LiFePO4 Battery Depth of Discharge

LiFePO4 batteries are often compared with conventional lithium-ion batteries and lead-acid batteries. LiFePO4 brings a lot to the table, such as a longer lifespan and higher safety.

One of the major alluring factors pushing users towards LiFePO4 is the Depth of Discharge. Depth of Discharge is vital when evaluating any battery. This article will discuss the Depth of Discharge of LFP batteries and all the important information you can learn from this value.

What is a LiFePO4 Battery?

LiFePO4 stands for Lithium Iron Phosphate battery. A LiFePO4 battery has LiFePO4 as the cathode material and a graphite anode. LiFePO4 battery is the best among all rechargeable batteries, with one of the longest lifespans. It has applications in many areas, such as solar panel, marine, leisure vehicles, UPS, electric vehicles, etc.

What is Depth of Discharge?

Depth of Discharge (DoD) is the degree to which you can discharge a battery. It is expressed in percentage (%). Discharging the battery beyond its depth of discharge is possible. However, it is detrimental to the battery.

Take the case of a battery that comes with an 80% Depth of Discharge. You can discharge 80% of the battery capacity. The battery should have a minimum  State of Charge (SoC) of 20%. After this, it should be connected to a charger.

If the battery is used below a 20% charge level, the health and lifespan of the battery will be negatively affected.

Importance of Depth of Discharge

Depth of Discharge is a great value to prolong the battery’s lifespan and keep it in best shape. Using batteries below the depth of discharge pushes them into a state of over-discharge. Over-discharge creates irreversible battery reactions in the battery. It leads to irreparable damage to the battery.

Batteries generally come with a Depth of Discharge chart to show how long the battery will last at a particular DoD. Charging batteries at a lower DoD prolongs the lifespan. A typical DoD charge for an LFP battery can look something like this:

DoD	Cycle Life
80%	3000 Cycles
70%	4000 Cycles
50%	5000 Cycles

Depth of Discharge also tells you how much of the battery capacity you can use without charging it. For instance, consider a lithium-ion battery with 100Ah capacity and 80% DoD. You can only discharge the battery to 80% of its capacity. Therefore, the usable capacity is 80 Ah (80% of 100Ah).

Discharge Depths for Different Types of Batteries

Different types of batteries come with different Depth of Discharge Ratings. This rating is the recommended maximum value to which the battery can be discharged. DoD of different batteries does not correlate to their cycle life. If LFP batteries last for 5000 cycles at 80% DoD, it doesn’t mean lead acid batteries will fare the same way.

Let us go through the discharge depths of different batteries one by one:

LiFePO4 Batteries

LiFePO4 battery cells have a maximum discharge depth of 98% to 100%. This is longer than any other battery technology currently in the market. This means that you can safely discharge these batteries to their full capacity. However, most manufacturers recommend still using a 80% DoD for these batteries to prolong their lifespan. Even if you occasionally use 100% of the battery capacity, the battery will not get harmed.

Lithium-ion Batteries

Li-ion batteries have a maximum discharge depth of 80%. Discharging beyond that will damage the Li-ion battery. It is a good idea to recharge these batteries once they reach an SoC of 30% (DoD of 70%).

Lead Acid Batteries

Lead acid batteries have the worst DoD among any batteries. They have a maximum DoD of 50%. This means you will have to recharge the batteries once their charge level drops to 50%. This is why the capacity of a lead acid battery is never fully utilized. If a lead acid battery comes with a 100 Ah rating, you will only use 50Ah continuously without charging.

Discharge Rate of LiFePO4 Battery

The discharge rate of a battery is very different from its depth of discharge. While the DoD represents the maximum capacity of battery you can use, the discharge rate represents the rate at which you can use the battery power.

What is the Maximum Discharge Rate of a LiFePO4 battery?

You can safely discharge a LiFePO4 battery to 100% of its capacity without any damage to the battery. This means a maximum DoD of 100%. The maximum discharge rate on these batteries is commonly listed as 1C.

