GaN chips for monitoring density and temperature of lead-acid batteries

Lead-acid batteries play a key role in energy storage systems due to their high energy density, low cost, and excellent reliability. However, overcharging and over-discharging can significantly shorten their lifespan and pose safety hazards. In recent years, methods for estimating the battery state of charge (SOC) via electrolyte density have attracted attention because of their higher accuracy compared to voltage or internal resistance methods. Nevertheless, traditional handheld densimeters rely on visual interpretation, making real-time and accurate monitoring challenging. Additionally, excessively high charging voltage or current can easily lead to issues such as battery overheating and thermal runaway. Therefore, developing a dual-functional sensor capable of simultaneously monitoring electrolyte density and temperature is crucial for ensuring battery safety.

This paper presents a GaN-based dual-parameter sensing device that enables simultaneous detection of electrolyte density and temperature through monolithic integration of light-emitting diodes (LEDs) and photodetectors (PDs). The device is encapsulated with silicon rubber 704, offering electrolyte corrosion resistance. Density detection relies on the total internal reflection effect at the sapphire/electrolyte interface, converting the photocurrent response of PD into a density signal. Temperature sensing uses the linear relationship between the LED’s forward voltage and temperature. The sensor is vertically mounted 1 cm above the electrode plate and fully immersed in the electrolyte. Experimental results demonstrate its excellent measurement accuracy and reliability.

This study systematically investigates the effects of temperature and electrolyte density on the PD photocurrent and LED forward voltage. The LED forward voltage initially decreases due to self-heating effects and then stabilizes, while the PD photocurrent increases monotonically with rising temperature. This behavior primarily results from the combined effects of improved responsivity of the PD, spectral broadening of the LED emission, and reduced electrolyte density. Quantitatively reveals a temperature sensitivity of the photocurrent, STP, of 0.515 μA/°C, and a temperature sensitivity of the forward voltage, STV, of -1.07 mV/°C, providing key parameters for subsequent decoupling analysis.

Density response tests were conducted using electrolytes with densities ranging from 1.09 to 1.29 g/cm³ prepared at 25°C. The PD photocurrent decreases with increasing density, while the LED forward voltage remains largely unchanged. Fig. 2d further measures a density sensitivity of the photocurrent, SDP, of -29.1 μA/(g/cm³), with the voltage density sensitivity, SDV, being nearly zero.

Both temperature and density variations collectively influence the optoelectronic signals. Based on these findings, a dual-parameter decoupling method was established. After substituting the sensitivity parameters and measured values into the decoupling equations, the relative errors between the derived density and temperature values and their reference values were both below 0.8%, validating the high-precision measurement capability of this approach.