How to Charge Lithium Iron Phosphate Batteries

In the past decade, Lithium-ion batteries have become a prominent part of our daily lives, not only in personal devices such as laptops and cell phones but also in the industries energy storage sector. These batteries continue to make an impact on the storage unit industry by providing consumers with safer and more reliable energy storage products.

Lithium-ion batteries are ideal because they are lightweight have a high energy density, and have stable electrochemical properties. This is especially when it comes to Lithium Iron Phosphate (LiFePO4) that we are going to discuss in this article. 

So how do you charge these batteries and what is the difference when it comes to charging them as compared to lead-acid batteries. 

How do Lithium-Ion Batteries Work 

Lithium-ion batteries are made up of a Lithium Anode and a Carbon Cathode. They work by transferring lithium ions between the anode and cathode. When the battery is discharged, the lithium ions move from the anode to the cathode. When the battery is recharged, the lithium ions move from the cathode to the anode. This process is repeated many times, which is why lithium-ion batteries can be recharged multiple times.

Lithium iron phosphate batteries are a type of lithium-ion battery that uses lithium iron phosphate as the cathode material to store lithium ions and graphite as the anode material.

This chemical makeup of LFP batteries is what gives them a high current rating, good thermal stability, and a long lifecycle.

LiFePo4 C Rating 

A battery’s C rating is a measure of its ability to supply current. It is expressed in terms of the battery’s capacity (in ampere-hours) divided by its current rating (in amperes). For example, a battery with a capacity of 10 ampere-hours and a current rating of 2 amperes has a C rating of 5. 

An LFP battery has a charging rating of 1C and a discharging rate of up to 25C. This means that you can use 100 amps to charge a 100 Ah battery. However, you should charge the battery according to the manufacturer’s recommendation. The manufacturer should give a percentage of the C rating to use.  Also for this rating, some conditions must be met. 

Balancing 

A lithium battery (LB) is a battery consisting of a single electrochemical cell, with nominal voltages of 3.6 or 3.7 V. A single-cell LB is unsuitable for most applications, so batteries are composed of multiple cells connected in series, parallel, or series-parallel.

Balancing a lithium battery means distributing the charge (or current) equally among all the cells in the battery. If they are not balanced during charging, the battery cells can become unbalanced and the battery will suffer from a shorter life span and reduced performance.

LFP Charging Stages 

Lead-acid batteries go through three charging cycles; Bulk, Absorption, and Float. Lithium Batteries go from bulk mode with a constant current until a target voltage is achieved of around 60%. 

The voltage in this stage continues to be constant even though the battery is still charging. Therefore a lithium battery does not necessarily need to go to absorption mode. If so only for a few minutes. 

The float charge is only necessary if the battery is being used and requires frequent topping off. The basic is to ensure the bulk voltage is set at around 14.4V, 14.6 for the absorption stage, and 13.5 or 13.6 for float.

Cannot Be Charged in Freezing Temperatures

LiFePO4 batteries can safely charge between 0°C to 45°C (32°F to 113°F). They cannot also be discharged below Freezing or 32°F. Some batteries have an in-built self-healing feature that allows them to be used in Freezing temperatures. 

You can also ensure the battery is kept in a warm enclosing. If stored above charge controllers and inverters they can provide some warm air during cold seasons. 

On the opposite side, LiFePO4 batteries perform well in high temperatures. High temperatures can even increase the capacity of the battery by up to 10%. 

Awakening a Dead Lithium Battery with a Charger 

When you store a lithium battery pack in a discharged state for a long period of time self-discharge slowly depletes any remaining charge in the battery. The protection circuit will turn off in this case making the battery unusable.

Some chargers and charge controllers have a reactivation feature to awaken this ‘sleeping’ lithium battery.  It does this by applying a small charge current to activate the protection circuit and when a correct cell voltage is reached it then starts a normal charge ‘waking up the battery’.

  1. How do I store lithium batteries?

    It is recommended to store Lithium-ion batteries at a 40% state of charge (SOC). This is the stage of charge in which most phones are stored including iPhones. However, you will need to cycle the batteries at least once every 6 months. Never store batteries that are discharged. Ensure the battery is stored indoors in an enclosed space. 

  2. Do I need a special charger for lithium-ion batteries?

    You may not need a special charger if you have a charge of any type of SLA battery; Gel or AGM.  AGM battery chargers are mostly compatible with LiFePO4 batteries but may need a few modifications. 
    If you using a charger for any other type of lead-acid battery it may not charge your battery. A charger with a higher voltage will not charge as the BMS is set to protect the battery from a high voltage. On the other hand, a charger with a low voltage may not fully charge the battery. 

  3. Can I charge a lithium battery with an alternator?

    Lithium batteries are very efficient when it comes to charging and an LPO battery bank can take up to 1C or 100% of its capacity when charging without slowing down. This is great when charging through solar or a generator as you have limited time but it may cause issues for your alternator. 
    Alternators are not made to put out full amperage constantly they may end up burning out when charging lithium batteries. It is this or charging up to 70% charge of your battery. One solution is changing the alternator to get one that can pump enough amps to your battery. 
    Another is getting a Battery Isolator or a DC to DC charger. It can be set for Lithium batteries and allows your engine’s alternator to take a break and saves it from wearing out.

  4. How Long to Charge a Lithium Iron Phosphate Battery 

    An LFP battery goes through the bulk stage where a constant charge is applied until it reaches 60% State of Charge. This takes around one hour. The second stage is the absorption stage where there is a constant voltage and this completes the remaining 40%. This should take two hours. So you can fully charge a LiFePO4 battery in 3 hours. 

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