TECHNOLOGY NEWSLETTER No. 9 September 2013
THE ACID STRATIFICATION PHENOMENON A frequent cause of modern full calcium battery failure is the sulphating of the active mass as a result of acid stratification. This edition of “Newsletter� deals with this phenomenon in detail.
Recognition of acid stratification. Acid stratification can be recognised when the measured open circuit voltage fails to correspond with the measured acid density (acid density + 0.84) x 6 = open circuit voltage.
The reasons for acid stratification. Acid stratification in full calcium batteries derives mainly from cyclical loads or charging at lower voltages, especially after deep discharge (e.g. lights left switched on). As a consequence of the insufficient intermixing of the electrolyte, strata are formed with differing levels of acid density that are low on the top (e.g. 1.20kg/l) and very high at the bottom (e.g. 1.34kg/l) of the battery.
The open circuit voltage always shows the voltage in relation to the highest acid density. In the diagram below, the open circuit voltage of 13.08V is formed by the high acid density of 1.34 kg/l on the battery floor. It is therefore assumed that the battery is sufficiently charged.
A non-stratified, full charged battery has an acid density of approx. 1.285 kg/l (corresponds with 12.75V). When this is discharged, the acid density sinks and both the positive and negative mass turns to lead sulphate. During charging, the positive plate is transformed back into lead oxide and the negative plate into lead. Moreover, charging also results in highly concentrated sulphuric acid with a correspondingly high level of acid density of approximately 1.80 kg/l, which as a result of gravity sinks to the bottom of the battery, where it only partly mixes with the diluted acid in the cell (diagram – phases 1-3). From acid to mass stratification. If the high acid density on the bottom (phase 4) remains for a longer period, this leads to lasting damage of the active masses. Above all, the negative mass sulphates and as time passes coarse lead sulphate results, which causes irreversible structural losses in the negative mass. These coarse lead sulphate crystals in the lower part of the plate are difficult to dissolve and therefore charging is obstructed and only occurs in the upper, nonsulphated area of the plate. This insufficient charging means that full capacity and cold start capacity are no longer achieved.
However, if the acid density were to be measured, one would determine that the concentration in the upper area only amounts to 1.20 kg/l and that acid stratification has occurred. Rapid, electronic testers find this defective pattern difficult to recognise and show high voltage during a somewhat limited cold start. The acid density in batteries with welded lids cannot be measured, but in such cases a load test would point to limited capacity. The effects of acid stratification. Nonetheless, following a stationary period of 2-3 weeks, the electrolyte is subject to natural mixing and an acid density of, e.g. 1.25 kg/l is created, which corresponds with a voltage of 12.54V (phase 5). As a consequence, self-discharge is frequently wrongly seen as the problem. Acid stratification creates the false impression that the battery is fully charged, which can subsequently lead to vehicle starting problems.
Banner GmbH, A-4021 Linz-Austria, Postfach 777, Banner Strasse 1, Tel. +43/(0)732/38 88-0, Telefax Sales +43/(0)732/38 88-21599, e-mail: office@bannerbatterien.com Author: Thomas Langthaler, Marketing
Battery technology as a cause of acid stratification. As a result of their charging behaviour, after deep discharge full calcium batteries require a higher charging voltage. This is also needed in order to facilitate gassing owing to the higher gassing voltage. However, in the case of a standard charge balance, the normal charging voltage in the vehicle is naturally sufficient. Special charging programs offer assistance. Unless mass damage has already occurred, in the majority of cases, stratified batteries can be fully recharged. A charging voltage of 16V over a period of 24 hours is important in this regard and the charging voltage should amount to at least one-tenth of the rated capacity (e.g. 10A for a 100Ah battery). The increased charging voltage creates a minimum amount of gas and the resultant, rising gas bubbles mix the electrolyte and thus reduce the tendency to acid stratification. Recommended chargers. In its accessory programme, Banner has charging devices that can be used to recharge deep discharged full calcium batteries with acid stratification. With the Keepower XL-pro, Accucharger Prof. 35A and the Banner Profilader a choice of three chargers are available and their technical descriptions can be downloaded from the Banner website. http://www.bannerbatterien.com/banner/ produkte/batterien/zubehoer/index.php