This article describes a multi-point optical fiber-based sensor for the measurement of electrolyte density in lead-acid batteries. It is known that the battery charging process creates stratification, due to the different densities of sulphuric acid and water. In order to study this process, density measurements should be obtained at different depths. The sensor we describe in this paper, unlike traditional sensors, consists of several measurement points, allowing density measurements at different depths inside the battery. The obtained set of measurements helps in determining the charge (SoC) and state of health (SoH) of the battery.

In battery applications, particularly in automobiles, submarines and remote communications, the state of charge (SoC) is needed in order to manage batteries efficiently. The most widely used physical parameter for this is electrolyte density. However, there is greater dependency between electrolyte viscosity and SoC than that seen for density and SoC. This paper presents a Quartz Crystal Microbalance (QCM) sensor for electrolyte density-viscosity product measurements in lead acid batteries. The sensor is calibrated in H₂SO₄ solutions in the battery electrolyte range to obtain sensitivity, noise and resolution. Also, real-time tests of charge and discharge are conducted placing the quartz crystal inside the battery. At the same time, the present theoretical “resolution limit” to measure the square root of the density-viscosity product ( ) of a liquid medium or best resolution achievable with a QCM oscillator is determined. Findings show that the resolution limit only depends on the characteristics of the liquid to be studied and not on frequency. The QCM resolution limit for measurements worsens when the density-viscosity product of the liquid is increased, but it cannot be improved by elevating the work frequency.

In order to manage batteries efficiently, it is necessary to know its state of charge. One of the physical parameters with information about the state of charge is the electrolyte density. This paper presents a plastic optic fibre sensor developed for measuring in real time the electrolyte density in lead-acid batteries. The multisensor consists of several optic fibre placed inside the battery at different positions in order to make measures at different heights. An electronic circuit was designed for controlling the light emitters and detectors in order to minimize the effects of the ambient light and temperature

In this work a QCM sensor system based on the phase shift detection method is implemented. The system is characterized for liquid measurement applications. The experimental calibration of the sensor system is performed with solutions of increasing viscosities, obtaining the equation of the sensitivity of the output voltage to the changes in the square root of the density-viscosity product of the liquid.

In measurement systems, it is essential to have a procedure to obtain not only the calibration curve but also its associated uncertainty for the entire operating range. To achieve it, different alternatives should be taken into account according to the characteristics of the measurement, the way they are taken, the repeatability and reproducibility of the experiments or the nature of the measurement itself. This work presents a step-by-step procedure, to obtain simultaneously the calibration and uncertainty curves of a measurement system, which takes advantage of the maximum likelihood estimation (MLE), the weighted least squares (WLS) method, and the ISO-28037 recommendations. As an application, a U-Bent plastic optical fiber (POF) sensor measuring the sucrose solution density related to the refractive index (RI) is implemented and calibrated according to the proposed procedure.