DC electronics help

Ormy

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Beyond A-level Physics my electronics knowledge is pretty shaky, I thought it would be enough to see me through this very basic DIY project but I'm having trouble with ammeters (I know, should be simple right?)

I bought a 25W solar panel that I'm planning to wire into my car to keep the battery topped up during long periods without any driving. Wired a diode to stop the battery discharging through the solar panel when it's dark, used my cheap but trustworthy multi-meter to check how much current is delivered to the battery in different light conditions. An amp or so when in bright midday sun, few dozen mA even when heavily overcast, zero reverse current in total darkness. All working as expected, so far so good.

Then I decided to install a small standalone LED voltmeter/ammeter unit into the circuit so I could continue to monitor how the solar panel is performing long-term. I found one on ebay, £6, bargain. See pictures. It has 5 connections, two thicker wires (red and black) for the ammeter part, then 3 thinner wires (red, yellow, black) that measure voltage and power the device itself.

Screenshot_20200611-205333.jpgScreenshot_20200611-205321.jpg

The problem is I couldn't get it to work properly. I connect the three thin wires and it powers up and measures voltage fine, but when I connect the thicker wires into the circuit it either turns off completely (I think due to a short somewhere), or shows a very incorrect current and voltage.

Here are the two configurations I tried (sincere apologies for the awful untidiness of my diagrams.)

IMG_20200611_205201.jpgIMG_20200611_205209.jpg

It seems to me like the ammeter circuits and voltmeter circuits are not electrically isolated within the device, so I set my multi-meter to measure continuity, connected it to the thin black wire and thick black wire on the device (see another badly drawn diagram). In my head there should be no continuity here but there is, zero ohms between these wires. Should that be the case?

IMG_20200611_205219.jpg

I noticed an instructional wiring diagram on the ebay description showing an independent DC supply being used to power the device, and current measured between a separate DC supply and load. But nowhere did it say that the DC supplies had to be separate, or is that just common sense?

So, which of the below is my problem?
1.) I'm an idiot and made a mistake with my wiring. This unit will work fine if I wire it correctly. Please give a circuit diagram of the correct layout.
2.) This unit can only be used to measure the current of a circuit that is completely independent of the circuit used to power the unit itself. To draw device-power from the same circuit I'm measuring current from I need a more capable/expensive unit. (Any suggestions on units that can do this?)
3.) This is a fundamental limitation of all digital/powered ammeters. It is impossible to measure the current in a circuit and draw power from the same circuit to power the ammeter itself.

Assuming my problem is no.3, I can see two solutions:
A) Use a 9V battery to provide power for the ammeter/voltmeter unit and a manual switch so the unit is only powered on for brief periods.
B) Use a simple moving-coil analogue ammeter. (I much prefer this solution because of simplicity)

Any help is appreciated. Thanks.
 

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