jamesh avatar image
jamesh asked

Victron MPPT 75/15 and Smart Shunt Data

TLDR:Current Output current data from MPPT controller is around 30% higher than SmartShunt current data

Hi, I am relatively new to the use of Victron products and recently upgraded my solar charge controller to a Victron MPPT 75/15 and installed a SmartShunt in my campervan. The reason for the upgrades was to enable me to manage the power usage of my leisure battery while I am away, I love the products I think they are great.

The setup I have is a small portable 100w panel from Renogy which is connected to the MPPT the negative output from the MPPT is connected to the load terminal on the SmartShunt, the terminal labelled battery on the SmartShunt is connected to the negative post on the Battery (105 Ah). The positive output from the MPPT is connected to the positive post on the battery.

All of the equipment and battery are stored within the same space and are connected using small cables of the correct rating for the maximum current.

I have included a couple of screenshots taken within the same minute using the IOS app showing the difference in data between the two devices. It is difficult to get an exact snapshot due to having to switch between the devices within the app but the output from the MPPT has been consistently the same for around 15 minutes as has the reading on the SmartShunt. There is no current being drawn from the battery - verified by disconnecting the panel and checking the data on the Shunt.

I don't have a clamp meter to check the actual current on the MPPT output cable and I thought before I go an buy one I would post on here.

I have tried changing cables and using a different battery / solar panel but consistently see the numbers are around 30% different between the two.

The settings on both devices are basically factory defaults, I changed the charged voltage on the smart shunt to 13.6 volts and the tail current to 1.5 with an 8 minute detection as the shunt kept setting the SOC to 100% despite the MPPT being in the absorption stage. Changing these settings has no impact on the detected current.



MPPT ControllersSmartShunt
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1 Answer
kevgermany avatar image
kevgermany answered ·

Usually this is because there's a load you don't see between shunt and panels.

Is anything else except the shunt connected to battery negative?

Try disconnecting all the positive connections from the battery except the input from the MPPT.

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jamesh avatar image jamesh commented ·
There is no load reading on the shunt with the panel disconnected. Your comment did give me an idea though which has led me to understand the cause of the issue which has now led me to a new issue.

The vehicle has a voltage sensing split charge relay installed. The relay opens the circuit if the vehicle battery voltage is >16.5v. What seems to be happening is that when the vehicle battery is >16.5v and the split charge relay opens the circuit and the panel is connected the panel is charging the leisure battery and the vehicle battery. To test the theory I turned on the ignition and switches the vehicle lights on until the excess voltage was used and the relay closed the circuit, at which point the shunt and mppt readings match.

This makes me wonder if there is a fault with the split charge relay, I don’t fully understand how they work and if they are designed to allow current to pass both ways, if so it seems like a design flaw as with the panel connected the vehicle battery will never drop below the threshold.

I wonder if this is a well known scenario ….

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kevgermany avatar image kevgermany ♦♦ jamesh commented ·

If you have a battery negative connection direct to earth (doesn't go through the shunt), the shunt cannot measure the current through that earth.

A wiring diagram would help. Especially showing shunt, all battery connections, panels, MPPT, split charge relay.

Doesn't sound right that the relay needs such a high voltage to switch, most 12V alternators are limited to about 14.4V.

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