Every day we get questions about charging LiFePO4 batteries and lithium battery chargers. We’ve reviewed products that work well with our Battle Born LiFePO4 batteries. To begin the series we will discuss the basics of charging batteries, and the products will be covered in other segments.
Charging a LiFePO4 Battery
Charging a LiFePO4 battery basically means applying an external voltage to drive current from the anode to the cathode of the battery. The lithium battery charger acts as a pump, pumping current upstream, opposite the normal direction of current flow when the battery discharges.
When the charger’s applied voltage is higher than the open-circuit battery voltage, then the charging current flows. During this process, the battery’s open-circuit voltage increases, approaching the applied voltage of the charger.
Bulk Vs. Float Charge
The lithium battery charger can behave in several different ways during the charging process. First, the charger can steadily increase its voltage in order to keep the current flow constant. This is the first stage of the charging process – typically called the “bulk” charging stage. During this stage, the charger adjusts its applied voltage to deliver the maximum current to the battery.
For example, a 10 amp charger will deliver its maximum of 10 amps during this bulk charging stage, and the applied voltage will increase up to a maximum voltage, or “bulk voltage.”
Once the bulk voltage is reached, the charger enters a second stage, called the “absorption” charging stage. During absorption, the charger applies a constant voltage, called the “absorption voltage.” As the battery’s open-circuit voltage approaches the absorption voltage, the current flow steadily decreases down to zero.
At this point, the battery is fully charged. However, a lead-acid battery will rapidly lose charge when the charger is disconnected. So, instead of turning off, the battery charger enters a third stage called the “float” stage, in which the charger drops to a lower voltage and holds at that voltage. The point of this stage is to keep the battery topped off, and account for the fact that lead-acid batteries tend to drain, even when there is no load connected.
The Charging Algorithm
These stages combined sequentially form what is commonly called the “charging algorithm.” A battery charger may generally be classified by a charging current (i.e. the max charging current) and a target battery voltage (12 V, 24 V, 36 V, 48 V, etc.). But battery chargers may also include multiple charging algorithms (typically classified as “AGM,” “SLA,” “Gel,” “Wet,” etc.). A closer look reveals that each algorithm has its own unique parameters, including:
• Bulk voltage
• Absorption voltage
• Absorption time
• Float Voltage
There is a wide variation among values for charging algorithms for lead-acid batteries. The bulk and absorption voltages typically vary between 14.0 and 14.8 V, and the float can vary between 13.2 and 13.8 V.
The 12V Battle Born Batteries sit comfortably right in the middle of these ranges. We recommend a bulk and absorption voltage of 14.4 V. A float is unnecessary, since li-ion batteries do not leak charge, but a floating voltage under 13.6 V is fine.
Here are a few FAQ videos that talk about charging LiFePO4 batteries.
In this blog series, we will post the results of our tests for a variety of LiFePO4 chargers – including converters, inverter chargers, and solar charge controllers. In each case, we will report on the uniqueness of the charging algorithms for each brand, explore the efficacy of using the factory default settings for charging Battle Born Batteries, and determine what can be done to achieve the optimal settings.
If you have any questions or concerns about charging LiFePO4 batteries, please contact us at any time at (855) 292-2831 or email us at [email protected].
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