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LTC4020 CC/CV charge cycle restart with LiFePO4 battery

I'm using an LTC4020 to charge an 8-cell lithium iron phosphate (LiFePO4) battery pack. I have a DC2134A demo board that I modified as follows:

  • Swapped out the resistive dividers on the VFB and VFBMAX pins to set the float voltage to 8 * 3.6V = 28.8V (R10 = R11 = 21.5kOhm, R8 = R9 = 226kOhm.)
  • Connected the TIMER pin to ground to use C/10 charging mode.

This works fine for the first charge period: it gets up to the 28.8V float voltage in CC mode, briefly switches to CV until reaching C/10, then finishes the charge cycle. The problem is that as soon as the charging stops, the battery voltage drops to about 27.2V and the charger automatically starts another cycle, voltage immediately goes back up to 28.8V, the cycle stops, voltage goes back down to 27.2V, charging starts again, and it oscillates like this forever.

According to page 19 of the datasheet, the voltage on VFB at which charging restarts is fixed at 97.5% of the float voltage, or 28.08V. I assume this is appropriate for LiPo batteries, but it is not for LiFePO4 because the voltage spikes up so sharply at the end of the charging cycle, causing the resting/zero-current voltage of the battery at 100% state of charge (27.2V) to be significantly lower than the float voltage it's charged at (28.8V.) As soon as the battery reaches a resting state the LTC4020 restarts the charging cycle.

Do you have any suggestions for how to prevent this from happening? Ideally I would be able to adjust the restart voltage independently of the float voltage so I can pick a state of charge where the charging restarts. I can add a latch circuit to manually pull the RNG/SS or NTC pin to ground and disable charging once the first cycle is complete, then restart it at will, but it seems like there must be a better solution to automatically restart when the battery is low. I know I'm not the first person to use this IC with LiFePO4...

  • Hi ed-me,

    Apologies for late reply!

    IR drops can affect the sensing of your real BAT voltage. Internal resistances multiplied by the charging current will give you an offset that is not visible to the Vfb. Another source of IR drop would be the cable resistance of your battery.

    That is the reason why the DC-DC converter prematurely stop its CV mode. Account for your IR drops (cell internal resistance, cable resistance) and recompute and see if the problem persists.

    Thanks!

    Regards,

    Audi

  • Should have closed this. In case anyone else is having this problem, I fixed it by adding a resistor between VFB and STAT1. STAT1 is pulled low when charging (in CC/CV mode) and high impedance otherwise, so this lets me adjust the resistive divider ratio to add more hysteresis between starting and stopping the cycle.