When you receive your new LiFePO4 (Lithium iron phosphate) battery, you might wonder about the battery’s voltage and state of charge (SOC). This article will show you the LiFePO4 voltage and SOC chart.
Let’s get started!
Manufacturers are required to ship the batteries at 30% state of charge. This is to limit the stored energy during transportation. It is also a good state of charge for the battery to sit at. This is because a lithium battery has a low self-discharge rate (less than 3% per month).
So when you receive a battery, it will be around 13 volts.
Now, you need the charge and discharge information of the battery. That’s why you are here. Your charger or charge controller requires more information to charge and discharge the battery properly.
Before giving you a voltage chart, you need to know that the discharge rate affects the voltage. If we discharge a battery at 1C, the voltage will be lower than at 0.2C. That’s why you should measure the voltage in an open circuit or with no loads attached.
This is the complete voltage chart for LiFePO4 batteries, from the individual cell to 12V, 24V, and 48V.
Download the LiFePO4 voltage chart here (right-click -> save image as).
Where to measure the SOC of the battery?
Always measure the state of charge at the battery terminals. This is because the voltage drop to other components like a charge controller or inverter can be high with such minimal fluctuations in voltage.
If the terminals of the inverter measure 13V but your battery terminals measure 13.2V, that’s a difference of 60% capacity!
What are the bulk, float, and equalize voltages of LiFePO4?
Some charge controllers do not have a dedicated Lithium setting. Therefore you can adjust the lead-acid parameters to match the lithium characteristics. It’s important to know that lithium only has bulk charging. It charges as much as possible, and when the battery is full, it stops.
The Bulk charge will be set at 100% SOC. The battery will be charged until this voltage is reached.
The float function for lead-acid batteries is there to keep the batteries topped up at a certain voltage because lithium has a very low self-discharge rate we can set it at 50% capacity. Because that’s the point where lithium is happy. You can also set it at 80-90% SOC. If you can disable it, do that.
The equalize function is also something for lead acid. If you can disable it do that, but if you cannot then use the recommended voltages in the table below.
Increase the lifespan of your LifePO4 battery
Lithium batteries are known for their long cycle life of up to 6,000 cycles. After those 6,000 cycles, there will still be 80% of the capacity left. To put that into perspective, that’s 16 years of daily cycling of the battery.
Now, those 6,000 cycles are reached when you don’t discharge below 10% and charge over 90%. That means a 100Ah battery has a usable capacity of 80Ah to reach 6,000 cycles. If you use the whole capacity of the battery, then you can get 4,000 cycles, which still is a lot.
You need to keep the temperature of the battery cool too. Going over 40°C or 104°F will make your battery age faster, thus decreasing its cycle life.
The same goes for the charging and discharging rate of the battery. As you might already know, the C-rate is used for a battery’s discharge and charging speed. A 100Ah at 1C discharge is getting a load of 100Amps. The battery can handle this, but it’s not ideal.
Try to have a charging and discharging rate of 0.2C for your battery. This is a 20amp charge or discharge current for a 100Ah battery. One way to increase your charging and discharging rate is to increase your battery’s Ah (capacity). Buy a 280Ah cell, and you can charge and discharge at 56Amps for maximum cycle life.
You might have heard about compressing the cells for increased lifespan if you build your batteries yourself. This is done at 300kgf (kilogram-force) or 3000 newtons. This doesn’t necessarily mean that you need to compress them. Using nylon tape around the cells is a great option. The idea here is to make it almost impossible for the cells to expand to limit gas formation.