Optimal Charge Settings for Lithium Iron Phosphates - LifePo4
Lithium batteries are fast becoming the default energy storage system for most homes and industries. The last 2 years as seen an explosion in demand for Lithium. This increased demand is complimented by a sharp drop in pricing of lithium batteries. All of this has lead to a state where lots of people are being exposed to lithium batteries for the first time... As with the case with new technology, there is a lot of ignorance on usage.
This blog post hopes to share some best practice I have adopted particularly around how to use lithium battery to ensure safety and durability.
Before I proceed, it is important to understand the voltage range of a lithium cell and how lithium cell make up a bank. If you are new to lithium find time to read the snippet on:
- Lithium Battery Voltage Curve
- How Lithium Battery cells make up a Battery Bank
TLDR
Recommended LFP Charge Voltage
- Absorption: 3.45vpc i.e 12v= 13.8, 24= 27.6, 48v = 55.2
- Absorption time: 1 hour
- Float Voltage: 3.35vpc i.e 12v= 13.4, 24v = 26.8, 48v = 53.6
- Low Voltage Disconnect: 3vpc
Recommended Charge - Discharge Current
- Max Charge current: 0.2c - 0.3c i.e Battery capacity x 0.2c e.g 280Ah x 0.3c = 84A
- Max Discharge Current: 0.3.5C
How Lithium Battery cells make up a Battery bank
When it comes to home energy storage, such as for car batteries, inverters, or solar systems, battery banks are commonly available in 12V units. This is true for most traditional batteries, including tubular batteries, which are often used for home energy storage.
These batteries are typically rated at 12V, meaning a 12V inverter system will consist of a single 12V battery or multiple 12V batteries connected in parallel. For systems requiring higher voltage, batteries can be connected in series to achieve 24V (using two 12V batteries) or 48V (using four 12V batteries).
Lithium batteries, specifically LiFePO4 cells, follow a similar principle. A single LiFePO4 cell usually has a voltage of 3.2V. To create a 12V LiFePO4 battery bank, four 3.2V cells need to be connected in series. For a 24V setup, eight 3.2V cells are required, and for a 48V system, 16 cells would be needed.
Charging Lithium Batteries
Charging of Lithium batteries involves 2 things, the charge voltage and the charge current rate. This 2 factors a very critical to the health, durability of your batteries and to the safety of your life and property.
Optimal Charge Voltage for Battery Longevity
In other to understand the optimal charge voltage for LFP batteries. It is important to understand the voltage curve of an LFP battery. Voltage curve is simply the battery voltage relative to it's capacity or state of charge.
The diagram below illustrate a typical LFP charge curve. This diagram is a snap shot of capacity test performed on some LFP batteries I got from china using the ZKE Smart Battery Tester. The voltage range of LFP is from 2.5v to 3.65v.
- 2.5v being when battery is at 0% State Of Charge (SOC) ie completely discharged (represented by "1" in screenshot)
- 3.65v being battery at 100% SOC
From the diagram 1 can see a sharp decline to 2.5v in voltage once the battery is past 3v. On the flip side there is a sharp raise in voltage between 4 - 5 when the battery is just past 3.4v.
What does this all mean?
It means the operating range of a lithium battery is between 3v and 3.4v. These voltage range represent 90% of battery capacity. 2.5v-2.9v and 3.4-3.65v represent about 10% of battery capacity.
Avoiding the danger zone.
- Don't charge your battery to 100%
LFP does like to be charged to 100%. Constantly cycling the battery to 100% full is actually dangerous for the battery and degrades its capacity rapidly. The reason for this is heat. At 100% state of charge or near the top voltage curve, ie 3.5v - 3.65v LFP generates heat and this heat impact the battery negatively causing it to degrade faster. - Don't discharge to 0%
On the flip side, Lithium batteries don't also like to be fully discharged. Fully discharging the battery can allow the cells to be unbalanced which can impact general stability of the battery bank
Sweet Charge Sport
An LFP battery should not be discharged below 3v or charged above 3.45v. This represent the sweet spot which ensures you can use 90% of your battery capacity while ensuring longevity.
On the charge side. The batteries should be charged to 3.45v. This means on your inverter / solar charger.
- Absorption Voltage: Set the Absorption / Boost / Bulk voltage to 3.45.
This Translate to 13.8v for a 12v battery system, 27.6 for a 24v battery system and 55.2 for a 48v battery system. - Absorption time : The length of time the battery spends in Absorption (or Boost / Bulk as some inverter solar charge calls it) should be set to 1 hour. This is to allow all the cells in the battery enough time to balance.
- Float Voltage: Once a battery is fully charged. The voltage should be dropped down to float state of charge. Generally float voltage should be 3.35v per cell. This translate to 13.4v for 12v battery system. 26.8 for a 24v battery system and 53.6v for a 48v battery system.
- Low Voltage Disconnect: Most inverters have LVD or Low Voltage Disconnect setting. LVD should be set to 3vpc x number of cells your battery has. ie 12v for a 12v battery system etc
Optimal Charge Current for Battery Longevity.
An average LFP battery is designed to be charged at 0.5C. This translate into half of its rated capacity in amp hours. A lot of people have been mislead by this charge rate into charging their batteries at or close to 0.5C. A closer look at the wording of an EVE LFP cell shows that 0.5C is to be done in a condition where ambient temperature of the battery. Most places in Nigeria have an average ambient of 30C or more. This means that for durability, battery charge current need to be much lower than 0.5C. I personally use 0.25C for my batteries. This translate to 70A for a 280Ah battery pack (280 x 0.25)
At a charge / discharge current which represent half of battery spec, I am cycling the battery at a much lower rate given my higher than test condition ambient temperature.