1）Batteries are external short-circuited or overcharged, reverse charged (forced over-discharge)
2）Continually being overcharged at high rate current will cause electrode pole expand and positive poles touch directly.
3）Internal or slight short circuit, e.g. burrs in positive and negative plates touch together if penetrated through separator, or positive and negative plates are wrongly placed causing the two plates to contact together, or the positive plate gets in touch with the outside steel case, or negative material fall into separator, or separator is defective, or positive current collector contacts with negative plate.