“Short Discharge Time”: a tool to compare cells and batteries in power applications

When selecting cells for a power battery, it is useful to have the ability to quickly compare various cell technologies, and calculate the resulting pack resistance and efficiency, independently of capacity, voltage or chemistry.

Here we propose a way of doing so, using the “Short Discharge Time”, the theoretical time required to discharge a full cell (or battery) through a short circuit. This constant is a characteristic of each battery cell technology, regardless of capacity, voltage or chemistry.

A lower Short Discharge Time indicates that the cell or battery is more efficient, which is important in power applications.

Short Discharge Time is calculated as:

short_discharge_time [h] = capacity [Ah] * resistance [Ω] / voltage [V]

Where:

  • Capacity is the nominal capacity of the cell or battery
  • Resistance is the DC resistance of the cell or battery at 50 % SOC
  • Voltage is the open circuit voltage of the cell or battery at 50 % SOC

Seconds is a more practical unit of short discharge time than hours:

short_discharge_time [s] = 3600 * capacity [Ah] * resistance [Ω] / voltage [V]

Short Discharge Time ranges from about 20 s (high power pouch cells) to about 200 s (Li-sulfur).

Of course, Short Discharge Time is just one parameter to consider. Others include energy density, cost, ease of connection and maximum rated current.

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