Producing 8230 watts, but batteries can only handle 120amps

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smcewan
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My RE system: 16 x 106ah nano carbon batteries, flexmax 80, flexmax 60, 8 x 320w panels, 6 x 270w panels, mate3, 4048a radian inverter

Producing 8230 watts, but batteries can only handle 120amps

Post by smcewan »

I have 16 outback 106ah nano carbon batteries. These are in 4 strings of 4 batteries at 48v. I currently have a system that can produce 4990w (or 104amps at 48v). I'd like to expand this and the engineer who does all my solar work says that its possible. I want to upgrade to a system producing 8230w (or 171amps at 48v). The batteries say the recommended max charging amperage is 30amps per string (times 4 strings = 120). That means I could be producing 171amps while the batteries can only handle 120. 3 different chargers will do this (2xFlexmax60 and 1xFlexmax80). Will the chargers know to limit total amperage to 120amps? I have it set to HBX (High Battery Transfer). Say I turn on an airconditioner. Is it ok to run more than 120amps over the top of these things if I have the wiring for it? My engineer assures me all is well but I feel like it is dangerous. Is it just that at 30 amps (120amps for 48v) these things will go to absorb voltage automatically? Thanks for any help/direction.
raysun
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by raysun »

smcewan wrote: Sat Jan 09, 2021 4:30 pm I have 16 outback 106ah nano carbon batteries. These are in 4 strings of 4 batteries at 48v. I currently have a system that can produce 4990w (or 104amps at 48v). I'd like to expand this and the engineer who does all my solar work says that its possible. I want to upgrade to a system producing 8230w (or 171amps at 48v). The batteries say the recommended max charging amperage is 30amps per string (times 4 strings = 120). That means I could be producing 171amps while the batteries can only handle 120. 3 different chargers will do this (2xFlexmax60 and 1xFlexmax80). Will the chargers know to limit total amperage to 120amps? I have it set to HBX (High Battery Transfer). Say I turn on an airconditioner. Is it ok to run more than 120amps over the top of these things if I have the wiring for it? My engineer assures me all is well but I feel like it is dangerous. Is it just that at 30 amps (120amps for 48v) these things will go to absorb voltage automatically? Thanks for any help/direction.
Its a complex question and the answer isn't black and white.

Without going into all the complexities, the simple, safest answer is the total charging current should be equal or less than the total current the battery will accept.

Now, launching a long and certainly convoluted discussion on the subject...

The following discussion is predicated on charging sources being configured to manage proper battery voltages during the charge cycle.

A battery-based power system with solar Photovoltiactic (PV) charging is a complex amalgam of current sources (PV panels), current sinks (loads), and combination source/sink (battery). All three are connected on a common bus, generally referred to as the battery bus.

In modern systems, when the PV array is generating power, it flows through the charge controller(s) onto the battery bus. The portion of the current that finds its way into the battery, v.s. directly to the load (usually an inverter) depends on the battery state of charge (SoC).

The simplest case is a battery at low SoC and zero load. In this case, 100% of the harvested charge current is sent to the battery. Lead-acid batteries at low SoC have low internal impedence and will permit a large charge current. Typically, they will permit more charge current than they are designed to safely accept. In the event of continued overcurrent, the battery internals can heat up, causing damage, sometimes catastrophic.

As charge is added to the battery, the internal impedence slowly rises, causing the battery voltage to rise as charge continues to be added. This is the typical curve seen as the Bulk phase progresses and the voltage rises to the Absorb voltage.

During the constant voltage charging phases - Absorb and Float - the rising and high internal impedence causes the battery to accept significantly less charge.

Due to the declining demand, the charge controllers will harvest less power from the PV array.

The scenario becomes somewhat more complex when loads are applied to the battery bus.

Charge current flows into the battery and the load simultaneously and in proportion to their relative impedences. For example, if the PV array is producing 200A of charger current, and the load is demanding 100A, then the remaining 100A flows into the battery. If the charge current continues at 200A, and the load demand falls to 50A, then 150A flows into the battery. If the battery is rated at 120A charging current, the first case is fine, the second case can lead to battery damage.

Sizing PV arrays for safe battery charging and meeting load demands while minimizing use of auxiliary power sources like generators or the grid, is complex proposition. Part of the challenge is PV production is highly variable. A 10kW array can produce anywhere from 0W to well over 10kW at any given moment.

The safest proposition is to specify the array such that it never produces in excess of the battery maximum charge current. The drawback to this is the PV array will usually be producing far less. The next design iteration is to "overpanel" the array, and rely on charge controller capabilities and configuration to keep the maximum current at safe thresholds for the battery.

