NOVELTY - A liquid detection device comprises at least one liquid-activated hydrogel battery (110) configured to generate an electric current responsive to contact with a liquid, where the battery comprises hydrogel (115) comprising hydrophilic polymer and electrolyte (120); and anode (130) and cathode (125) in contact with the hydrogel, where hydrogel is hydrated responsive to contact with a liquid, supporting ionic communication between the anode and the cathode, where the communication generates an electric current; and circuit in operative communication with the battery.    USE - As liquid detection device for detecting water selected from ground water, industrial water effluent, drinking water, rain water, brackish water, surface water, mineral water, salt water, fresh water, distilled water, and deionized water; biological fluid selected from blood, plasma, vomit, serum, urine, sweat, and saliva; and protic solvent; in the diaper and floor covering i.e. pet training pad to detect urine voided; for detecting the liquid in the environment sensitive comprising electronic device, dry goods, a wooden structure, moisture sensitive equipment, computing device, a computer, a tablet computing device, a telephone, a cell phone, an air conditioner, and electronic laboratory equipment. The perspiration-activated material is used in fabric; bag; footwear insert comprising shoes, sneakers, skates, running shoes, wrestling shoes, cleats, bowling shoes, work boots, and hiking boots; and sports equipment selected from shoulder pad, a thigh pad, a helmet, and a glove for controlling perspiration odor and eliminating causes of the perspiration odor selected from bacteria, fungi, and viruses, and for evaporating perspiration absorbed in fabric (all claimed).    ADVANTAGE - The liquid detection device detects thawing of a liquid in a frozen state; detects a plumbing leak and roof leak; and conserves power and minimizes maintenance.    DETAILED DESCRIPTION - A liquid detection device comprises at least one liquid-activated hydrogel battery (110) configured to generate an electric current responsive to contact with a liquid, where the liquid-activated hydrogel battery comprises at least one hydrogel (115) comprising at least one hydrophilic polymer and at least one electrolyte (120); and at least one anode (130) and at least one cathode (125) in contact with the hydrogel, where the electrolyte supports ionic communication between the anode and the cathode responsive to the hydrogel being hydrated with the liquid and does not support the ionic communication responsive to the hydrogel being dehydrated by an effective absence of the liquid, where the ionic communication generates an electric current for the liquid-activated hydrogel battery; and at least one circuit in operative communication with the liquid-activated hydrogel battery and configured to be energized by the electric current, where a detection event is initiated responsive to the circuit being energized. INDEPENDENT CLAIMS are included for the following:    (1) preparing the liquid detection device, involving providing at least one liquid-activated hydrogel battery configured to generate an electric current responsive to contact with a liquid; and arranging at least one circuit in operative communication with the liquid-activated hydrogel battery, where the circuit being configured to be energized responsive to the electric current, where a detection event is initiated responsive to the circuit being energized;    (2) detecting liquid using the liquid detection device involving providing the liquid detection device; and exposing the liquid detection device to liquid such that the liquid hydrates the hydrogel of the liquid-activated hydrogel battery;    (3) a perspiration-activated material comprising a first layer comprising a conductive layer configured to receive perspiration in contact with it; and a second layer in contact with the first layer, where the second layer comprises at least one liquid-activated hydrogel battery configured to generate an electric current responsive to contact with the perspiration received from the first layer, where the electric current is transferred to the first layer and is dispersed within the conductive layer; and    (4) detecting a level of a liquid using the liquid detection devices, involving providing the liquid detection devices; arranging the liquid detection devices at height configured to indicate a level of the liquid; exposing the liquid detection devices to the liquid such that the liquid hydrates the hydrogel of the liquid-activated hydrogel battery of at least one of the liquid detection devices; and determining the level of the liquid based on a height of a highest liquid detection device associated with a detection event.    DESCRIPTION OF DRAWING(S) - The figure shows view of liquid-activated hydrogel battery comprising a hydrogel in a hydrated state.    Liquid-activated hydrogel battery (110)    Hydrogel (115)    Electrolyte (120)    Cathode (125)    Anode (130)