# MAINT-11 - start or stop the maintenance cycle per battery

**Status: for review.** Tests: `test_maint11.py`. No option added.

**The fault.** At two or more batteries the page had one "start a battery cycle"
button, and `_maint_button()` chose the battery. An operator could not cycle a
chosen one (inverter B, new after storage, needed its first cycle).

## Rule

1. `maint_start` and `maint_abort` take an optional `unit`. With no unit they
   run the old code unchanged. The Batteries screen sends the unit from a
   per-battery Start cycle and Stop cycle button, at two or more batteries only.
2. **A start while any battery holds the maintenance slot is refused, not
   queued.** The message names the battery that is cycling. One slot per site
   stays the rule, and the scheduler is untouched.
3. Also refused, with a plain message: an unknown unit, a start on a battery
   that is switched off (battery modes on), and a stop on a battery that is not
   cycling. Nothing changes on a refusal.
4. A stop on the cycling battery is today's abort: the cycle ends, then every
   inverter goes to 0 W.

## Why refuse and not queue

A queued start is a cycle that begins hours later, unseen, after the operator
has moved on. A refusal is immediate and says why. The scheduler already
queues by waiting time, so nothing is lost.

## What this does not cover

- Today's bare button never refused a battery that is refusing to give power
  (SAFETY-73); its drain waits. The per-battery start does the same.
- A start on the battery that is already cycling is refused as "busy", where
  the bare button restarts its drain. The bare button is unchanged.