DC Power Reading from Cerbo GX

Question

question

Tommy asked Apr 12, '22

DC Power Reading from Cerbo GX

I have a CERBO, a 712 monitor, and a MPPT solar controller.

I read that I am supposed to have the negative from the solar controller connected to the load side of the shunt.

1. When the Solar controller is connected to the LOAD side of shunt: The DC power, shows the NET usage. If I'm reading this correctly, here I'm using 267W. The battery is providing the other 91W, so my net DC Power is 176W

2. When the Solar controller is connected directly to the battery: The DC power, shows the actual usage. If I'm reading this correctly, here I'm using 373W. The battery is providing the other 284W.

Am I reading these numbers correctly? In my mind, scenario 2 makes more sense to me because I can see my actual DC load use. I guess the problem with scenario 2 is that I wont be able to see exactly how much the solar controller is putting into the battery?

How do you guys have yours set up?

cerbo gx BMV Battery Monitor

pvtoshunt.jpg (227.3 KiB)

pvtobattery.jpg (251.7 KiB)

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Modifications Space

Node Red Space

Answer 1 · 2022-04-13T00:34:55Z

klim8skeptic answered · Apr 13, 2022 at 12:34 AM

1. When the Solar controller is connected to the LOAD side of shunt:

You are drawing 91w from the battery, drawing 85w from the mppt, and your load total is 176w.

91 + 85 + 176.

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 2 · 2022-04-13T01:37:40Z

shaneyake answered · Apr 13, 2022 at 01:37 AM

You should only have the shunt connected to the battery and everything else connected to the Shunt.
if you wire the MPPT directly to the battery, you will confusing the shunt and Cerbo and all your numbers will be off, including SOC.

DC Power is your load, not net. It is the total load, except inverter if you have one.
Battery Power is the power in and out of the battery.
MPPT is the solar power.

So as klim8skeptic said.
Battery + MPPT = DC Load, in scenario 1, all numbers displayed are correct.

In scenario 2.
You load is equal battery power.
The DC load is totally wrong as it is adding Battery Power and MPPT for 373W which is wrong since your load is actually only 284W.

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 3 · 2022-04-13T12:06:15Z

Tommy answered · Apr 13, 2022 at 12:06 PM

Thank you. I guess I was reading it incorrectly. I just hooked it back up with the mppt negative on the load side of shunt (scenario 1). Based on what you're saying, here it looks like I'm using 134W total; the battery is providing 35w and the mppt is providing 99w.

I understand and that makes perfect sense, but I'm not fully convinced. Here's why, when I turn off all my 12v dc items (lights, TV, etc) I get a NEGATIVE DC power reading, which isn't possible?

cerboposuse.png (1.3 MiB)

cerboneguse.png (2.0 MiB)

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 4 · 2022-04-13T13:56:39Z

Tommy answered · Apr 13, 2022 at 01:56 PM

This is why I it appears the DC power is "net", rather than total:

Here is a screenshot with solar running (and connected to load side of shunt) and showing a dc power use of 111W.

However as soon as I disconnect the solar, my DC power is 146w.

My dc load didn't increase at the same instant I disconnected solar.

Here is a video of me disconnecting solar. Watch the DC power go up as soon as I disconnect it.

This is so frustrating. Something is off and I cannot figure it out.

solarconnected.jpg (134.9 KiB)

solardisconnected.jpg (138.5 KiB)

1

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

shaneyake commented · Apr 13, 2022 at 04:45 PM

DC power isn't going to be very accurate. So your expectations are currently not inline with what it can actually provide.

DC power is a calculated value not a measured value.

What you are seeing is a mismatch in timing between measurements.

Like the MPPT data and Shunt data are not synced in when they take their measurements.

So the Shunt data might update before the Mppt data or vice versa.

So in your case shunt is reporting 153W and MPPT is still reporting 114W because it hasn't updated yet. So 114-153=-39W, once the new mppt value comes through you will see it go back positive.

There is always this measurement sync error. Measurements can be 5 seconds apart.

Also there is measurement error stack up. MPPT might be +/- 10W and Shunt might also +/- 10W so the total error will be +/- 20W. Once you disconnect the solar like you did then you will error reduce or increase as now only the shunt measurement is at play.

If you would like acratute measurements then you can install another Smartshunt and have only your loads connected to that one. This one would connect to the load side of your battery shunt. It will give accurate load metering but most people don't need that which is why this feature exists.

0 ·

Answer 5 · 2022-04-13T16:28:25Z

pwfarnell answered · Apr 13, 2022 at 04:28 PM

The DC load is a calculated value. The GX knows the power in/out of the battery accurately from the BMV 712, this has to be all loads / supplies to accurately monitor SOC. The system knows the power from the MPPT and can estimate the DC power in/out of the Multiplus. The GX then estimates the DC load as being what is not accounted for, including for example if you have charging from an engine alternator. Therefore, unless you have a separate shunt on your DC power, the DC power is an estimate, I think you are trying to read too much into it. Also, if properly wired up, the power draw of the GX will be under DC load. It also means that the DC load is where any inaccuracies in the other equipment measurements ends up.

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 6 · 2022-04-13T16:28:27Z

Kevin Windrem answered · Apr 13, 2022 at 04:28 PM

The value in the DC Power tile is a calculation. It is what remains when all KNOWN DC sources and loads are factored out. Unfortunately, the values used in the calculation don't change at the same instant in time so the calculation can be off substantially for "a while". If loads or charging sources are changing, the DC Power number could be off for most of the time.

The fact that the DC Power number went negative means that the UNKNOWN sources exceeded the UNKNOWN loads, and/or the DC numbers were in a state of change.

A better way would be to introduce a shunt for all DC loads and charging sources and use that for the tile. That way the number would be an actual measurement rather than a calculation. Some work is bring done to allow shunts to monitor charging sources and loads but the calculations for the DC Power tile are not yet complete. It also means multiple SmartShunts would need to be added to the system increasing costs.

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 7 · 2023-07-11T00:09:35Z

johnm1 answered · Jul 11, 2023 at 12:09 AM

If the "DC Power" is just a calculation and not always accurate, how do I turn off the DC Power icon/tile?

2

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

shaneyake commented · Jul 11, 2023 at 12:45 AM

On the GX, Settings -> System Setup -> Has DC System

1 ·

Kevin Windrem commented · Jul 11, 2023 at 05:44 AM

Unless you have a shunt connected to measure the power used/generated by other devices it is a calculation. BUT it is still useful to show what energy is going out or coming in even if it is an estimate.

I added a shunt in my RV to measure the DC loads in the system.

1 ·

question