I'm the case of an 80A charge controller, the maximum PV power it can accept is nominally 80A x 48V = 3840W. More input power runs the risk of overloading and damaging the controller, battery, or both.

There's lots more discussion around these issues, but this is enough to get started.

Your sense of caution is a good "survival instinct". The design engineer, me, or anyone else can provide opionion, advice, and guidance, but bottom line, the only one footing the bill for getting it wrong is you.

FWIW. For 5 years, I used the big brothers of the battery blocks you have. I got to know their performance and idiosyncrasies quite intimately. Threat them right, and they will give good service. Abuse them, and you will know about it in short order.
SandyP
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by SandyP »

smcewan wrote: Sat Jan 09, 2021 4:30 pm ../.. Will the chargers know to limit total amperage to 120amps?
Nope, the charge controllers come set to their maximum output, however you can manually set the maximum output for each of the FM charge controllers to limit their output.
wrote: ../.. Is it ok to run more than 120amps over the top of these things if I have the wiring for it?
As it is only FM output current in excess of what is being consumed by loads that is sent to the batteries for charging then, yes, as long as your wiring is capable then it should not be an issue.
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SandyP
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Outback FM60 MPPT (max output lowered to 55amps)
12 x 2V Hoppecke GEL 612 Ah C24 - 24V System
Outback VFX3024 Inverter/Charger
Victron BMV-602s
Honda 5.5kW Genset
Location: Victoria, Australia

Re: Producing 8230 watts, but batteries can only handle 120amps

Post by SandyP »

smcewan wrote: Sat Jan 09, 2021 4:30 pm ../.. I want to upgrade to a system producing 8230w (or 171amps at 48v). The batteries say the recommended max charging amperage is 30amps per string (times 4 strings = 120). That means I could be producing 171amps while the batteries can only handle 120. 3 different chargers will do this (2xFlexmax60 and 1xFlexmax80). ../..
So long as you use the 3 FM charge controllers (max output of 200amps) then the 8230W array if appropriately split to the 3 FMs (each string's amp out put being less than the maximum of its charge controller) will not cause issues for the charge controllers.
The main benefit of this will be that with the larger array you should get better output in low sunlight / cloudy conditions.
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fcwlp
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by fcwlp »

raysun wrote: Sat Jan 09, 2021 5:52 pm Threat them right, and they will give good service. Abuse them, and you will know about it in short order.
@raysun, how did you threaten them? :grin:
raysun
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by raysun »

fcwlp wrote: Sun Jan 10, 2021 11:23 am
raysun wrote: Sat Jan 09, 2021 5:52 pm Threat them right, and they will give good service. Abuse them, and you will know about it in short order.
@raysun, how did you threaten them? :grin:
😅😅

Threaten? Me? No way!

I babied them. Especially in the last 6 months. I knew I was upgrading as soon as I collected enough aluminum cans off the side of the road to pay for it, so they had to last until then.
smcewan
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by smcewan »

Thanks everyone! My concern was not so much the charge controllers. We will split the panels so they won't go over their max amps. It was more for the batteries. I imagine a scenario where all three controllers are charging the batteries in BULK, so that there is no limiting of amperage, and the batteries themselves receiving more than the 120amp maximum recommended charge amperage. It may be that at 120 amps they will reach the absorb voltage and the charge controllers will limit the amperage automatically. Or it may be that until they reach that voltage they can handle more than the 120 amps because they are so empty. I don't understand well enough to know. I'll keep an eye on them and manually reduce the amperage if I need to. They are old batteries and I'm tring to suck the juice out of them before I install some new lithium batteries that can handle up to 300 charging amps. Thanks for the help. Any further insight is welcome!
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by sbrownian »

There is a thread on here discussing putting >only< the charge controllers in 'grid tied' mode and using the FNDC to limit the max amps into the battery.

They have to be in grid tied mode so they can use some routines built in to the firmware for the purpose of shared current limiting.

