Technical details of the charging control system

Technical details of the charging control system

This article provides some detailed technical insights into how the Charge HQ app uses the charging settings to decide when to charge or not to charge.

In order to understand how Charge HQ controls charging, the most important thing to know is that by default, EVs want to charge when they're plugged into a charger, and they will charge unless something tells them not to charge. So all of the control by Charge HQ is based on "stop charging", or "limit charging rate" commands. When charging starts, it hasn't so much issued a "start charging" command but rather it has ceased to issue a "stop charging" command.

For any public charging network, or a wall socket charger when you're travelling this is exactly the behaviour you want, connect the charger and the car starts charging. When charging from rooftop solar or trying to utilise cheap off-peak power, it isn't.

If you enable Charge HQ, and assuming you don't have any charging schedules defined in the car, when you connect it to your home charger it will start charging.

Charge HQ then decides when to stop or limit charge rates based on a priority of:

  1. Is Charge Now or Stop mode selected?
  2. Does a Scheduled Charging (Always Charge or Do Not Charge) window apply?
  3. Is a Do Not Charge Price, Always Charge Price or Minimum Renewables Limits met?
  4. Is there a Solar Tracking constraint?

The highest priority controls in the program are the Charge Now and Stop modes.

These two modes will override any other controls (e.g. scheduled charging, solar tracking, price & renewables limits).

Next, if Charge HQ is in Auto mode it will continue down the list and scheduled charging periods will be considered. If there's an Always Charge period, charging will occur (by absence of a stop signal) and no other controls will be considered. Likewise if there's a Do Not Charge period, charging will be stopped for the duration and no other conditions will be considered.

If no scheduled charging periods are defined, it will next consider Price Limits and Minimum Renewables Limits. Charge HQ will send a stop charging signal if either of these limits apply. Otherwise charging will continue.

Last up is Solar Tracking. Charge HQ will allow charging to occur at the set maximum limit unless constrained by solar tracking. When enabled, it will either a) stop charging outside of solar generation hours or when below set minimum solar/charing limits, or b) limit the charge rate based on the excess or gross solar generation during solar production hours to match the power available.

Let's consider some practical examples of how the controls play out.

Behaviour with local solar tracking

The most common use of Charge HQ currently is to use energy from rooftop solar to charge the EV. Once Solar Tracking is enabled, Charge HQ tells the charger to stop charging whenever there is no solar generation.

In practice, this means it will stop charging either based on the Min Solar Generation figure set by the user, or, if this is not set, based on the minimum supported by the charger which varies between 1.2 kW and 4.3 kW.

So, with Solar Tracking turned on, the default "always charge" behaviour is actively stopped whenever there's too little sun.

But, based on the control priorities above, we'll first check if the Minimum Renewables Limit, or Do Not Charge Price Limits are met. If they are, we'll stop charging. If the energy price is below the Always Charge Price Solar Tracking charging will occur at the Default Charge Limit rate. Importantly: all of these settings will override Solar Tracking.

Note: A little counter intuitively, this currently means that even if you have excess solar, if the renewables on the grid falls below your minimum renewables limit charging will stop. This behaviour is pending review, see discussion below.

Note also: When charging from rooftop solar (based on a minimum of 1 KW), the Do No Charge Price Limit will reference the Feed in Tariff rate (not the Grid Price) as this is the effective cost to the user of using excess solar.

Likewise, if you configure a scheduled charging period to alway charge or never charge during solar generation hours, it will override local Solar Tracking.

Finally, telling Charge HQ to Charge Now or Stop will also override solar tracking. Charge HQ will remain in the selected mode until the charger is disconnected, and will default back to Auto when it is next plugged in.

Behaviour with grid only charging

If you have an EV but don't have a rooftop solar PV system, Charge HQ will always charge when connected unless it first encounters a scheduled do not charge period.

After this it will look for Minimum Renewables Limits or Do Not Charge Price Limits. If neither of these are triggered it will keep charing until it reaches the charge limit set in the app, or that defined by the vehicle.

These controls can be used to align your EV charging with periods of high renewable generation, or, for customers on variable rate or time of use tariffs, to align with periods of cheaper energy.

If you do not have a rooftop solar PV system, the Always Charge Price will not impact charging behaviour.

Why do Minimum Renewables and Grid Price Limits apply to Solar Tracking?

The logic in the current behaviour is as follows:

  • Minimum Renewables Limit: We're taking a grid wide view and considering your rooftop solar as part of the broader renewables generation fleet. By applying the renewables limit when charging from rooftop solar we're stopping the flexible EV charging load from using power during when renewables generation is low, and hopefully delaying it to a future time when there is more renewable energy being generated.

    Doing this reduces energy generation from fossil fuels, and increase demand and price support for renewables generators.
  • Do Not Charge Price Limit: the price limit considers the Feed in Tariff price when charging from excess solar, which is the value not earned by exporting power. The price limit control is mostly of use to customers exposed to wholesale prices via Amber, and there may be a situation where the FiT price becomes so high that it is better to export energy and charge at another time.

    We're looking at changing this behaviour in future.