Cycles and Depth of Discharge

The working life of a battery is usually measured by the number of cycles that it is subjected to as it performs a function. A battery ‘cycle’ refers to a single discharge and a charge phase.

The number of cycles that determine battery life also vary by the amount of energy that is discharged during each cycle. The amount of energy discharged in a cycle is referred to as the depth of discharge (DOD). The DOD is expressed as a percentage of the total available energy that the battery is able to provide. A fully discharged battery is said to have a DOD of 100%. Likewise if a battery has had half of its energy used it is described as being discharged to 50% DOD.

Generally speaking, batteries that involve applications with high Depth of Discharges have shorter lives than those that face smaller depth of discharges.

Battery capacity loss less than 20% over 10 years - only 40km range reduction.

Therefore when referring to the life of batteries both the terms Cycle Life and Depth of Discharge are used. Therefore a battery may be quoted with a life of ‘n’ Cycles at 80% DOD, or ‘3n’ Cycles at 20% DOD.

Generally, the capacity (or ability to store energy) of a battery decreases gradually as it continues the charging and discharging cycles. Over this time the ability of the cell to provide energy decreases gradually when compared to a new battery. Rather than simply waiting until a battery becomes completely unable to accept charge or provide energy the service life of a battery may be said to be finished at a certain Relative Capacity.

When a battery life is quoted it will be in the form of:

‘4n’ Cycles at 20% DOD (80%). This would mean that the battery is still performing at 80% of its capacity when compared to a new battery, after performing ‘4n’ cycles with each cycle drawing off 20% of the battery energy.

Charge & Discharge

The rate of energy that is discharged or recharged to and/or from a battery also affects the life of a battery. The rate of discharge or recharge refers to how fast the energy contained in a battery is used or how quickly the battery is recharged again after being used. The rate of energy flow is measured in Amps. When referring to these rates of energy the term ‘C Rating’ is used.

A size and performance of a battery may be described by two variables; the voltage (Volts) and the storage capacity (Amps). For example an old large lead acid battery might be a 12 Volt, 100 Amp, which could be found in a truck are farm tractor. Discharging this battery at 100 Amps is described at a discharge at 1C. This is a very high discharge for a lead acid battery as these batteries do not perform well or last long at these discharge rate applications. Lead acid batteries operate comfortably around 0.2C or less.

The evMe contains a lithium polymer power module that has a voltage of around 400 Volts and an Amp of 100 Amps. The high quality lithium polymer batteries used in the evMe are comfortable at charge/discharge rates of around 8C or discharges of 800 Amps.

Similar batteries to the evMe are used to start the Apache (AH-64) helicopters where discharge rates of 1,200 Amps or over 30C are required. When referring to the life of batteries the higher the charge / discharge rates or C Rates the cycle life of batteries reduces if the cycles involve higher C rates.