When batteries age they sluff off positive plate material. If this debris settles on the plates it can short out. This bridging will result in the cell voltage spiking when you charge the battery. The spike in voltage tells the charger that the battery is full charged and the charger will shut off before the battery receives any charge. That is why it is important to watch the voltage with the charger plugged in. If the voltage climbs very fast the cells or cell is shorted.
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