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April 2016

Depth of Discharge vs. Cycle Life

One of the more important considerations in the maintenance and life of a battery is its depth of discharge (DOD). How a battery is discharged affects its lifetime. In more technical terms, discharging the battery by 50% will result in more life cycles than discharging it by 80%. The figure below shows the projected life cycle based on DOD’s of 100%, 50% and 30% taken from www.pveducation.org.

Fig 1. DOD vs. Cycle Life

The graph above is theoretical and you should ask your battery manufacturer for the actual DOD vs. cycle life of their battery. For Motolite, we asked their technical support engineer and this is what they gave:
30% DOD 750 cycles
50% DOD 450 cycles

Fig. 2. Voltage vs. State of Charge (SOC). Source: Homepower E-Magazine

The voltage setting to determine the appropriate DOD depends on the discharge rate. For 50% DOD, at C/10, the cut-off voltage is about 12V. But at the higher rate of C/5, the cut-off is 11.5V. So for a 100Ah battery, if the discharge rate is 20A, the cut-off is 11.5V. But for a discharge rate of 10A the cut-off is 12V.

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