Table of Contents
If you’re shopping for home backup or solar batteries, you may have encountered the term “depth of discharge” and wondered what it means.
When you start digging deeper into measuring a battery’s performance, it’s easy to get lost in all the technical jargon.
But, depth of discharge is an essential metric to consider if you plan to invest in a large off-grid solar solution or home backup battery.
Fortunately, it’s actually not a hard concept to grasp…
Let’s demystify the depth of discharge once and for all.
So, What Is Depth of Discharge?
You probably already know what battery storage capacity is.
Capacity refers to the total amount of electricity a battery can store when fully charged. It’s expressed as a unit of power (usually watts or amps) consumed over time (hours).
Kilowatt-hours (kWh) are what you see on your household electricity bill, and watt-hours are also commonly used to measure consumption and storage. You’ve probably seen milli-amp hours (mAh) or amp hours (aH) as a spec for smartphones or other battery-powered consumer electronic devices.
Storage capacity is the primary measure of how much electricity a battery can store when fully charged. But, the recommended depth of discharge for a battery is an essential indicator of how much of that stored electricity you can actually use.
But first things first.
Depth of discharge (DoD) measures the percentage of a battery’s total storage capacity that’s been consumed.
Let’s say you use a DELTA Pro portable power station as part of your smart home ecosystem.
Out of the box, DELTA Pro offers 3.6kWh of electricity storage. If you use 1kWh of electricity, your DELTA Pro’s depth of discharge is 27.7%.
Electricity Consumed / Total Storage Capacity *100 = Depth of Discharge Percentage
But why is DoD so essential to evaluating battery performance?
We’re getting to that…
What Does State of Charge (SoC) Mean?
You may have also heard the term state of charge (SoC). SoC measures the percentage of a battery’s storage capacity remaining at any given moment.
State of charge is essentially the flipside of the same coin as depth of discharge. Using our previous example, a DELTA Pro that has discharged 1kWh of its 3.6kWh capacity has a DoD of 27.7%. The unused storage capacity is 72.3%.
Therefore, your DELTA Pro’s current SoC is 72.3%.
Capacity Remaining / Total Storage Capacity *100 = State of Charge Percentage
How Does DoD Impact Battery Health?
Here’s where things get interesting.
Different types of batteries have different recommended depths of discharge. Recommended DoD varies based on numerous factors, but battery chemistry is the most essential.
Lead acid batteries — the oldest form of rechargeable battery technology — typically have a recommended DoD of around 50%. Continuing to consume electricity from a lead acid battery at a DoD of more than 50% will permanently damage the battery and shorten its lifespan.
Sticking with DELTA Pro as an example, a traditional lead acid battery with 3.6kWh of storage will suffer irreparable harm if you consume more than 1.8kWh of electricity at any given time.
The more electricity you use past the 50% DoD threshold — the greater the negative impact on a lead acid battery’s performance and longevity.
On the other hand, lithium-ion batteries have a recommended DoD of up to 80%. Lithium iron phosphate (LiFePO4) batteries — a newer subset of Li-ion batteries found in most EcoFlow products — can offer even greater DoD, along with higher energy density with less weight.
Electric Vehicle (EV) manufacturers are increasingly adopting LiFePO4 (LFP) batteries over traditional Li-ion for these and numerous other reasons.
Getting back to the DELTA Pro. It’s powered by a LiFePO4 battery, and the recommended DoD is 80% — or 2.88 kWh.
If DELTA Pro used a lead acid battery with a recommended DoD of 50%, its usable storage capacity (without causing permanent damage) would be 1.8kWh. The actual DELTA Pro has recommended DoD of 80% — meaning your functional storage is 2.9kWh.
Technically, the lead acid and LiFePO4 batteries have the same storage capacity. But unless you don’t mind irreparably harming your system, the LiFePO4 battery has a usable storage capacity that’s greater by 60%.
Recommended DoDs don’t just vary based on battery chemistry. Every reputable manufacturer will supply a DoD recommendation based on stringent laboratory testing. But as a rule of thumb, Li-ion and LiFePO4 batteries have a vastly superior depth of discharge capacity — allowing for deep cycles — to lead acid.
Newer sealed lead acid (SLA) battery types, like AGM, may offer deeper cycles than wet-cell lead acid batteries. But when put head to head with Li-ion and LFP on depth of discharge, even newer lead acid technologies can’t compete.
