A battery stores energy so we can use it when we need it. When we use that energy, the battery starts to discharge. This is a normal part of using any battery. But sometimes, a battery is used until its charge becomes very low, and that’s deep discharging. But when using lithium batteries, deep discharge becomes more critical to understand. There is no doubt that they handle many charge and discharge cycles. But that doesn’t mean you should always use all their stored energy. So what is deep discharge, and is it harmful to lithium batteries? All these questions might confuse you. In this guide, I will cover everything about deep discharge and its link to lithium batteries. So let’s get started.
What Is Deep Discharge?
Deep discharge means using a battery until a large amount of its stored energy is gone. It means discharging the battery much more than it normally would. For example, using 80% of the battery’s usable capacity is a deeper discharge than using only 30%.
But remember there is no fixed point of deep discharge for every type of battery. That totally depends on the battery type and its recommended limits. Deep discharge is when you use stored energy continuously without recharging. When a battery is being used, its state of charge (SOC) gradually goes down. If you use the battery for a short time and recharge it while it still has plenty of energy left, the discharge is shallow.
However, if you keep using it until only a small amount of energy remains, the discharge becomes much deeper. This is what we call deep discharge. However, deep discharge does not automatically damage the battery. Batteries like lithium-ion batteries are designed to discharge and recharge many times. The main concern is repeatedly taking the battery to very low levels or going below its safe discharge limit. Doing this too often increases battery wear and reduces its useful life.
What Does Deep Discharge Mean for Lithium Batteries?
For lithium batteries, deep discharge refers to how much stored energy you use between charges. When a lithium battery starts at a high charge level and is used for a long time, its stored energy keeps going down. If you keep using it until the battery is near its lower limit, you use a large part of its available capacity. This is what makes the discharge deeper.
You may see this in solar batteries, backup power systems, electric vehicles, and other devices that use lithium batteries. As I said above, lithium batteries are made for several charging and discharging cycles. However, that doesn’t mean you can use them to the point of complete discharge. There is a safe limit for how low its charge should go. Beyond that, deep discharging begins, and frequent deep discharges can affect battery performance.
The good news is that using a lot of a lithium battery’s power doesn’t mean it will be damaged right away. Lithium batteries are made to be charged and discharged many times. But doing a deep discharge again and again will ultimately damage it. That’s why it is better to follow the battery’s recommended discharge limit.
How Deep Can a Lithium Battery Be Discharged?
There is no single discharge level for every type of lithium battery. Up to what low extent you can safely discharge a battery depends on its chemistry, design, and the limits set by the manufacturer. Many modern lithium batteries are designed to use most of their stored energy. But that doesn’t mean you should always discharge them all the way to 0%.
So, basically, a battery that can safely use 90% of its stored energy will have different limits from another battery. To understand deep discharging in lithium batteries, you first need to know the term Depth of Discharge (DoD). DoD basically tells you how much of the battery’s energy you have used. For example, if you use 30% of the battery, the DoD is 30%. If you use 80%, the DoD is 80%.
The higher the DoD, the more energy you have taken from the battery before charging it again. You probably also notice that a lithium battery doesn’t always let you use every last bit of its energy. This is normal. A Battery Management System (BMS) is behind this. It stops the battery when it reaches a safe lower limit. This protects the battery cells from being deep discharged.
What Happens During a Deep Discharge of Lithium Batteries?
When a lithium battery is deeply discharged, its charge keeps getting lower as you continue using it. However, this does not happen all at once. The battery gradually moves toward its lower operating limit. So let’s discuss what exactly happens during deep discharging.
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Voltage Drops
One of the first things that happens is a drop in battery voltage. Voltage is simply the electrical pressure that helps the battery deliver power to the connected equipment. A small drop is normal when you use the battery. However, when the battery gets very low, it’s deeply discharged, and the voltage drops close to the battery’s minimum safe level. At this point, the battery can’t provide power as well as it did when it was more charged.
