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.