Deep Discharge
It is the condition in which a large part of the battery's capacity is drained. In lead-acid batteries going below 50 percent noticeably shortens the cycle life, and in very deep discharges permanent sulphation forms on the plates. It is the most frequent reason battery life is lost in motor driver applications.
The second and more immediate consequence of deep discharge is voltage collapse. As the battery empties its internal resistance increases; the high current drawn at start-up pulls the terminal voltage below the driver's undervoltage threshold. The system refuses to run or keeps shutting down at start-up.
Ways to protect against it:
- Sizing: Choose the capacity so that it uses at most half of the daily consumption.
- Cut-off threshold: The driver's undervoltage lockout is generally not designed for battery protection; in critical applications use a separate low-voltage disconnect.
- Measurement: Assess battery health not by the no-load voltage but by the voltage behaviour under a known load.
In solar-supplied field installations the risk of deep discharge increases in winter because generation drops. In this type of system, sizing the capacity for the worst season is far safer than making the selection according to values measured in summer.
The context in which this term is used: DC Motor Driver Selection Guide