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paul-campbell avatar image
paul-campbell asked

MK3 USB DC current path

I have a Multiplus 24/800/16-16 which is connected to the MK3 USB and from that to a RPI 3b.

The RPI3b is power from a USB charger running off the MPPT controllers "Load" output.

The issue is the DC return current is not coming back. It appears to be going back via the RPI3 USB to the MK3, through the inverter and back to the battery. The MPPT shows just random noise on the load current jumping around. Quite often it is reading 0W, 0Amps. However it is running a raspberry PI, a 2.4Ghz Wifi AP and 2xESP32s. It should be drawing about 4 watts.

First question is, can the MK3 run through a USB1.1 isolator or does it require USB 2?

Asides putting a 0.1Ohm resistor or two in the USB ground does anyone have any suggestions to prevent this?

MK3
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5 Answers
mvader (Victron Energy) avatar image
mvader (Victron Energy) answered ·

The MK3 USB is galvanically isolated.

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paul-campbell avatar image paul-campbell commented ·
Ah. Thanks. So it's not going back that way.


I think I have figured it out, there is only one other ground route.

There is an RS485 Wifi dongle on the EPEver MPPT. For some reason the 2410N would not power this from the RS485 5V and GND. Don't know why, but I ended up having to inject USB 5V into the dongle.

It's the only other route the ground can take. The MPPT DC Load is powering the USB power supply. That is in turn powering the RS485 dongle. The current then faces a choice, to return via the MPPT load shunt or return via the RS485 ground and experience no shunt resistor.

That RS485 dongle is probably the oldest surviving device in my system. I have it flagged for replacement anyway.

I maybe should also consider a small 10W 5V isolated DIN rail supply for the admin stuff.

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Mike Dorsett avatar image
Mike Dorsett answered ·

Powering the Pi from the AC output will be a problem - you will lose the pi functions when / if the power shuts down.

If the supply to the Pi is galvanically isolated - as it should be, then you need to bond the Pi's ground to the inverter battery negative terminal. This will help get rid of any induced noise.

I use a small 1A 5V dc/dc converter to power my Pi direct from the 24V system battery, you can get 48V input converters too. This provides the pi with continuous power, which is needed as it runs the BMS as well.

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paul-campbell avatar image
paul-campbell answered ·

Yeah, the MPPT "load" output is basically direct to the battery via a fuse and a current shunt. I think it's the R value of the later which means the DC current would prefer to go via the inverter "-" to the battery.

I use a 4 USB port charger which has a 9-60V input range. I can't remember if it's 5A total or 3A on the 5V side.

Maybe I should move that DC/DC converter to be directly connected to the battery with a pretty short "-" cable.

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paul-campbell avatar image
paul-campbell answered ·

I suppose the "Admin" power should either be directly across the battery prior to the BMS but with it's own cut off supply circuit... or ...

Can you recommend a reliable 5V 5A UPS? I don't mind it being a handful of 18650s. I got mixed reviews on the usual sources like ama and ali. For 1V voltage ripple.

I have a 50W panel and a 12V lead acid, but ... surely that is overkill?

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kevgermany avatar image kevgermany ♦♦ commented ·
There are 18650 based Pi UPS units. Got mine from eBay and I just feed it with a cheap DC:DC converter from AZ-Delivery.
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kevgermany avatar image
kevgermany answered ·

@Paul Campbell

Pi topics should be in the modifications area. Moving it there.

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