s 900w, 3 combiners (60a run to ZHC Solar 80 CC), to a bank(parallel) of 6 Renogy 200ah batteries(30a max charge current). 3 inverters 2 WZRELB 3000w and 1 ATS 8000w inverters. Plan to use no more than 50% load on the inverters, and no 220AC at all. All of this is interconnected with 1/0 cables with 250a and 300a breakers in front of the inverters. as well as 60a fuses on the lines coming in from the combiner boxes outside.

Are the Renogy batteries 12 volt batteries?

I gather you have a 12 volt system, correct?

If yes to both these questions then the only logical deduction is the 6 batteries are all parallel wired, correct?

6 parallel 12 volt batteries would be able to accept 6 x 30 amps = 180 amps. This wiring configuration, especially with AGM batteries is a recipe for a costly and premature replacement. Even if you could achieve perfect charging/discharging balance you will never save a red cent going with a battery based solar system. If you want to recoup your cost of a PV system you only have one choice, straight grid tie, WITHOUT batteries.

mcgivor · October 2018

As soon as batteries are introduced all savings are lost, grid energy is way cheaper, the cost of two sets of batteries will buy more Kwh than they could ever store/produce, even if not deeply cycled they will still have a limited life expectancy 5 years give or take. With the right equipment an AC coupled system without batteries would be an option, unfortunately the equipment described will not allow this.

Estragon · October 2018

The ampacity of 150a for 1/0 is from:
http://www.armstrongssupply.com/wire_chart.htm

You can run more current than NEC ampacity with bigger breakers, like the 250-300a mentioned, but I wouldn't. You're basically (at best) using batteries to heat up wires instead of running loads, or (at worst) making the wire do the job of the breaker.

If the batteries are 200ah@12v in parallel, the 30a charge rate is per battery, so as LH2 said, 180a total. In most locations you likely won't see more than ~50-60a from each 900w array.

As mentioned, having 6 batteries in parallel could be problematic. I'd highly recommend getting a DC clamp meter and using it regularly to check for even charge/discharge current under load.

jonr · October 2018

There are places that charge $.50+/kwh, making $.25/kwh (taken from Tesla info) or so for batteries possibly feasible.

Estragon · October 2018

Anytime I try to work out cost of off-grid battery power using reasonable life expectancies (batteries 5-7yrs, electronics 10-12yrs, panels 20-25yrs), I end up at $0.75-$1/kwh.

Some folks seem to get longer life (and I hope to as well) from components, but lots also seem to get less for one reason or another.

kelperis · October 2018

These are great comments. I am glad the max charge issue was just my misunderstanding. I live in rural Kentucky, and the electrical co-op is difficult to work with at best. So I do not want to ‘partner’ with them on a grid-tie system. I don’t have power everything to save $100 per month. I am topping off the batteries each night, so when the load kicks in, much of it will come straight from the panels. At some point during the morning/afternoon I will have a choice between letting it come from the battery, turn on the bulk charger to assist the panels. My backup power only needs to provide the bulk charger. I could have spent $10k on a backup generator for my house. Now I avoid most of that expense. Again, I enjoy your comments.

BB. · October 2018

Suggestions on battery paralleling:

http://www.smartgauge.co.uk/batt_con.html

-Bill

kelperis · October 2018

Thanks. Mine is using the 2nd method. I don’t have the physical space to use the last one.

Porschephanatic · October 2018

BB. said:

Suggestions on battery paralleling:

http://www.smartgauge.co.uk/batt_con.html

-Bill

Cool! Bookmarked for future reference.

BB. · October 2018

The Smartgauge site has a lot of technical information about batteries and other design issues. It is a cool site to browse when you have some free time.

-Bill

kelperis · October 2018

> @Estragon said:
> The ampacity of 150a for 1/0 is from:
> http://www.armstrongssupply.com/wire_chart.htm
>
> You can run more current than NEC ampacity with bigger breakers, like the 250-300a mentioned, but I wouldn't. You're basically (at best) using batteries to heat up wires instead of running loads, or (at worst) making the wire do the job of the breaker.
>
> If the batteries are 200ah@12v in parallel, the 30a charge rate is per battery, so as LH2 said, 180a total. In most locations you likely won't see more than ~50-60a from each 900w array.
>
> As mentioned, having 6 batteries in parallel could be problematic. I'd highly recommend getting a DC clamp meter and using it regularly to check for even charge/discharge current under load.
>
>
I double checked the Windy Nation chart again, and it says 285 as the max amperage for 1/0. Why would these two sources be do different? It is worrisome that there can be so much difference. My approach was to ensure that the cables didn’t overheat to unsafe levels. The equipment all have different levels which I use fuses or breakers, as recommended by the vendor. So many different answers make it difficult to keep all of vender recommendations in sync. I suppose that is what I get. Y not just buying a kit from a single vendor.

