# How to definite the total life cycle of battery under a depth-of-discharge(DoD)?

As we know that battery discharged with different DoD have different life cycle, which is usually described as lower DoD with more life cycle. And I found that there is a euqation to estimate the life cycle of battery shown as\

total life cycle = a*DoD^b

where a and b are special coefficient for different types of batteries.

I think the life cycle here means the charging/discharing times (not the times of fully-charge/discharge). Please just regard the life cycle as charge/discharge time here.

But the problem is, how do we definite the life cycle. To which condition we can think the battery is retired.

I mean that, if we say a battery can cycle 2000 times with a 50% DoD, does it means that the state-of-health (SoH) of this battery will decrease under 50% after 2000 cycle times? or it means that the SoH decrease to zero? or a special value setting as the 'retired level' by the manufactor?

• There is usually a defined end point in terms of reduced capacity - often 80% of original capacity. This point has nothing to do with the DoD figure. The battery will still work(with reduced capacity) and there is some talk of recycling old batteries from electric vehicles by using them as grid storage, where weight per unit of capacity doesn't matter.
– user16324
Jun 19, 2020 at 12:16
• @BrianDrummond If setting 80% as retired level, does it means that the DoD here is definied as "discharge amount / usable capacity"? Cause I usually define the DoD as "discharge amount / nominal capacity". I cannot understand how can a battery discharge with 100% DoD under a fewer SoH (85% as an example). Jun 19, 2020 at 12:32

The 'Life Time' of a battery is how you choose to define it. Often the manufacturer will have their own definition, and most folks go along with that, especially if the manufacturer is rating the things that are particularly important to their application.

There are several different aspects to the performance of a rechargable battery, all of which degrade as the battery is cycled.

• How much charge can be stored, the battery capacity
• How long it holds the charge, the retention time
• Its efficiency from charge in to charge out (important for PowerWall)
• Its internal resistance
• Its power output (important for EVs)

The most common one used to define lifetime is the capacity. End of life might be defined as 30% of original, or 50% of original. That might be the most important thing to you, or maybe charge retention is, or internal resistance. If you're a big buyer or a government or military, then you would be able to ask the manufacturer to specify and test the battery to your definition. If not, you'll have to do your own tests.

When you go to buy a battery, read the datasheet carefully. Reputable manufacturers will state their definition for the 'end of life' of the battery.

There is an SAE standard for electric vehicle battery lifetime testing, but I've not found a non-paywalled version.