How To Calculate Depth of Discharge?
We covered how to calculate depth of discharge in watt-hours above. Here’s the formula for calculating DoD in amps:
Discharge current (in amps) x length of time discharged (in minutes) / 60 minutes / nominal capacity (in amp-hours) = depth of discharge
Here’s an example:
Say your battery has a nominal capacity of 500 amp-hours.
If you discharge a load of 250 amps for 20 minutes, the calculation is:
250 amps x 20 minutes = 5,000
5,000 / 60 minutes = 83.33 (this gives you the state of charge)
83.333 / 500 amp-hours = 0.16667
That gives you a 16.667% depth of discharge.
What Is a Battery’s Cyclic Life?
Depth of discharge is also closely related to a battery’s cyclic life or cycle life. Cycle life is the most accurate way to measure a battery’s longevity. It refers to how many charge/discharge cycles a battery is expected to deliver without any noticeable decrease in performance.
For example, DELTA Pro’s estimated cycle life is 3,500 charges/discharges (10 years of regular daily use) before decreasing to 80% of storage capacity and 6,500 cycles before reaching 50% capacity.
Diminished storage capacity doesn’t mean the battery won’t continue to operate… But it does mean you’ll have to charge it more frequently.
Partially due to their sensitivity to DoD and overcharging, traditional lead acid batteries often have a cycle life of only 300-500. SLA batteries can last significantly longer.
What is the Impact of Temperature on Batteries?
Extreme temperatures (hot and cold) have a significant on battery performance. In one of the few instances where lead acid batteries may outperform Li-ion and LiFePO4, they can be charged at lower temperatures. However, all batteries operate best at room temperature — between 68°F – 86°F (20 – 30).
|Battery Type||Maximum Operating Temperature||Maximum Charging Temperature||Charging Advisory|
|Lead Acid||–20°C to 50°C||–20°C to 50°C||Charging takes longer at temperatures below freezing and above 30°C|
|Lithium-ion||–20°C to 60°C||0°C to 45°C||Do not charge below freezing|
|LiFePO4/LFP||–20°C to 60°C||0°C to 45°C||Do not charge below freezing|
Frequently Asked Questions
The recommended depth of discharge for a 12V battery depends on the battery chemistry and the manufacturer’s instructions. As a general rule of thumb, lead-acid batteries typically have a DoD of around 50%, while lithium-ion and LiFePO4 batteries can have a depth of discharge ranging from 70%-90%.
80% depth of discharge means that 80% of the battery’s total electricity storage capacity has been consumed, leaving 20% remaining. If a battery has a recommended depth of discharge of 80%, continuing to use the remaining 20% could irreparably harm its future performance and longevity. Typically, Li-ion and LFP batteries have greater DoD and are less likely to be damaged by exceeding recommended levels than lead acid.
Every currently available battery chemistry degrades with use. Li-ion and LiFePO4 batteries have a significantly longer cycle life than traditional lead acid and nickel-cadmium batteries. But their performance will also diminish after 5-10 years of regular use. Exceeding the manufacturer’s recommended DoD can significantly shorten a battery’s longevity and negatively impact functionality. Exceeding recommended DoD has a greater negative impact on lead acid batteries than Li-ion and LiFePO4.
Depth of discharge (DoD) measures how much of a battery’s total electricity storage capacity has been consumed. Depending on battery chemistry, DoD can vary widely — from 50% (lead acid) to 80% (Li-ion/LiFePO4). DoD significantly impacts how much electricity you can use without permanently damaging a battery. Along with storage capacity, it’s a critical indicator of battery performance.
Far too many consumers make the mistake of thinking that storage capacity is the sole indicator of how much electricity a battery can provide before it “dies.”
As outlined above, up to 50% of a lead acid battery’s storage can be effectively unusable without permanently damaging the device.
Considering that lead acid batteries tend to be significantly cheaper than deeper cycle chemistries like Li-ion and LiFePO4, failing to understand how recommended DoD can limit available electricity can be highly misleading.
Understanding depth of discharge is essential to making an informed purchase of home backup and whole home generator solutions.
When your family’s electricity supply relies on battery storage, a 60% increase in usable storage capacity can be like the difference between night and day.