The depth of discharge indicates the percentage of the battery that can be discharged in relation to the total battery capacity.
RV batteries are becoming a very common complement in self-consumption installations. Something that should not be surprising given the additional benefits they offer, such as: backup power when the network fails; increase your energy autonomy; contribute to increased savings thanks to intelligent energy management, among other benefits.
With an increasing number of companies offering storage options, you may be wondering how to choose a 12v lithium battery. And one of the important factors to consider when buying a backup battery for your home or business is depth of discharge (DoD).
Most battery chemistries (including lead-acid and lithium-ion) degrade as they are charged and discharged; this gradually reduces its ability to store energy until you can no longer store it; issue that affects the duration of the operating life of the battery.
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In this article we explain what depth of discharge or DoD is and why it is important for your lithium battery.
What is the capacity of a battery?
Before talking about the depth of discharge, it is necessary to clarify what the useful capacity of a lithium battery is. This indicates the amount of energy that you can store in your battery to then power the loads of your home or company.
It is measured in kilowatt-hours (kWh) and you can find lithium battery for solar panels with a capacity of between 2.4 and 15.4 kWh per module. Furthermore, most lithium solar batteries are scalable; that is to say, that you will be able to expand your storage capacity as your energy needs increase.
To give you an idea, here is the useful capacity of the main brands on the market in high-voltage lithium batteries.
However, in the real world beyond the datasheets, fully discharging a battery could result in irreversible loss of battery life and capacity. So how much can I discharge my battery? This is indicated by the depth of discharge.
What does the depth of discharge of a battery represent?
The depth of discharge (DoD) of a battery can be defined in two ways:
On the one hand, it refers to the percentage of energy that can be discharged in relation to the total capacity of the battery; In other words, it shows us to what extent the battery can be discharged. For example, if the manufacturer of a 10kWh battery recommends a maximum DoD of 80%, you should not use more than 8kWh of the battery without recharging. In this sense, a higher DoD means that you can use more energy stored in your battery.
On the other hand, it is identified with the percentage of current charge or fraction of the capacity extracted from the battery. It can also be measured in Ampere-hours (Ah); for example, for a 50Ah battery, 0Ah is full and 50Ah is empty. This second meaning is inversely related to the state of charge (SoC); i.e. if a battery is 70% charged (SoC), the battery’s DoD at that time would be 30%
However, in this article we will focus on the first meaning, which indicates up to what percentage we can discharge our lithium battery.
Currently, most high-voltage lithium batteries have a DoD of 100%; that is, they would be able to take advantage of all the energy stored in the battery.
What is battery life and how is it measured?
The lifespan of your battery indicates the number of charge/discharge cycles it can perform. It depends on the battery capacity you normally use; if you regularly discharge the batteries to a lower percentage, you will have more useful cycles than if you frequently discharge the battery to its maximum DoD.
The life of a battery is usually specified by two numbers:
The years that the battery can work in a floating state (ie fully charged)
The number of charges/discharges (cycles) it can perform while still delivering its rated performance.
The problem with this specification is that the energy capacity of the battery depends on the discharge current (in amps, A); and that the number of cycles in the useful life depends on the depth of discharge.
Therefore, it is important to read the specifications of a battery system very carefully, in order to properly understand what the manufacturers’ specifications really mean; This will make it easier to compare them with other manufacturers or technologies.
Why is battery temperature important?
Most solar batteries these days are guaranteed to last between 5 and 15 years. This depends, in part, on how often the battery is discharged and how many cycles it has performed. 12 volt deep cycle agm battery is widely used in our life. But there are other things that can have a big impact on battery life, such as the temperature at which it is kept.
Batteries in a hot atmosphere (over 30ºC) may overheat, shortening their lifespan. Very cold temperatures also have a negative impact on the battery as it has to work harder and at a higher voltage to charge. To maximize battery life, it is advisable to keep the battery in a relatively warm environment, not too hot and not too cold.
Why is depth of discharge important for a battery?
As we have seen, the depth of discharge of a battery tells us how much energy we have stored that we can dispose of. Having a greater depth of discharge allows for longer cycles, using more of your battery power.
However, having longer charge/recharge cycles is not always indicative of a longer lifespan. At 10% DoD, the life of a lithium battery can be up to 14,000 cycles; while at 100% DoD, the battery life cycle can be 4,000 to 6,000 cycles. Taking this into account, the data of the technical specifications of the manufacturers are usually taken in relation to a deep discharge of 100%.
Which lithium battery is the right choice for you?
As we have seen, depth of discharge is important when comparing battery options, but it is not the only factor to evaluate. You need to comprehensively consider what kind of battery you should buy according to your own needs and usage scenarios.