Why is Depth of Discharge Important?
- by Bryan Veldboom - updated on 8/4/2023
"Depth of discharge" is a phrase that comes up a lot when talking about batteries, but what does it actually mean? Depth of discharge impacts all rechargeable batteries, including those found in your car, your boat, your phone and more. Today we're going to talk about depth of discharge and the impact it has on a battery's overall cycle life. We'll also be taking a look at maximum discharge rate and how this varies between different types of batteries.
What is the Difference Between Depth of Discharge and Life Cycle of a Battery?
Depth of Discharge - Refers to the percentage of the battery that has been discharged relative to its overall capacity. So, if a battery currently has a 50% depth of discharge, it means that 50% of its overall energy capacity has been used.
Cycle Life - A battery can only be charged and discharged a certain number of times. Cycle life refers to the total number of charge and discharge cycles a battery can go through before it needs to be replaced. For instance, an average lead acid battery has a cycle life of around 200-300 cycles at 100% depth of discharge.
What is the Relationship Between the Depth of Discharge and the Life Cycle of a Battery?
Depth of discharge plays a significant role in determining your battery's overall cycle life. If you've only used 50% of a battery's capacity, you might assume that you have another 50% left to use, but that's not the case.
Rechargeable batteries are not made to discharge down to nothing. Discharging a battery too low has a negative impact on the battery. If you regularly discharge a battery lower than its recommended maximum discharge, you will deplete the active material in the battery's cells and shorten the battery's overall cycle life.
What is a Safe Depth of Discharge?
As mentioned above, battery manufacturers often provide a recommended maximum depth of discharge. This maximum depth of discharge varies significantly between different types of batteries.
For instance, a lead acid SLI battery is designed to provide a short burst of power necessary to start an engine. Because of this, SLI batteries have a much lower maximum depth of discharge than a deep cycle battery, which is designed to deliver continuous power over a much longer period of time.
What is the Depth of Discharge of Different Battery Types?
Here is a very general look at the average maximum depth of discharge for a few different types of batteries:
Wet (flooded) Lead Acid SLI Batteries
Uses: Starting battery in cars, trucks, boats and RVs
Maximum DoD: Shouldn't be discharged below 50% of total capacity
AGM Lead Acid SLI Batteries
Uses: High performance starting battery in cars/Dual purpose batteries in boats & RVs
Maximum DoD: Can take discharges of 50-80% total capacity
Deep Cycle Batteries
Uses: Powers golf carts, scrubbers/House battery on boats & RVs
Maximum DoD: 50-80% total capacity
Lithium Iron Phosphate Powersport Batteries
Uses: Starting battery in motorcycles, ATVs, snowmobiles, etc.
Maximum DoD: Some lithium batteries can discharge up to 90% total capacity
Lithium Iron Phosphate Deep Cycle Batteries
Uses: House battery on boats & RVs
Maximum DoD: Some lithium batteries can discharge up to 90% total capacity
Lithium Ion Batteries
Uses: Powers cell phones, tablets and laptops
Maximum DoD: Should only be discharged to 80% total capacity
How to Maximize Your Battery's Cycle Life
Paying attention to how deeply you discharge a battery will help you maximize its cycle life, but it's not the only factor that impacts a battery's total lifespan. If you're looking to get the longest life possible from a battery, basic battery maintenance is key. Here are a few basic tips to follow:
- Never allow a lead acid battery to sit in an undercharged state for too long as this can cause sulfation.
- Charge SLI batteries by driving your vehicle long enough that the alternator can fully recharge the battery.
- For deep cycle and phone batteries, be sure to charge them regularly using a compatible battery charger.
- Never overcharge a battery as doing so will definitely affect battery health.
- If you have a flooded battery that needs to be "watered," be sure to check the electrolyte levels in the battery's cells and add distilled water when the plates become exposed.