CITROËN C4 Handbook page 270

In the event of a breakdown
Some functions are not available if the
battery is not sufficiently charged.
The batteries contain harmful substances
such as sulphuric acid and lead. They
must be disposed of in accordance
with regulations and must not, in any
circumstances, be discarded with household
waste.
Take used remote control batteries and
vehicle batteries to a special collection point.
It is advisable to disconnect the battery if
the vehicle is to be left unused for more
than one month.
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Protect your eyes and face before
handling the battery.
All operations on the battery must be
carried out in a well ventilated area and
away from naked flames and sources
of sparks, so as to avoid the risk of
explosion or fire.
Do not try to charge a frozen battery;
the battery must first be thawed out to
avoid the risk of explosion. If the battery
has been frozen, before charging have
it checked by a CITROËN dealer or a
qualified workshop who will check that
the internal components have not been
damaged and the casing is not cracked,
which could cause a leak of toxic and
corrosive acid.
Do not reverse the polarity and use only
a 12 V charger.
Do not disconnect the terminals while
the engine is running.
Do not charge the batteries without
disconnecting the terminals first.
Wash your hands afterwards.
Do not push the vehicle to start the
engine if your vehicle has an automatic
gearbox.