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I've been working with a pack of batteries from a DIY electric vehicle. The batteries in the pack are LiFePo4 chemistry and were chosen for safety (greater resistance to thermal run-away), flat discharge curve (3.2-3.3v for 70% or so of the charge) and longevity vs other lithium chemistries. The flat discharge curve makes it difficult to determine state of charge (SOC) with a voltage reading alone since a reading of 3.3 volts could mean 50% charged or even 70% charged. The voltage does climb from 3.4 to 3.6 over the final 90-100% SOC levels which should mean that if you see a reading of 3.4v you are at 90% charged.

Most chargers are CC (constant current) / CV (constant voltage) chargers set at 3.6-3.65v. When this voltage is reached the amperage of the charger declines rapidly and stays low for the final 15-30min of the charge. This final phase of the charge could be considered similar to an absorption/float stage.

My question is - if most of the literature on this chemistry shows 3.6v as 100% SOC, then why after a round of charging do my batteries fall back to 3.33v after "resting" for 24hrs (could be the chemistry equalizing). Is this what is called a static voltage/open circuit voltage? Am I supposed to let the float stage of the charge go for longer at 3.65v so that my batteries stay at 3.4v after resting? I'd like to get them up to a 90% charge (3.4v) before doing a vehicle test run.

Thank you all.

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    3.2 nom, https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery Minimum discharge voltage = 2.5 V Working voltage = 3.0 ~ 3.3 V Maximum charge voltage = 3.65 V – Tony Stewart EE75 Mar 30 '18 at 03:22

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I may have found a possible answer: https://endless-sphere.com/forums/viewtopic.php?t=51586

The charging voltage is 3.6-3.65v for LiFePo4 (4v for the chemistry with the addition of yttrium) and a voltage of 3.6v is 100% charged (safely. This chemistry used to be charged to 4.2v max but it was discovered the this decomposed/oxidized the electrolyte). Voltage does decrease normally after charging has terminated, and this decrease is determined by how long you charged at 3.65v and how low the current was allowed to fall. The voltage decreases because the chemistry is still absorbing the electricity/energy from the charge. The voltage of the terminals at the end of the charge cycle may be higher than the voltage of the interior of the cell. As this equalizes within the cell, the voltage drops to match.

A fully charged LiFePo4 battery will stay at 3.6v (or close to it) even after resting for 24hrs. Cells that have gone through many cycles may fall to 3.33-3.4v due to cell degradation. 3.37v is considered 80% charged, 3.4 is 90% and 3.6 is 100%. There is very little useable energy between 3.4v and 3.6v with the LiFePo4 chemistry and the voltage drop will reflect this when the battery is in use. This cell under load will reach 3.3v quickly and then remain between.3.2 and 3.3v for quite a while (flat discharge curve coming into play).

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