Can you over-discharge a LiFePO4 battery?

Yes, it is possible to over-discharge a LiFePO4 battery. Over-discharging occurs when the battery power is consumed even after the battery is fully discharged. Therefore, any use of a LiFePO4 battery after 0% charge level will cause it to over-discharge.

How to Calculate the Depth of Discharge?

Calculating the Depth of Discharge and State of Charge of a battery is very easy. The first thing to understand is that the DoD and SoC are complimentary to each other.

DoD = 1 – SoC

Therefore, for a battery which is at a 60% charge level, the DoD will be:

DoD = 100 – 60 = 40%

You can also use the discharge current to find the battery’s DoD. Suppose you have a battery with a 100 Ah capacity. Now you connect this battery to a supply for 30 minutes, discharging it at 50 A. The discharged capacity is:

Discharged Capacity = 50 A x 30/60 h = 25 Ah

The Depth of discharge of the battery will be: 25/100 *100 = 25%

The State of Charge of this battery will be: 100% – 25% = 75%

What Causes LiFePO4 Battery Discharge?

Some users often question why their LiFePO4 battery gets discharged earlier or faster than normal. There are many reasons that can cause unaccounted LiFePO4 battery discharge. These are:

Connected Devices

It sometimes happens that an electrical appliance connected to the battery is left on by mistake. This can cause battery to lose charge slowly. If the appliance is of a higher power draw (like an oven), a faster discharge can occur.

High Discharge Rate

Every battery has a maximum discharge rate listed on the battery. Discharging the battery at a higher rate than this will cause an increased discharge. This can cause the battery to run out of power sooner than expected. The high discharge rate is generally because of too many devices connected to the battery.

Faulty Chargers

Faulty chargers can cause the battery to not reach its full capacity. This leads to the battery lasting lesser than it should. An important point is that this issue can arise even if the chargers are perfectly fine but the charging circuit is damaged. Another possible cause can be in the loose terminal connections.

Weather

The weather can greatly affect the performance of the battery. High temperatures can cause faster chemical reactions in the battery. Hot weather can speed up the chemical reactions in the battery and cause a faster discharge. Cold weather can provide a longer discharge time, but it is also detrimenatl to battery health. The battery should be kept in the manfuacturer recommended temperature range for best results.

Alternator

LiFePO4 batteries in vehicles are connected to the engine alternator. The alternator is responsible for battery charging. Any fault with the alternator or the circuit can cause battery not getting charged to full capacity. It will lead to a faster discharge than expected.

How Do You Prevent LIFePO4 Battery From Unwanted Discharge?

You can easily avoid LiFePO4 battery discharge by following the simple tips below:

• Use a Battery Management System (BMS) with the LiFePO4 battery. High-grade batteries such as those provided by Eco Tree Lithium have this feature inbuilt. A BMS disconnects the battery discharge whenever it reaches an unsafe zone.
• Advanced chargers are an optimal way of charging battery. Optimally charged batteries have a more efficient discharge rate.
• Whenever the battery operation is complete, ensure that you have disconnected all the appliances from the battery. Connected appliances continue to draw some power even when turned off.
• Ensure that the connections to the battery terminals are tight.
• Check the maximum discharge rate of the battery. Connect the appliance load accordingly.

Is Cycle Life Same as the Depth of Discharge?

No, cycle life is not the same as the Depth of Discharge. Cycle life pertains to how many charge and discharge cycles the lithium battery can last. Depth of discharge refers to the maxmum percentage of the lithium battery capacity you can use for the discharge cycles.

Endnotes

You can make a LiFePO4 battery last twice as long using the Depth of Discharge to your advanctage. At the same time, Depth of Discharge is the most disadvantageous factor with options like lead acid batteries. Always go with battery banks that have a higher DoD as they provide you with a higher usable capacity range and a longer battery life.