Maintenance & Care Technical Bulletin

Depth Of Discharge

In cycling applications many battery suppliers and system installers talk about DOD and how this can influence the life of the battery bank.

Read the manufacturers specification sheet to determine the correct usage of the product in your application.

I would like to unpack this term so that all concerned can understand the importance of DOD%.

There are various batteries available in the market place that can be used in a cycling application, all at different pricing structures and life expectancy. If one looks at the data sheets provided you will see that the no of cycles that a battery can deliver in its lifetime is proportional to the %DOD.

You will notice a big effect on cycle life for a deeper discharge cycle. This is due to the extra mechanical stress on the plates. A cycling battery is rated @ an hourly rate so that the rate of discharge is taken into account when
determining the time and energy required for an application. If you look at the discharge curve of lead acid products you will notice that the battery voltage is proportional to the State of Charge (SOC) of the battery and that at 50% DOD the voltage is always in the region of 2.00volt per cell and (12volt) (24volt) (36volt) (48volt) for respective bank sizes.

Most sealed batteries are designed for a 50% DOD or less as per manufacturer’s specification and for this reason one must set your alarm and cut off at the voltage to adhere to the DOD recommendation. If this is not adhered to the cycle life of the battery will be reduced and this could be considerable in some instances.

The table below will give you an indication of the voltage settings for various %DOD.

In Conclusion, if you want to set your system up for a 50% DOD the cut off voltage on a 12v block is 12.00 volts. If you want to set up your system for an 80% DOD and reduce life cycles and void
the terms of the warranty in some cases then the cut off will be 11.40 volts on a 12 volt block. A 30 % DOD to increase battery life will be set at
12.30 volts.

DOD setup is extremely important, don’t let it cost you !