Why does my battery stop at 10%?
Your battery holding some charge back isn't a fault. Here's why, and what to check if it really isn't charging or discharging.
Every home battery keeps some charge back. It stops discharging at a minimum level, protecting the battery and leaving some energy for a power cut. So a battery sitting at 10% or 20% in the evening, with the house using the grid, is usually doing exactly what it should.
Where the limit is set
| Your battery | What it's called | Where to see it |
|---|---|---|
| Fox ESS (EP6, EP12) | Min SoC (minimum state of charge) | FoxCloud app, in the battery or work mode settings |
| Duracell Energy (Dura5) | The discharge limit | Set when your system was installed |
| Tesla Powerwall | Backup Reserve | Tesla app, Powerwall settings |
On a Powerwall you can move the Backup Reserve slider yourself. A higher reserve gives you more for power cuts, but less to use each evening.
Usable and total capacity
The size on your paperwork is usually the battery's total (nominal) capacity. The part you can use day to day is a little less:
| Battery | Total | About usable |
|---|---|---|
| Fox ESS EP6 | 5.76 kWh | 5.2 kWh |
| Fox ESS EP12 | 11.52 kWh | 10.4 kWh |
| Duracell Energy Dura5 (each module) | 5.12 kWh | 4.6 kWh |
| Tesla Powerwall 2 or 3 | - | 13.5 kWh |
Why it may not fill up from solar
- Winter. Short, dull days mean your panels may only make enough to run the house, with little left for the battery. December can produce around a fifth of June. A cheap overnight tariff helps here: see Battery Optimisation Guide: Charging on Off-Peak Tariffs.
- A schedule is set. A charge or discharge schedule, or a work mode such as Feed-in Priority, changes what the battery does.
- Tesla is planning ahead. In Savings mode the Powerwall decides when to charge, and may sit idle while it waits for the best time.
When it might be a fault
Contact us if:
- the battery won't charge on a sunny day while you're exporting to the grid, or
- it charges from the grid, or sends power to the grid, when you haven't set it to, or
- the app shows an error or alarm about the battery.
A common cause is the sensor that tells the battery what your home is using being knocked or moved, often during a smart meter change. See My battery or app went wrong after a smart meter was fitted.