Battery overcharging

Lead-acid batteries - Gassing

In lead-acid batteries, as end-of-charge conditions are approached, electrolyte dissociation occurs, where H₂O molecules split into H₂ and O₂, a phenomenon called gassing.

This phenomenon is rarely explicitly treated in conventional models but remains fundamental. It consumes part of the charging current and induces voltage excess beyond the linear SOC-dependent behavior. Therefore, it affects overall system operation, particularly battery efficiency.

This excess voltage follows a predetermined "S"-curve ¹. Gassing current increases exponentially with excess voltage and progressively replaces charging current. The exponential "delta" coefficient was measured by a German team for various battery ages and is approximately 11.7 V^-1 (ref²). End-of-charge is a predetermined limit curve dependent on charging current (³), where all current is used for dissociation, allowing determination of the Io_gass parameter:

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

In PVsyst, gassing voltage dVGassing is specified as a predefined profile as a function of SOC (correction begins at SOC = 0.8). These parameters are specified in battery data but are not modified.

NB: Gassing consumes electrolyte water (variable MGass in the simulation). When batteries are frequently overcharged:

• With open batteries, distilled water must be regularly added,
• With sealed batteries, a mechanism recombines H₂ and O₂, but overcharging should be avoided. The overcharging voltage threshold should be set lower.

Li-Ion batteries: exponential increase of the internal resistance

In li-ion batteries, overcharging is characterized by significant internal resistance increase, leading to current-dependent voltage rise. This is described in detail on the internal resistance page.

NB: In our model, this SOC voltage does not reach the maximum voltage limit. In practice, forcing high voltage on a fully charged battery presents a high risk of battery destruction and explosion.