Try putting a dash (-) after/before the % in the statement. That cleans up the spaces in the jinja code. I find more consistent results with dashes:
icon_template: >
{%- if states.zwave.multisensor_chambre.attributes.battery_level|int < 20 -%}
mdi:battery-alert
{%- else -%}
mdi:battery
{%- endif -%}
You also have a quote at the endif which seems odd.