The thermal behaviour of VRLA/AGM cells and monoblocs

Valve-regulated Pb-acid/glass mat (VRLA/AGM) secondary cells and monoblocs were investigated concerning parameters influencing the onset of thermal runaway conditions. For this purpose, 2 V 1000 Ah and 2 V 300A h cells were specially assembled so as to include electrolyte starvation conditions similar to service life dryout. Key cell performance parameters determined experimentally indicate that also large VRLA/AGM cells show stable behaviour in increased temperature environments. When, however, dry-out or electrolyte volume level reductions of >15% are experienced, then unfavourable environmental conditions could cause a slow but steady escalation of the cell float current. The trapping of the current generated heat within the cell may yield thermal runaway conditions. The monitoring of the temperature differential "cell case-surrounding air temperature" could give a warning signal of unsafe thermal conditions to the battery plant operator.