Battery Discharge and its relation to the application

Discharging batteries is a function of your application. Below is a list of helpful items:

  • Shallow Depth of Discharges (DOD) will result in longer battery life. <30% DOD is recommended for general-purpose deep-cycle batteries
  • Lead acid Batteries in solar or renewable energy applications should be sized for no more than 50% DOD. 30% DOD sizing is preferable
  • 80% DOD is the maximum safe discharge for industrial semi-traction type deep-cycle flooded, AGM and GEL batteries
  • Do not continually discharge any lead-acid battery >80%. This will damage (or kill) the battery
  • Recommended maximum DOD for flooded batteries is 50% of capacity
  • A periodic equalization charge for flooded batteries is a must
  • Do not leave batteries deeply discharged for any length of time
  • Lead-acid batteries do not develop a memory and need not be fully discharged before recharging
  • Batteries should be charged after each period of use
  • Batteries that charge up but cannot support a load have most likely reached the end of their useful life.

How long can I discharge my Discover battery?

How long your Discover battery can be discharged depends upon its capacity and the amount of power consumed by the equipment connected to it. Generally, the faster you discharge the battery, the less power it will deliver due to the Peukert Effect. Conversely, the slower you discharge it, the more power it will deliver. A 100-amp hour battery supplies a current of 5 amps for 20 hours, during which time the battery's voltage remains above 1.75 volts per cell (10.5 volts for a 12-volt battery). If the same battery is discharged at 100 amps, the battery will only run for approximately 45 minutes before the voltage drops to 1.75 volts per cell, delivering only 75-amp hours of total power.

Connecting a load of only 1.2 amps to the same battery until it reaches a cell voltage of 1.75 will result in the battery delivering 120 amps of usable power. This is all the result of the Peukert effect. Discover Industrial Semi-traction deep-cycle Dry Cell AGM and GELL batteries are electrochemically designed to deliver 80% of published AH capacity above 1.9 volts per cell, thereby ensuring that a 100 amp hour Discover Semi-traction Dry Cell AGM or GELL battery will deliver a higher % of the 100AH available than general purpose deep-cycle batteries when discharged at higher rates resulting in an improved Peukert coefficient.

What is the Peukert Effect?

The Peukert effect describes how a battery's capacity is directly affected by the speed at which it is discharged or, in other words, the effect that different discharge rates will have on the available capacity of the battery. The Peukert value or exponent is directly related to the battery's internal resistance. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is discharged, its available ampere-hour capacity will be lower. Conversely, if it is drained slower, the AH capacity is higher. For instance, a 100 AH battery (rated at the 20-hour rate) will deliver a steady 5 amp draw for 20 hours before the battery voltage drops to 1.75 volts per cell. However, if discharged at a current of 100 amp, this same 100AH battery will only deliver approximately 47 total amp hours.

The Peukert formula for a battery's capacity at a given discharge current is: Cp = I n t, where Cp is the capacity available with any given discharge current; I = the discharge current; n = the Peukert exponent, which is a result of Time (T2 minus T1) divided by Current (I1 minus I2), which can be determined by carrying out two discharge tests and measuring the time to 1.75vpc with each being at a different discharge current (I). Each test should be dramatically different regarding the discharge current. One high and one low is best.

Does the depth of discharge affect cycle life?

Yes. Work it harder and longer, and it will fail sooner. But remember, if you do not work it at all or work it too often, the life cycle will also be reduced. Here are some published cycle life data available from a few manufacturers of quality products in the market today