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The Battery Has Less Available Energy
After the voltage has dropped, there is very little stored energy left inside the lithium battery. A fully charged battery runs a system for many hours. But the same lithium battery will provide much less operating time after losing most of its energy during deep discharge. This means the connected equipment will run for less time or even not work at all. But that does not mean the battery has suddenly failed. It means there is not enough stored energy available at that moment.
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The BMS Disconnects the Battery
When a lithium battery is deeply discharged, the risk of cell damage increases. To prevent this, the BMS comes in. It is a strong safety system that keeps an eye on the battery. It checks things such as voltage and protects the battery when conditions become unsafe. During deep discharge, the battery voltage falls too low. Now it is the BMS’s job to disconnect the battery from the equipment. This stops the battery from discharging further. As a result, the battery suddenly appears to have no power, even though it still stores some energy. This shutdown does not mean the battery is damaged. It is the BMS protecting the battery from going beyond its safe limit.
How to Prevent Deep Discharge in Lithium Batteries
Deep discharging is common in solar batteries, backup power systems, and electric vehicles. If you ignore it and it happens too often, the battery can get damaged. But you can prevent it with just a few simple settings and regular checks. How? Here are a few adjustments that will help you prevent deep discharging in lithium batteries.
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Set a Safe Low-Voltage Cutoff
The first thing you can do is set the low-voltage cutoff. It tells the battery system when to stop taking power from the battery. When the battery voltage reaches this set point, the system cuts off the load immediately and prevents further energy use.
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Use a Proper BMS
A Battery Management System (BMS) is the safety part of a lithium battery. It monitors the battery and protects its cells from problems such as very low voltage, overcharging, and excessive current. So always make sure the lithium battery you buy has a BMS.
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Monitor Battery State of Charge
State of Charge (SOC) means how much charge is left in the battery. For example, 80% SOC means the battery has about 80% of its usable charge remaining. But at 20%, it means the battery may start to discharge. So, before that happens, keep an eye on the SOC so you can recharge before it drops too low.
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Avoid Leaving the Battery Empty
Try not to leave a lithium battery at a very low charge for long. If the battery has been heavily used, recharge it according to the manufacturer’s recommendations. This becomes more important if the battery will not be used for some time. A battery that sits at a very low charge becomes harder to recover over time.
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Follow the Manufacturer’s Limits
Every lithium battery has its own operating limits. The manufacturer normally provides information about the recommended discharge level. So these instructions should be your major concern. Don’t use the lithium battery beyond the safe limits the manufacturer provides.
Frequently Asked Questions
Can a Deeply Discharged Lithium Battery Be Recharged?
Yes, a deeply discharged lithium battery can be recharged if it has not gone below its safe limit. Many lithium batteries stop working because the BMS has switched them off for protection.
But after deep discharging, use the correct charger. Also follow the battery manufacturer’s charging instructions.
Does Deep Discharge Damage Lithium Batteries?
Deep discharge does not damage a lithium battery immediately. But doing so too often can put extra stress on the battery cells. This can shorten its useful life over time, and in some cases the battery may be permanently damaged.
Can a lithium battery recover from deep discharge?
Yes, in many cases, a lithium battery can recover after a deep discharge if it is still within its safe operating range. If it has a BMS, it may simply have disconnected the battery to protect it. After charging, the battery will start working normally again. However, if a battery has been deep discharged for a long time, it may not recover safely.
Conclusion
Deep discharge is not something to fear when using a lithium battery. It just means using a large part of the battery’s stored energy before recharging it. Deep discharging does not suddenly damage the battery. However, the real problem arises when you take the battery too low too often. However, a little care can reduce the deep discharge risk. For example, keep an eye on the battery’s charge level. Use a proper BMS and a safe low-voltage cutoff. Try not to leave the battery empty for long. Most importantly, follow the limits given by the battery manufacturer.