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Acid Measurement

A Jumpstart for Acid Measurement in Battery Production

| Author / Editor: Kornelia Froech, Gerald Schwarz* / Dominik Stephan

The stand-alone density sensor L-Dens 3300
The stand-alone density sensor L-Dens 3300 (Source: Anton Paar)

Lead-acid battery production: Optimized formation and fast production change with process concentration measurement — A reliable inline sulfuric acid concentration measurement ensures the quality of the formation process and the final H2SO4 concentration in the battery. It also reduces the shutdown time during production change at the filling station.

The lead-acid battery is the oldest and most proven type of rechargeable battery. Because of its low price and the relatively large power-to-weight ratio it is mainly used as an automotive starting, lighting, and ignition (SLI) battery, despite its very small energy-to-weight ratio. The main components of a lead-acid battery are anodes made of lead, cathodes made of lead dioxides as well as diluted sulfuric acid (H2SO4) as the electrolyte. During the production process of a lead-acid battery sulfuric acid in different concentrations is required. The acid concentration not only depends on the production step, but also on the type and size of the battery.

The first production step in which sulfuric acid is required is the formation of the plates. During the formation, alpha and beta PbO2 are formed on the positive plates. The ratio between alpha and beta PbO2 has a direct impact on the current efficiency of the battery. The H2SO4 concentration during formation is one important parameter to achieve the correct ratio.

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After the tank formation, the batteries are assembled, filled with the correct acid concentration and charged. After the container formation, the electrolyte is replaced (two-shot process) or the acid is adjusted (one-shot process). At the end of filling and loading, the acid concentration and electrolyte level must meet the specified value.

A Matter of Concentration

In aqueous sulfuric acid solutions density measurement is perfectly suited to determine the concentration of H2SO4 up to 90 %. In the concentration range between 0 % and 55 % (1.4453 g/cm3 at 20 °C), which is important in lead battery production, density shows a steep and almost linear dependence on acid concentration. Highly concentrated sulfuric acid (98 %) is delivered to production sites mainly by trucks. On site the concentrated H2SO4 is diluted to the different required concentrations. The dilution of sulfuric acid is a strong exothermic process and requires constant cooling. Therefore, the temperature of the sulfuric acid can change rapidly during the dilution process. Anton Paar’s high-­precision process density sensor L-Dens 3300 Version GLS can easily follow these changes as the wetted parts are made of glass. All plastic-coated sensors are thermally inert and are not able to follow fast temperature changes (e.g. most conductivity sensors).

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Measurement Setup

Small to medium-sized plants store all kinds of required concentration in tanks while bigger sites can also have a two-stage dilution process. The first step is to dilute and store the delivered H2SO4 in intermediate concentrations and the second step is the final dilution at the filling station. A product change, for example when introducing a new type or size of battery, may cause a concentration change at the filling station.

Adapting the concentration of the filling tank can take up to 40 minutes if only supported by lab concentration measurements. Anton Paar’s process density sensor L-Dens 3300 enables automatic control of the concentration change which reduces the downtime to a fraction of the time needed for the manually controlled change.

During formation the concentration of the electrolyte will increase. The acid concentration measurement and adjustment are important tasks in order to achieve a constant formation with high quality. In the recirculation method the acid concentration is measured and adjusted during formation. The L-Dens 3300 GLS is a very compact process density sensor with its wetted parts made of glass. It consists of an integrated controller, a high-quality display with a user interface and capacitive keys, providing powerful measurement solutions to energy storage specialists.

* * The authors are working for Anton Paar’s Product Management and Product Competence line, respectively, Graz/Austria.

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