How to activate battery charging when excess solar

Applications
1

I have had solar panels for over 7 years now with a traditional inverter setup, had no batteries fitted when it was installed as I could not make the maths work.

Now we are several years on and there are plugin battery products legal in the UK, like the Anker SOLIX, I am keen to get a home battery.

The issue I face is how to tell the battery when to charge and when to discharge. I already have EmonPi with CT clips so know when I am selling back to grid, how can I use this information to tell the battery when to charge and when to discharge? Can I do this with MQTT or another method. I don’t really want to have to fit another power monitor into my consumer unit.

2

It will depend on what battery you decide to use, this video shows a dji battey unit connected to a solar pv system via a Shelly monitor

3

Are they currently legal in the UK? According to this, it does not imply they currently are - but they intend it to be within the next few months.

4

And here:
https://www.theiet.org/media/press-releases/press-releases-2026/press-releases-2026-january-march/24-march-2026-iet-urges-households-to-check-electrical-safety-before-using-plug-in-solar-products

along with words of caution.

5

Thanks a lot. I have looked at the Shelly monitor and I know I can do it with one of those. I guess was I was trying to do was use the EmonPi to do what the Shelly monitor is doing.

I already have the Pi and the monitoring, already have CT clips, can already see the data. If I can’t do with the Pi, I might as well sell it and buy a Shelly monitor instead as no need to see the energy use if I am not needing to optimise device use to the sun due to batteries.

6

No longer available as a kit, but the documentation is still here in Docs, is Robin Emley’s Mk2 PV router. The idea behind his design was to take advantage of the backlash in a mechanical Ferraris energy meter, and divert (route) excess PV into (typically) a water heater, using a burst-fire triac as the control switch.

Clearly, something like this cannot be used directly to supply pulsed mains to an intelligent battery charger, but if you have a battery charger that has an input that will control the charge rate, then you can control the charger from the output of your emonPi. The worst case you can expect is the charger’s control input is analogue, in which case you’ll need to add something (possibly another RPi) to accept a digital message to set a voltage or current signal to the charger. If the charger has a data input with one of the common sorts and protocols, then you can probably do it with one of the software tools you have in mind and a USB to *** adapter.