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Battery overcharging

Lead-acid batteries - Gassing

In lead-acid batteries, when we approach the end of charge conditions, we observe the apparition of a phenomenon of electrolyte dissociation, of the H2O molecules into H2 and O2, named Gassing.

This phenomenon is rarely explicitly treated in the usual models, but it is still fundamental. On the one hand it consumes a part of the charging current, and on the other hand it induces an excess voltage with respect to the linear behaviour depending on the SOC.Therefore this affects the global operating of the system, in particular the operational efficiency of the battery.

The shape of this excess voltage is a predetermined "S"-curve 1. The "gassing" current increases exponentially with the excess voltage, and substitutes itself progressively for charge current. The "Delta" coefficient of the exponential has been measured by a German team for various batteries of different ages, and is established at about 11.7 V-1 (ref2). The end of charge is another predetermined limiting curve, depending on the charging current (3). It corresponds to the situation where the whole current is used for the dissociation, and allows to fix the Iogass parameter:

\(Igassing = Io_{gass} * exp ( Delta * dVgassing)\)

In PVsyst, the Gassing voltage dVGassing is specified as a predefined profile, as a function of the SOC (correction beginning at SOC = 0.8). THe corresponding parameters are specified in the battery data, but never modified.

NB: The gassing phenomenon consumes electrolyte water (variable MGass in the simulation). When you often overcharge your battery:

  • With open batteries you have to frequently add distilled water,
  • With sealed batteries, there is a mechanism for recombining the H2 and O2, but it is not recommended to overcharge too much. The overcharging voltage threshold should be specified at a lower value.

Li-Ion batteries: exponential increase of the internal resistance

In the Li-Ion batteries, the overcharging is characterized by a significant increase of the internal resistance, leading to a voltage increase depending on the current. This correction is described in detail in the internal resistance page.

NB: In our model, this SOC voltage doesn't attain the maximum voltage limit. In the reality, if you force a high voltage when the battery is fully charged, you will have a high risk of destruction of the battery, and even of explosion.

  1. Watsun-PV - User's Manual and Program Documentation
    Watsun Simulation Laboratory, University of Waterloo, Waterloo, Ontario N2L 3G1, 1992. ↩

  2. Bopp, Gabler, Sauer, Jossen, Höhe, Mittermeier, Bächler,Sprau, Willer, Wollny.
    A Systematic Effort de Define Evaluation and Performance Parameters and Criteria for Lead-acid Batteries in PV Systems.
    13th European Photovoltaic Solar Energy Conference, Nice, Oct 1995. ↩

  3. J.B. Copetti, F. Chenlo and E. Lorenzo
    Comparison between Charge and Discharge Battery Models and Real Data for PV Applications.
    11th European Photovoltaic Solar Energy Conference, Montreux, Oct 1992, pp1131-1134. ↩