HEMS Wiring Update

Late last year I started adding the ability to optimise my electricity price by shifting some electrical loads around in response to a dynamic electricity tariff. My electricity price changes half-hour-by-half-hour and day-to-day, with the prices for the day ahead published each afternoon. I already had the ability to manage the same electrical loads to maximise use of the output of my own solar panels for some 3 years. The first load smartly controlled to follow my electricity costs was my electric car charging, but I have subsequently added optimisation of water heating and storage battery behaviour.

However, time has revealed an occasional issue arising when both the bought electricity price was low and a solar suplus was available, so both sources sought to enable the car charger; but in practice the vehicle didn’t charge. The issue here is that the car charger does a sanity check on the radio signal indicating that it should be operating, which fails since the combination of two signals driving a common radio transmitter can lead to excessive duration of the ‘on’ signal which fails the sanity check.

Revised HEMS wiring

My chosen solution is to disable one of the two signals sources when the other wants the car charger on. I’ve chosen to make the price signals via the HEMS the master, so when the HEMS wants to charge the car HAT #4 opens so that the ImmerSUN no-longer has influence over the car charger, and HAT #1 is used to control car charger behaviour. When the price is relatively high HAT #4 remains in the normally closed position and HAT #1 is open allowing the ImmerSUN to control charging behaviour via its output relay to use any surplus solar. (HAT refers to HArdware on Top – accessory circuit boards that mount on top of a Raspberry Pi. In my case board with 4 output delays. HAT #3 is currently unused.)

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