When selling lithium batteries, it's important to make sure customers know how to charge them properly. Since charging times and voltage requirements vary by battery chemistry, it is critical to use the correct settings for the battery type on each charger to avoid battery damage due to overvoltage and achieve a full charge. When charging a LiFePO4 battery, make sure you are not using a charger for other Li-Ion chemistries, which are usually set at a higher voltage than the LiFePO4 battery requires.
The ideal way to charge a LiFePO4 battery is to use a dedicated LiFePO4 battery charger, as it will set a proper voltage limit, almost all major and well-known brands of lithium iron phosphate chargers limit the charging voltage to 14.6V. This is very important for limiting the voltage supplied to the entire battery and keeping the maximum voltage of a single series battery at 3.65V which is the charging voltage limit of LiFePO4 cell.
One thing that confuses a lot of people is whether we can use a lead-acid charger to charge a LiFePO4 battery? The simple answer is: this is POSSIBLE if the voltage is set within the range of the LiFePO4 battery charging profile, but you have to be very careful because in some cases it can endanger your battery life and it will undercharge, thus you can never expect the battery to be fully charged to its rated capacity. In any case, there are many similarities between lead-acid batteries and lithium iron phosphate battery charging curves. Both of them require a Constant current/Constant voltage (CC/CV) charge type with Bulk, Absorption stages. Nevertheless, lead acid batteries will require a final stage: Float stage.
Charging profile:
LiFePO4 batteries require a simple Bulk/Absorb charging profile to be fully charged and then by a state commonly referred to as floating, but unlike the role of the lead-acid charger float stage, which maintains the voltage at 13.6V or so and keep any load drawing from the battery. Step 1 Bulk charge uses constant current (CC) to reach a state of charge (SOC) of about 95%; when the charging voltage of each battery reaches 3.65V, which is the upper limit of the effective charging voltage, then perform the second Absorption stage: also called constant voltage charging. This is what we call the CC/CV charging algorithm.
The dedicated LiFePO4 battery charger first continuously increases the voltage to charge at the highest possible amperage (constant current), then keeps the voltage constant (constant voltage) and reduces the amperage to fully charge the battery. The charger limits the amount of current to a preset level until the battery reaches a preset voltage level. As the battery is fully charged, the current decreases. The system allows fast charging without the risk of overcharging and is suitable for lithium iron phosphate batteries.
For example, in this first constant-current charging phase, a 12V (charging voltage 14.6V)) 20-amp charger will provide a maximum of 20 amperes of current, and the applied voltage will increase to the maximum voltage or "bulk voltage".
Once the bulk voltage is reached, the charger enters the second stage, which is called the "absorption" charging stage. During the process, the charger applies a constant voltage called "absorption voltage". The current will steadily drop to zero, at which point the battery is fully charged. Absorption stage is extremely short as the voltage increases to 14.6V very quickly at the end of the charge cycle.
However, a lead-acid battery will quickly lose power when the charger is disconnected. Therefore, the lead-acid battery charger does not turn off, but instead enters a third stage called the "float" stage, where the charger drops to a lower voltage and remains at that voltage. The focus at this stage is to keep the battery topped off and take into account the fact that lead-acid batteries tend to drain even when no load is connected (due to its high self-discharge rate). So a lead acid charger continues to put a small current into the battery – we call trickle charge regardless of whether there is a load or not.
A 100% fully charged 12V LiFePO4 battery will hold a voltage of about 13.3-13.4V, and its lead-acid cousin is around 12.6-12.7V. The lead-acid battery charger can partially charge the lithium battery, but only up to 60-80% of the lithium battery capacity. A lead-acid charger is considered "full" once it reaches the lower voltage associated with a full lead-acid battery. So if a lead-acid battery charger is used on a lithium battery, it can only be charged to 60-80% of the capacity of the lithium battery.
What’s more, many lead-acid chargers come with desulfurization and equalization modes, which will pulse a high voltage of 15.3-15.8V into the battery. On some chargers, the equalization mode may be automatic and cannot be turned off manually. The desulfurization and equalization modes are very important to properly charge and maintain lead-acid batteries, avoid electrolyte stratification, and ensure battery voltage equalization, but they are definitely not suitable for LiFePO4 batteries. Applying an equalizing charge of 15v+ to a lithium battery can severely reduce battery life or cause irreversible damage to the battery due to periodic overvoltage charging.
This is possible if you want to use a lead-acid charger on a lithium battery, but it should not be used if the lead-acid charger has an automatic "equalization mode" that cannot be turned off permanently. A lead-acid charger that can be set to charge no higher than 14.6v can be used for regular charging, and must be disconnected when the battery is "fully" charged.
All in all, using a LiFePO4 battery charger with a specific lithium battery charging algorithm is the best choice to achieve the maximum performance and service life of the LiFePO4 battery.