(DE19724007-A1)      Linear electrochemical functional element has an ion or electron conductive strip which is accessible at both ends and which is preferably enclosed by a solid insulating sheath. Preferably, the strip consists of an ion exchange membrane, a hydrogen diffusion electrode or a half-cell formed of an ion exchange membrane and a hydrogen diffusion electrode. Also claimed is a process for fabricating a half-cell for a charge state indicator device with the above functional element, the process comprising: (a) deep drawing, pressure die casting or mould casting a cup of metal (especially copper or lead) which is resistant to sulphuric acid at the potential of a lead/lead sulphate electrode; (b) introducing a negative material paste of tribasic or tetrabasic lead sulphate and sulphuric acid; (c) converting the paste into lead sponge by electrochemical reduction in sulphuric acid while the cup is connected as negative pole in a conversion cell; and (d) introducing the linear functional element as a galvanic key in the cup and deforming the cup to enclose the element. Further claimed is a method of using a charge state indicator, having the above functional element, for controlling an accumulator.    USE -   Especially as a hydrogen reference electrode for measuring the pH and/or concentration of a solution, e.g. in a starter battery or fuel cell, or as an indicator for the state of charge of lead accumulator cells.    ADVANTAGE -   The functional element is shape stable, pH stable, flexible and thin so that it can be precisely fitted even in a confined cell space, e.g. in the plate assembly of a starter battery.    (DE19751126-A1)      Linear electrochemical functional element has an ion or electron conductive strip which is accessible at both ends and which is preferably enclosed by a solid insulating sheath. Preferably, the strip consists of an ion exchange membrane, a hydrogen diffusion electrode or a half-cell formed of an ion exchange membrane and a hydrogen diffusion electrode. Also claimed is a process for fabricating a half-cell for a charge state indicator device with the above functional element, the process comprising: (a) deep drawing, pressure die casting or mould casting a cup of metal (especially copper or lead) which is resistant to sulphuric acid at the potential of a lead/lead sulphate electrode; (b) introducing a negative material paste of tribasic or tetrabasic lead sulphate and sulphuric acid; (c) converting the paste into lead sponge by electrochemical reduction in sulphuric acid while the cup is connected as negative pole in a conversion cell; and (d) introducing the linear functional element as a galvanic key in the cup and deforming the cup to enclose the element. Further claimed is a method of using a charge state indicator, having the above functional element, for controlling an accumulator.    USE -   Especially as a hydrogen reference electrode for measuring the pH and/or concentration of a solution, e.g. in a starter battery or fuel cell, or as an indicator for the state of charge of lead accumulator cells.    ADVANTAGE -   The functional element is shape stable, pH stable, flexible and thin so that it can be precisely fitted even in a confined cell space, e.g. in the plate assembly of a starter battery.    (US6045938-A)      Linear electrochemical functional element has an ion or electron conductive strip which is accessible at both ends and which is preferably enclosed by a solid insulating sheath. Preferably, the strip consists of an ion exchange membrane, a hydrogen diffusion electrode or a half-cell formed of an ion exchange membrane and a hydrogen diffusion electrode. Also claimed is a process for fabricating a half-cell for a charge state indicator device with the above functional element, the process comprising: (a) deep drawing, pressure die casting or mould casting a cup of metal (especially copper or lead) which is resistant to sulphuric acid at the potential of a lead/lead sulphate electrode; (b) introducing a negative material paste of tribasic or tetrabasic lead sulphate and sulphuric acid; (c) converting the paste into lead sponge by electrochemical reduction in sulphuric acid while the cup is connected as negative pole in a conversion cell; and (d) introducing the linear functional element as a galvanic key in the cup and deforming the cup to enclose the element. Further claimed is a method of using a charge state indicator, having the above functional element, for controlling an accumulator.    USE -   Especially as a hydrogen reference electrode for measuring the pH and/or concentration of a solution, e.g. in a starter battery or fuel cell, or as an indicator for the state of charge of lead accumulator cells.    ADVANTAGE -   The functional element is shape stable, pH stable, flexible and thin so that it can be precisely fitted even in a confined cell space, e.g. in the plate assembly of a starter battery.