# Solar Charging and Battery Priority

Battery priority decides how far the station may draw a house battery down to
keep a vehicle charging. It is part of ECO+ mode and is controlled by two
thresholds, Stop SoC and Start SoC. Available from firmware 2.3.81 on all
station models.

## Requirements

Battery priority is evaluated only when all of the following hold:

* ECO+ mode is active. With ECO+ off, battery priority has no effect at all.
* The station detects a battery on the installation. Without a detected
  battery there is nothing for the function to protect.
* Firmware 2.3.81 or newer.

## Stop SoC and Start SoC

The two values are not two conditions on the same event. They are the two
halves of a hysteresis, and each one is evaluated in a different state:

* **Stop SoC** — the battery state of charge at which the station **pauses**
  vehicle charging. At or below this level the battery is no longer drawn
  down for the vehicle. While charging is running, this is the only threshold
  that is evaluated.
* **Start SoC** — evaluated **only while such a pause is active**. Charging
  resumes once the battery has recovered to this level.

While the station is charging, Start SoC has no effect. The only question
asked during a running session is whether the battery has reached Stop SoC.

## What this means during a session

With battery priority active, a negative surplus — that is, import from the
grid — is deliberately ignored and bridged from the battery instead. The
session continues until Stop SoC is reached.

Two consequences that are easy to misread, both with Stop SoC at 60 % and
Start SoC at 95 %:

* **A cloud passes while the battery is at 90 %.** Charging continues without
  interruption. That 90 % is below the Start SoC of 95 % does not matter,
  because no pause is active.
* **Evening, battery at 90 %.** The session is not ended. The station keeps
  charging from the battery until it reaches 60 %, and pauses only then. The
  battery is not held at 90 %.

## Choosing the two values

* **Start SoC must be higher than Stop SoC.** The configuration interface does
  not enforce this. If Start SoC is set at or below Stop SoC, the resume
  condition is already satisfied the moment the station pauses, and the
  station alternates between pausing and charging.
* **A high Start SoC means a long pause.** At 95 %, charging resumes only once
  the battery is practically full. That is the intended behaviour — the
  battery has priority — but it is a longer wait than a resume threshold just
  above Stop SoC would give.