For quite a while I had designed a control box which I wanted to place in between all my electrics and the solar control box. My own box would provide me with voltages and currents, so I could finally see how much the solar panel would charge the battery, and how much my electrics would use. Here is the latest plan of this concept:
From left to right the solar controller basically has 6 inputs: +/- from solar panel, +/- from/to leisure battery, and +/- to electric load. My own box would have the same 6 inputs, would then do some switching and voltage/current measurement, then have the 6 outputs which would then connect to the solar controller. In addition I planned to connect the vehicle battery/alternator for charging while driving, then an external 12V source in case I have external power, and an input for an external battery charger. The above schematics allows all this: automatic switching to external 12V, when such a source is connected; manual choice of charging the battery; integrated split charge relay for automatic switching to charging from vehicle while engine is on; automatic disconnect of the solar panel while driving; manual override of split charge relay for explicit linking of the batteries even when not driving (for example for charging both the leisure and the car battery from the solar panel). And the design allows to show the voltages and currents in the three circuits: the solar panel, the leisure battery, and the connected load. Furthermore I wanted an external high-current output, bypassing all controllers in case I connect a high-powered device (e.g. a microwave oven).
Over the time I had bought all the necessary components: 3 Volt/Ampmeters, relays, wires, and a suitable box.
So I began to build. First added a little board inside the box, so I could mount the components onto it. Here is the volt/ampmeter with the required shunt resistor (a low ohm resistor which is used to measure a voltage across, which is then interpreted as a current).
I tried various layouts of the backplane in the box:
The front plane would have three data displays plus two switches: one for switching on the external battery charging, another one for bridging the split charge relay.
Then everything would come together...
... and I realised that the box was too small. The thick wires (for up to 40 Ampere) and the bulky terminals did not leave any wiggle room. While in theory all the relays and the (overly large) shunt resistors would fit, in practise I was not able to close the box without the risk of creating a short circuit somewhere or damaging those very fine and sensitive Volt/Ampmenter wires. So I decided to leave the box open for now and connect it to the vehicle anyway. At the front there is now also a switchable circuit breaker for the electric consumer loads.
And - it works! The above picture shows the voltages and currents as the solar panel is loading (left v/a meter), the battery provides some output current as well (center v/a meter), and the refrigerator is on and drawing current (right v/a meter). So for the first time I can see what is actually going on in the circuits . great! Theoretically the currents of the solar panel and the battery should add up to the load - but they are 200 mA short; not sure why that is... possibly the solar panel controller uses a bit of power (each of the v/a meters uses 20 mA) ... but it also could be because the controller regulates current and voltage in the battery and in the load circuits, so that it is not the current that should add up, but the power. Based on this, there is a leakage of around 400 mW.
Now I can temporarily mount this box on the vertical panel and have finally a more empty work top. The external wiring is not yet final and will be shortened eventually.
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