EV Charging and Home Battery Interaction

EV Charging and Home Battery Interaction

In many instances when your EV charges from grid energy, if you have a home battery system, the battery will discharge energy whilst the car is charging.

There's a view that charging your EV battery from your home battery is sub-optimal as:

  • It may be inefficient due to the losses incurred in charging and discharging the home battery
  • The energy in the home battery may be best kept to meet home loads when energy tariffs are higher
  • The EV will often use much more energy on a daily basis than is available in the home battery, thus draining and leaving less power for a) higher tariff periods, and b) backup power during outages
  • Batteries have finite cycles and using them for EV charging is a waste
  • It may cause the battery to cycle more than desired, depleting during off-peak periods and then charging again from grid energy
  • It just feels wrong using one battery to charge another

Conversely, some users may not care since:

  • Maximising the use of the home battery reduces energy generation from fossil fuels (if it's able to be charged from solar)
  • They're on a flat rate tariff and so both home energy use and EV charging is billed at the same rate
  • Maximising the use of the battery minimises their power bill

That said, let's say you don't want your home battery to be used to charge your EV, let's explore why it happens and what the options are.

How home battery discharging is controlled

Home batteries are commonly configured in two modes:

  1. A solar self-consumption mode - the battery charges using excess energy from rooftop solar until it is full, and then discharges to meet home loads until empty.
  2. A tariff aware mode - again charging from solar, but now the decision of when to discharge energy is controlled by a program that prioritises the use of the energy in the battery for times of the day when energy is more expensive.

Home battery systems decide what rate to charge or discharge energy at by monitoring the flows of energy at different points in the home. In the common case where solar PV is installed this involves monitoring

  • consumption of energy from the homes grid connection
  • production of energy from the solar PV system

Importantly, when you're "using" energy from a grid connected battery, the battery management system is choosing to discharge energy at the same rate that you're using it.

Note that where the battery is using a local control system to optimise charge and discharge behaviours, these behaviours may change on a seasonal basis. Consider a home with high overnight heating loads in winter but with minimal cooling requirements in summer. There may be sufficient demand to discharge a full battery in winter but not enough to use the stored energy overnight in summer. The local control system will make different choices when optimising against tariffs between the seasons.

How EV charging interacts with home batteries

When your EV is charging, from the perspective of your home battery monitoring system, it looks exactly the same as any other device in your home that consumes energy.

This is mostly a problem for off-peak overnight charging, or if your EV charging rate during the day exceeds the excess energy produced from your solar PV. Charge HQ can reliably charge your EV during the day from your excess solar without drawing power from your battery.

Currently, Charge HQ can not stop actively stop your battery discharging whilst your EV is charging overnight but we have some work arounds.

To actively stop your EV charging from your home battery Charge HQ would need to be able to control the battery. Improving the EV charging functionality of the app is a higher priority at the moment, but it's technically possible for many batteries and may be considered in future.

Options for stopping EV charging from your home battery

Automated battery controls

Some home batteries allow configuration of charging and discharging behaviour by the owner. In such cases you may be able to instruct the battery not to discharge at specified times overnight when you schedule your EV to charge.

See our guide for preventing the Tesla Powerwall being used for EV charging.

We're not aware of methods to do this for other batteries, but if you are please let us know.

Integrated Solar, Battery and EV Chargers

If you're purchasing a new solar inverter, home battery and EV charger at the same time, you may find an integrated unit from vendors such as SolarEdge or SMA. In such a configuration the solar inverter controls the charging behaviour of both the home battery and the EV charger and can co-ordinate the two.

Manual battery controls

Different battery models will offer different levels of control over when the battery will charge and discharge.

You may be able to configure your battery to not discharge at all during a period that matches a scheduled charging window for your EV.

For the Tesla Powerwall, you can do this by manually adjusting the backup reserve level to 100%. This approach will require manual adjustment at both morning and night however.

Exclude your EV charging circuit from your home battery energy monitoring

Home batteries rely on monitoring of solar production and home energy consumption to correctly control charging and discharging. The most common configuration we see is that the home consumption monitoring includes any energy used for EV charging.

This usually places the CT clamp for monitoring immediately after the utility meter.

An alternative is to wire the CT clamps used to monitor the home consumption downstream of the EV charger but upstream of all other home loads. This means that any consumption from EV charging is hidden from the battery preventing it from discharging to match any EV charging consumption.

Since the battery can't see EV charging loads during the night for off-peak charging, it also can't see it during the day. If the battery is configured to charge from excess solar it won't take into account any of the solar production being used for EV charging when the sun is out. Charge HQ has an advanced Consumption Excludes Charging control to allow for this configuration, and will always prioritise home battery charging before using any excess solar.

Maximising load

A sub-optimal and partial solution that you might consider having exhausted all other options is to simply charge as fast as you can.  Home batteries have a maximum discharge rate (often 3-5kW), once you exceed this any excess energy must be supplied from the grid. If for example your battery can only discharge at 5kW and you have a 22kW charger, at a maximum the battery can only supply around 1/4 of the energy used for charging your EV.

The same idea could be used by stacking loads, if you have multiple EVs, and your grid connection supports it, charge both EVs at the same time. If you have other high off-peak loads such as hot water, charge your EV in parallel.

Noting that maximising peak demand is generally a bad thing from the perspective of the grid, if you're doing it during off-peak periods whilst EV ownership is still relatively low it should be fairly inconsequential.

If you've come across any other solutions we'd love to hear them, please let us know!