Deep discharge protection for battery packs with cell groups

A battery pack, comprising two cell groups (2a, 2b) which are connected in series, each group having an identical quantity of cells (3);

• and a monitoring circuit (5) arranged in its entirety inside the battery pack for preventing deep discharge of the battery pack (1) with series-connected cell groups (2a, 2b) by polarity reversal of an individual weakest cell (3) by carrying out a difference measurement step (8) which is repeated over time for detecting an event (Θ) indicative of a probable reversal of polarity and in which a change over time of a 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 (SD) that reliably distinguishes a fast polarity reversal of an individual weakest cell (3) in exactly one cell group (2a, 2b) from a slow fluctuation of a voltage difference between two cell groups (2a, 2b) during an ordinary discharge characteristic curve without polarity reversal of an individual weakest cell (3).

A battery-powered hand-held power tool, comprising a battery pack (1′) with an average voltage contact (19) which is arranged electrically between two cell groups (2a, 2b) which are connected in series and each have the same number of cells (3);

• an electric driving motor (20);
• and a monitoring circuit (5′) for preventing deep discharge of the battery pack (1) with the series-connected cell groups (2a, 2b) by polarity reversal of an individual weakest cell (3), by carrying out a difference measurement step (8) which is repeated over time for detecting an event (Θ) indicative of a probable reversal of polarity and in which a change over time of a 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 (SD) that reliably distinguishes a fast polarity reversal of an individual weakest cell (3) in exactly one cell group (2a, 2b) from a slow fluctuation of a voltage difference between two cell groups (2a, 2b) during an ordinary discharge characteristic curve without polarity reversal of an individual weakest cell (3), the monitoring circuit (5′) being integrated in its entirety within a motor control of the electric motor (20).