BB. · October 2018

There are lots of things to consider when rating current capacity of wiring... Not only the material (copper, aluminum), but the insulation type, conduit fill vs open air, ambient temperature, and even conditions (oil, water, UV/Sunlight, vibration, flexing, American Wire Gauge vs SAE wire gauge, etc.).

Just a couple of charts:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

Just a few notes from the West Marine link:

These simple graphs assume:

105°C insulation rating: All Ancor wire uses 105°C insulation rating. Lower temperature insulation cannot handle as much current
AWG wire sizes, not SAE: All Ancor wire uses AWG wire sizes. SAE wire sizes are 6%–12% smaller, carry proportionally less current, and have greater resistance
Wires are not run in engine spaces: Maximum current is 15% less in engine spaces, which are assumed to be 20°C hotter than non-engine spaces (50°C vs. 30°C).
Conductors are not bundled: If three conductors are bundled, reduce maximum amperage by 30%. If 4–6 conductors are bundled, reduce maximum amperage by 40%. If 7–24 conductors are bundled, reduce amperage by 50%.

The 1/0 wiring from NEC chart (simplified, not full chart/deratings from NEC manual) give the range of current for 1/0 cable to be 125 to 175 Amps (assumed to be in conduit?).

The 1/0 wiring from the Marine Chart is 285 Amps maximum... HOWEVER, lots of deratings (bundling, ambient temperatures, etc.) bring that 285 Amp rating down towards the NEC ratings.

And, just for grins, the "FUSING" current for various sizes and materials of wiring:

https://www.powerstream.com/wire-fusing-currents.htm

Note from LInk: These currents are estimates for how much current it takes to melt a wire in free air. Of course anything that helps dissipate or concentrate the heat, such as insulation, will change the wire fusing current, so these are only estimates.

In the end, many times for off grid solar DC (and AC inverter output) systems, the NEC ratings are much more conservative and are a good place to start because it also keeps wire losses down... We typically design systems in the 1% to 3% losses for wiring. Note in the Marine Wiring link, they even have a 10% loss table.

-Bill

Estragon · October 2018

In addition to Bill's excellent answer, note that there are also the connections to consider. Heating and cooling of wire and connections can be an issue, even at much lower than fusing temps/currents. I use an IR thermometer to occasionally check for hotspots and developing problems.

BB. · October 2018

To confirm Estragon's information... Most of my electrical failures have been because of poor connections. Aluminum House wiring (out of the 60's and 70's), loose binding screws, and water that has followed the wire to the binding screws/wire nuts (took 10+ years and maybe a couple of teaspoons of water to take out my utility meter sockets in two different (older) homes.

On other major issue I have seen--And why I try to avoid parallel power circuits. In electronics, it is common to parallel 2-5+ wires in a ribbon cable to bus the low voltage DC from one board to another.

Over time/vibration/thermal cycling, the connectors between the cables and the boards would eventually go a little high resistance (even the gold plated ones)... What happened was not that the high resistance connections started to overheat, it was the low resistance (the "still good" connections that overheated). I would have a piece of test gear fail and have to pull it apart. And I would find a ribbon cable that looked like it was "unzipping" from black to brown to normal insulation on the paralleled power connections. The reason that the "good connections" started overheating was because of Power=Current^2*R.... It turned out the high current was getting 4x the heat in the good connections and frying it. Then it failed, and the next best connection carried the current and it failed.

That is why I never assume that 5x wires in parallel will carry 5x the current. Also, the "resistance" of the cable plays a part too... That supplies "ballast resistance" which helps keep current flow "equal" between the parallel paths (i.e., a 0.01 Ohm connector with a 0.1 Ohm cable, the cable is "steering" most of the current flow, not the variation in connector resistance).

That is a reason why with parallel battery connections, "too large" of AWG cable can actually cause an imbalance of current flow... The resistance of the cable is less than the resistance of the batteries and the connections.

-Bill

Multiple Charge Controllers

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