BU-409b: Charging Lithium Iron Phosphate

Lithium Iron Phosphate (LFP) has identical charge characteristics to Lithium-ion but with lower terminal voltages. In many ways, LFP also resembles lead acid which enables some compatibility with 6V and 12V packs but with different cell counts. While lead acid offers low-cost with reliable and safe power, LFP provides a higher cycle count and delivers more than twice the capacity of lead acid. These advantages with reduced size and weight compensate for the higher purchase price of the LFP pack. (See also BU-808: How to Prolong Lithium-based batteries.)

Both lead-acid and lithium-based batteries use voltage limit charge; BU-403 describes charge requirements for lead acid while BU-409 outlines charging for lithium-based batteries.

Compatibility of a 12V pack between LFP and lead acid is made possible by replacing the six 2V lead acid cells with four 3.2V LFP cells. While the voltage total is similar, the lead acid charger applies a float charge when the battery is fully charged to compensate for self-discharge and parasitic loads, a feature that lithium chemistry cannot tolerate.

Optimal stress with lithium batteries occurs at high voltage as the battery reaches full charge. The high-voltage stage during charge should be kept short and the charge currents must be completely turned off when the battery is fully charged. Maintaining lithium-based batteries with a float charge would shorten the life span and even compromise safety on some lithium battery systems. A Battery Management System (BMS) for LFP packs may include built-in provisions to protect the battery when serviced with a lead acid charger.

To compensate for parasitic loads and self-discharge, some Li-ion chargers apply a recharge when the terminal voltage decreases to a set level. Table 1 illustrates recommended voltage levels for lead acid and lithium-based cells. Table 2 transfers these voltages into a 12 volt pack. The values are nominal. Higher or lower values may be chosen to improve performance or prolong battery life.

1 2.30V to 2.45V; See BU-403: Changing Lead Acid
2 Commonly used for UPS; float charge voltages for automotive may be higher
3 Prolonged float charge on LFP batteries will shorten battery life
4 Voltage may vary according to architecture
5 LFP will not be fully charged but has more capacity than lead acid
6 Device may terminate end of discharge at higher voltage levels to assure power