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Open Roads Forum  >  Tech Issues

 > question on interpreting MPPT controller display

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outwestbound

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Posted: 09/03/17 01:58pm Link  |  Quote  |  Print  |  Notify Moderator

Hi. This is intended to be general. If an answer can't be made without knowing the exact unit, then please let me know. My question may be poorly phrased.

Does the amount of amps shown on the controller as coming in from the panels depend on the battery bank's state of charge?

I have an 830AH AGM batt bank and 960 watts of PV panels. When I originally tested the system, the bank was DOWN 250 AHs and the controller was bringing in about 55 amps from the panels in BULK mode. Now, I'm only DOWN about 65 AHs, and in similar sun conditions, the controllers are indicating only about 24 AHs in FLOAT mode.

Does the controller always show what the max coming in can be, or does it show what the battery bank is capable of accepting, depending on the banks state of charge and thus, the mode the controller operates in (e.g., bulk, absorption or float).

Hope this makes sense.


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2oldman

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Posted: 09/03/17 02:00pm Link  |  Quote  |  Print  |  Notify Moderator

outwestbound wrote:

Does the controller always show what the max coming in can be, or does it show what the battery bank is capable of accepting, depending on the banks state of charge and thus, the mode the controller operates in (e.g., bulk, absorption or float)..
Any decent controller will show what's actually going to the batteries. Mine shows that, and what's coming in.

time2roll

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Posted: 09/03/17 02:04pm Link  |  Quote  |  Print  |  Notify Moderator

The controller can only report what is actually happening.

The potential amps cannot be known except in bulk mode as that is when all is running at max. You could briefly load the battery and watch the controller ramp up.


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pianotuna

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Posted: 09/03/17 02:13pm Link  |  Quote  |  Print  |  Notify Moderator

outboundwest,

It may depend on what is selected for set points. What controller do you have? Have you changed the stock settings?


Regards, Don
Full Time in a 28 foot Class C, 256 watts solar, 556 amp hours of AGM in two battery banks 12 volt batteries, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

BFL13

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Posted: 09/03/17 02:14pm Link  |  Quote  |  Print  |  Notify Moderator

The amps showing at the controller depends on the MPPT's output watts. You divide the output watts by the battery voltage to get the amps. So yes, the SOC of the battery affects the amps you see.

EG output watts 200:
Battery v 13, amps 200/13 = 15.38
Battery v 14, amps 200/14 = 14.29

Not all those amps go to the battery. Any load gets first crack at the amps, with any left over going to the battery. (Same as with any battery charger when the battery is connected to loads while also being charged--as with an RV converter)

To get the amps to the battery, you need a second ammeter just for that, downstream from the controller. EG, a Trimetric monitor with the controller's output going through the Tri's shunt.

Of course there remains the usual tapering of the amps to the battery when they get nearer to being full. The SOC when the amps start to taper depends on the charging rate. Higher rate, lower SOC when amps taper. Solar usually is at a low rate so amps would not taper from that until SOC in the 90s percent range.

You also have the IV curve for your panel, where as battery voltage goes up amps start to taper a bit until they fall right off at the knee of the IV curve, but this will be a higher voltage than what you would set the controller to get to, except with MPPT controllers using Vmp before they go to PWM after the Bulk stage.

* This post was last edited 09/03/17 02:36pm by BFL13 *   View edit history


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outwestbound

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Posted: 09/03/17 02:34pm Link  |  Quote  |  Print  |  Notify Moderator

pianotuna wrote:

outboundwest,

It may depend on what is selected for set points. What controller do you have? Have you changed the stock settings?


the set points are per my batt manufacturer. I used some simple MPPT controllers made by BZ Products in the US. I don't have displays inside my camper. My Magnum 3012 display, because it has the batt monitor, gives me a net amp figure. This net figure is the gross on the controller, less the load (devices + phantom), which is fine for me. I wanted simplicity.

Someone answered my question I think. If I'm under good sun and if in BULK, harvesting 55 amps, but then in FLOAT, the controllers show only 25 amps, in the same sun, then that answers my question.

I thought the controller always showed the max potential harvest, when evidently, it is only displaying the usable amps based on the needs of the batteries.

Lwiddis

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Posted: 09/03/17 02:34pm Link  |  Quote  |  Print  |  Notify Moderator

"Does the amount of amps shown on the controller as coming in from the panels depend on the battery bank's state of charge?" Yes, panels "send" what controller "requests" up to panel specs and available light.

"or does it show what the battery bank is capable of accepting," Shows what batteries are accepting. This is why solar system isn't pumping out its max that much.


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outwestbound

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Posted: 09/03/17 02:36pm Link  |  Quote  |  Print  |  Notify Moderator

2oldman wrote:

outwestbound wrote:

Does the controller always show what the max coming in can be, or does it show what the battery bank is capable of accepting, depending on the banks state of charge and thus, the mode the controller operates in (e.g., bulk, absorption or float)..
Any decent controller will show what's actually going to the batteries. Mine shows that, and what's coming in.


Mine has only one number and if I flip a switch, it will show volts. I went with a simple controller that doesn't have much display and no external display panels. I didn't want one.

outwestbound

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Posted: 09/03/17 02:54pm Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

The amps showing at the controller depends on the MPPT's output watts. You divide the output watts by the battery voltage to get the amps. So yes, the SOC of the battery affects the amps you see.

EG output watts 200:
Battery v 13, amps 200/13 = 15.38
Battery v 14, amps 200/14 = 14.29

Not all those amps go to the battery. Any load gets first crack at the amps, with any left over going to the battery. (Same as with any battery charger when the battery is connected to loads while also being charged--as with an RV converter)

To get the amps to the battery, you need a second ammeter just for that, downstream from the controller. EG, a Trimetric monitor with the controller's output going through the Tri's shunt.

Of course there remains the usual tapering of the amps to the battery when they get nearer to being full. The SOC when the amps start to taper depends on the charging rate. Higher rate, lower SOC when amps taper. Solar usually is at a low rate so amps would not taper from that until SOC in the 90s percent range.

You also have the IV curve for your panel, where as battery voltage goes up amps start to taper a bit until they fall right off at the knee of the IV curve, but this will be a higher voltage than what you would set the controller to get to, except with MPPT controllers using Vmp before they go to PWM after the Bulk stage.


Thanks. I had a competent fellow install this stuff, then had to set the camper aside for 10 months. But I recall what you're saying. I've had math beyond calculus, so the workings of the MPPT and getting my voltage up to allow enough headroom for the MPPT to cook, is something I recall discussing before.

I had specified a pair of Morningstar controllers with internal displays, but after talking to my consultant, decided to go with a more simple controller that doesn't generate as much heat or have internal displays. I just put the batt meter on the Magnum 3012 and so I'm getting a read out at the battery of what's going in. I don't have a display of what's coming in on the controllers inside my fifth wheel, but I'm ok with that.

I was just confused on what the controllers where telling me but I think I got it now. Thanks.

Mandalay Parr

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Posted: 09/03/17 03:13pm Link  |  Quote  |  Print  |  Notify Moderator

To keep it simple, it's what the batteries will accept.
Thus it will diminish as the batteries become charged.


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