Your profile doesn't include a FNDC, so I'm not sure if that will work or not...
raysun
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Suniva 330 watt panels (12 - 6 strings of 2 in series)
Hyundai 355 watt panels (6 - 3 strings of 2 in series)
Honda EU7000is gas fuel generator

Re: Producing 8230 watts, but batteries can only handle 120amps

Post by raysun »

smcewan wrote: Sun Jan 10, 2021 1:03 pm Thanks everyone! My concern was not so much the charge controllers. We will split the panels so they won't go over their max amps. It was more for the batteries. I imagine a scenario where all three controllers are charging the batteries in BULK, so that there is no limiting of amperage, and the batteries themselves receiving more than the 120amp maximum recommended charge amperage. It may be that at 120 amps they will reach the absorb voltage and the charge controllers will limit the amperage automatically. Or it may be that until they reach that voltage they can handle more than the 120 amps because they are so empty. I don't understand well enough to know. I'll keep an eye on them and manually reduce the amperage if I need to. They are old batteries and I'm tring to suck the juice out of them before I install some new lithium batteries that can handle up to 300 charging amps. Thanks for the help. Any further insight is welcome!
The scenario under Bulk is one in which the charge needs to be externally throttled to make sure the combined current does not exceed the battery rating. There is no condition within the battery that will current limit, not even Absorb.

At the very least, set each charge controller's Current Limit such that the sum of the three is <= 120A.

Thinking one can "keep an eye on it" and manually intervene will be an excellent way to speed up the acquisition of that shiny new lithium battery.

If the system has an FNDC, the maximum charge current can be set through dynamically controlling all charging sources.
smcewan
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by smcewan »

Thanks. This I think is what I will do. Manually setting their combined amps to <= 120amps will help me get the most out of the panels while not risking my batteries.
raysun
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My RE system: Flexpower Two: (2) FXR3048A, (2) FM80, MATE3s, FlexNetDC
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Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 6 strings of 2 in series)
Hyundai 355 watt panels (6 - 3 strings of 2 in series)
Honda EU7000is gas fuel generator

Re: Producing 8230 watts, but batteries can only handle 120amps

Post by raysun »

smcewan wrote: Sun Jan 10, 2021 2:50 pm Thanks. This I think is what I will do. Manually setting their combined amps to <= 120amps will help me get the most out of the panels while not risking my batteries.
120A x 50V = 6000W. Thats a good amount of charging power. Unless sited somewhere unusually cool and unusually sunny, it won't be leaving a lot of power "on the table" as most arrays don't often deliver their rated power.
smcewan
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by smcewan »

Great point! My array is rated at 4990w but tends to deliver more on the range of 3600-4200 except on rare occasions. 6000w would rarely leave much power on the table, it would only protect the batteries. Hadnt thought of that! Ok, plan set! You all are awesome. Thank you so much for helping me think through this!
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by pss »

What you are asking for is a system that has a computer like program with IF OR THEN LOGIC. Your PV system does not. Simply put, you can limit the charge controller amperage to the batteries to the nameplate or less, but that value is then fixed, not variable based on state of charge, pv production or load.

So the question becomes do I have enough PV to charge my batteries and power my loads in the day? Do I have enough battery capacity to power my loads without PV or do I consume utility power or generator power?

If you go to 8300 watts of PV, you can support a bigger battery bank, close to 900 amps on good sunny days. If you want more amps than 60+60+80, use a forth charge controller and array.

Also understand this: if battery SOC is say 80 percent, most all current from pv will be used to charge batteries and little to load. If batteries are say SOC 98 percent, almost all current will be used to power loads if load exists, otherwise wasted.

So the final equation is: PV production should be greater than battery capacity + day time loads & battery capacity should be equal to or greater than nightime loads if off grid.
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Location: Victoria, Australia

Re: Producing 8230 watts, but batteries can only handle 120amps

Post by SandyP »

smcewan wrote: Sun Jan 10, 2021 2:50 pm Thanks. This I think is what I will do. Manually setting their combined amps to <= 120amps will help me get the most out of the panels while not risking my batteries.
With the added benefit that your charge controllers will be working well below their maximum output so they should last longer!
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smcewan
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Re: Producing 8230 watts, but batteries can only handle 120amps

Post by smcewan »

I thought I would update this in case it is helpful to anyone in the future. I limited the overall amperage on the charge controllers to the spec-recommended 120 amps combined total. However I noticed that even so there was gassing and the batteries got quite hot. Since they are almost 3.5 years old I imagine the recommended 120 amp total has dropped. I experimented a bit until I came to the conclusion that with 85 amps I have no noticeable gassing, no temperature problems, and I feel I can leave the batteries unmonitored. This is quite a bit lower than I had hoped, but still acceptable for my usage and setup.

The history of these batteries is that for the first 3 years I used them every night until 50% SOC. For the last 4 months I have worked that down to somewhere around a 90% DOD every night. 4 months ago they ran until 4-5am depending on a few variables and now they tend to run until 2-3am using an average of about 0.7-1.2 kw/h. In the next month or two I plan to switch them out for lithium batteries.
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