A method of preventing deep discharge of the battery pack (1′) with series-connected cell groups (2a, 2b) by polarity reversal of an individual weakest cell (3), includes a difference measurement step which is repeated over time for detecting an event indicative of a probable reversal of polarity in which the change over time of the voltage difference between two cell groups (2a, 2b) having identical numbers of cells is checked to determine if the change in the voltage difference has exceeded an amount corresponding to a difference threshold value which reliably distinguishes a fast polarity reversal of an individual weakest cell (3) in exactly one cell group (2a, 2b) from the slow fluctuation of the voltage difference between two cell groups (2a, 2b) during an ordinary discharge characteristic curve without polarity reversal of an individual weakest cell (3); and a battery pack (1′) and a battery-powered hand-held power tool (7) with a monitoring circuit (5′) for implementing the method.