Can or should I tie wall brick ground to battery ground?

Question

Apologies for the "sort of schematic" made up of symbols and larger components:

I'm new to drawing schematics. What I really wanted to draw was a set of connections between assemblies, not basic circuit components.

I have a working DIY UPS. On the output side, I'm powering a bunch of devices from a 12V battery bank -> inverter -> AC/DC wall bricks or possibly internal DC PSUs.

I'm also using a SparkFun Pro Micro powered directly from the battery bank so that I can use it to sense the DC voltage of the battery (I didn't show the voltage divider in the "schematic", but it's there.) In order to get accurate voltage readings, I need the Pro Micro's GND to be the same as the AGM battery, but the point of the Pro Micro is it uses a USB connection to report data back to some of the devices that are being powered by the inverter. So, those grounds are not the same.

Can I tie these grounds together? Should I? Measuring with a DMM, I've seen no more than 0.5V difference between those two GNDs.

Note that the AC GND from the inverter isn't connected to anything in my example because the wall bricks are a two prong style, but some of the things in "NAS or Other equip" does use a three prong plug to an internal PSU.

Answer 1

The question is really not if you can or if you should.

They are the same ground already, as there are no separate grounds on Arduino Pro Micro.

It does not have separate grounds.

You don't need to do anything and they will already be connected through the Arduino.

Answer 2

Using the USB ground as a return for anything else invites signal integrity problems. However, in this setup, tying the Pi "DC In -" to the SparkFun "GND" is a good idea: they are circuit common.

For safety, it is best to connect the "AC Gnd" to an earth ground, or whatever common conductor (vehicle chassis, boat hull, ...) is in the vicinity. If the inverter isn't double insulated, you should connect DC - to earth also.

Answer 3

You're worried about gound loops, the ungrounded isolated power supplies used by the Raspberry pi and the NAS will prevent ground loops by that path.

Connecting both the USB and the DC to the arduino probably connect them together, but this will not be problematic, because the raspberry pi supply breaks the loop.

Connecting the NAS to the raspbeery pi via ethernet will not be a problem because ethernet includes transformer isolation.

If you're only measuring voltage with the arduino you might be better off with something like INA231 which doesn't need a separate power supply, will give a more precise reading, and can also measure current.