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.

EV charging and battery interaction

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 your battery discharging whilst your EV is charging overnight. But we are thinking about it.

Workarounds

For Charge HQ to stop your EV charging from your home battery it 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.

Here are some thoughts on some ways that you might be able to work around this problem independently of Charge HQ.

  1. A solar inverter that is in control of both your battery and your EV charger should be able to do this. We suggest looking at SolarEdge or SMA.
  2. In theory, a Tesla Powerwall in a home with Tesla EVs could do this if Tesla built the software. It doesn't appear to work this way currently but it's possible it will change in future, we suspect they're busy with other priorities also.
  3. You could prevent your battery from discharging during a period that matches a scheduled charging window for your EV by setting the reserved capacity to the maximum percentage during this period.
  4. If your battery allows optimisation based on time of use tariffs, you could encourage your battery to prioritise discharging periods outside of your EV charging period by ensuring a) the off-peak periods are defined, and b) your scheduled EV charging window aligns with this off-peak period.
  5. Consider maximising your charging rate. 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.
  6. 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.
  7. 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!