The absorbance of the samples is measured via a multi-wavelength optical sensor (415-910 nm) (AS7341 ams-OSRAM, Austria) mounted on a breakout board (Pimoroni, UK). Further details of the optical sensor, including an optical channel summary with channel numbers, center wavelengths, and full width at half maximum, can be found in the product datasheet . The light source is a white LED Backlight module (12 mm × 40 mm, Adafruit, USA) with a diffuser. A microfluidic chip is used as the flow cuvette (Fluidic 620, Microfluidic Chipshop, Germany). The chip has 16 separate channels of varying depth (length/width/depth, 18/2/0.8-0.1 mm). For absorbance measurements, the depth of the chosen microfluidic channel is the path length of the flow cuvette. In this work, we only used the 0.15 mm deep channel to account for the high absorbance of the posolyte in the 40-50% SoC range. The microfluidic chip is connected via Mini Luer rubber fittings with standard PTFE microfluidic tubes (OD/ID, 1.6/0.8 mm).