How do PV charger/inverters 'control'?

WIAWSNB

And, I'm still confused as to how the AC-C knows to not charge up from the Grid (unless you ask it to), but to do charge from the PV, and then release to the house!

I take it it's all automatic, but would love to understand - in basic terms - how!

Reed_Richards

I think it's just a feedback loop. Your inverter/charger can control the amount of power being used to charge the battery. And it can monitor the power being discharged to the grid. So if it sees, say, 1 kW of power being discharged to the grid it starts to charge the battery with 1 kW so the power being discharged to the grid drops to zero. And it does that second by second. If it sees power being drawn from the grid it will discharge the battery to balance that.

My inverter and battery combination does not respond instantaneously. So, for example, if I boil a kettle at night drawing power from my battery, when the kettle turns off the battery output ramps down over several seconds. During that time it is sending power to the grid (as the power has nowhere else to go).

WIAWSNB

https://forums.moneysavingexpert.com/discussion/comment/81976816#Comment_81976816

That makes complete sense - thank you.

Heedtheadvice

...and in simplistic terms your electrician was correct.

To do that balancing described above the inverter increases ( or decreases ) it's output voltage to push the current in the desired direction and quantity.

The Grid voltage varies ( within it's spec ) over time so, for an AC connected battery

The battery inverter is constantly monitoring the grid voltage and current ( internally for voltage, and usually externally by a current transformer ) . Each circuit that one requires to control needs a CT to measure it or by maths can be calculated if it is the only other circuit connected. Thus it satisfies the instructions to import, export or do nothing. The more circuits of interest the more CT units are required.

The delay that Reed mentions is usually to change over from charge to discharge ( or the other way) with a relay switching in/out the charging and discharging circuits plus a delay to prevent too much switching.

CTs are also used to limit power such as DNO specified export or as in the case of a Zappi EV charger an option to limit car charging if the whole house load would otherwise exceeds safe levels ( think TOU tarrifs with a short period to charge car, fill battery, house loads etc all at the same time ).

CTs in their basic for are simply a type of transformer that when loaded by a resistor produce a voltage a monitoring circuit. In the case of the CTs usually used domestically the resistor is built in to the clamp so produces a voltage itself. This can be very useful so as to have an extended length of cable obviating the need for wifi ( if a cable can be routed)

WIAWSNB

Fab, thank you all.