For Constant-Voltage Trickle Charger RTCs

Charge/Discharge Cycles, Hours to Discharge, and Total Hours
  IBAT   Depth of Discharge Cell Capacity




  Hours   Cycles   Total Hours
This calculator determines time to a given depth of discharge, based upon load current and Cell capacity, and estimates the maximum number of charge/discharge cycles.

For Manganese Lithium (ML) cells, the maximum number of charge/discharge cycles (cell life) is affected by the depth of discharge (DoD). This calculator estimates ML cell lifetime based upon a given depth of discharge. The calculator uses an estimated cell load to determine approximate backup time versus discharge depth.

As a ML cell is cycled, the capacity at full charge decreases. This means that that it will take less time to reach a given depth-of-discharge as the cell is cycled. This calculator does not take into account the reduced capacity during use, but is intended to serve as a first approximation of cell life.

This calculator models the charge/dischage cycles versus dischage depth using the formula

y = (a+b*x)/(1+c*x), or cycles = (a+b*DoD)/(1+c*DoD)

where a, b, and c are constants based upon cell test data. Hours to the discharge depth are calculated using the formula

y = (a+b*(Depth of Discharge)) / (1+c*(Depth of Discharge))

Total hours are calculated by multipling hours by the number of cycles.

Where the inputs are:
  • IBAT--The current drawn by the device when powered by the battery supply input.
  • DoD--Depth of Discharge. The amount of cell discharge as a percent of total capacity.
For details on the formulas used in the calculations, refer to application note 3779.

ML Cell Charge/Discharge Cycles vs. Depth of Discharge Figure 1.