NOVELTY - The device (D) has a photon source (SP) that is arranged for delivering of photons. A prism (PR) presenting a refraction index comprises a wall (P1) that is placed in immediate vicinity of a lower face (FIB) of a case (BB) of a battery (BA) i.e. standard lithium ion battery. The prism includes an external face (FE1) and an internal face (FI1) for reflecting the photons towards the interior of the case. A detection unit (MD) detects photons completely reflected by the internal face approximately in a pre-defined direction and delivers an output signal representative of the detected photons.    USE - Detection device for use in a housing of a battery i.e. standard lithium-ion battery, of a vehicle (claimed) to detect presence of undesired liquid e.g. condensation water, cooling liquid such as standard water-glycol, air conditioning circuit cooling liquidate and electrolyte escaped from a battery cell. Can also be used for a Ni-Mh battery, lithium-metal polymer type battery and or lead battery.    ADVANTAGE - The detection unit detects photons completely reflected by the internal face approximately in the pre-defined direction and delivers the output signal representative of the detected photons, thus allowing detection of liquid even in small quantity. The device is inexpensive and is not very cumbersome. The device provides warning in the event of need for an intervention to evacuate the detected liquidate, and provides optional warning to replace a defective element inside the case. The device minimizes risks of short-circuit, thus providing improvement in terms of reliability and safety of the batteries.    DETAILED DESCRIPTION - The battery is a refillable multicellular battery. An INDEPENDENT CLAIM is also included for a battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic cross-sectional view of a part of a case of battery in a vertical plane with a detection device in the absence of liquid.    Battery (BA)    Case (BB)    Undesired liquid detection device (D)    External face (FE1)    Internal face (FI1)    Lower face (FIB)    Detection unit (MD)    Wall (P1)    Prism (PR)    Photon source (SP)