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We are designing a buck controller to charge a LiFePO4 battery (4 series 9 parallel cells, each of 3.2 V). Hence, we would like to charge the battery pack with 13.90 V but the buck output has got a ripple of 0.18 V. We were wondering if this will impact the battery life. enter image description here

I added BMS between MPPT and Battery. BMS Specs are given figureenter image description here

Battery specification: enter image description here

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    \$\begingroup\$ Where's your batter's datasheet? Why are you using constant voltage rather than constant current to charge it? \$\endgroup\$
    – Puffafish
    Commented Oct 25, 2023 at 10:43
  • \$\begingroup\$ @Puffafish We are designing a MPPT solar buck controller keeping the output voltage fixed. We added battery specs. Please find the question Datasheet for battery masterbattery.es/pdfs/32700-6000mAh.pdf \$\endgroup\$
    – Vishal Jha
    Commented Oct 25, 2023 at 10:48
  • \$\begingroup\$ Is the simulation result with the battery (model) in place or something else? I would expect the voltage ripple to be very low with a low impedance battery connected but the current ripple would be there. \$\endgroup\$
    – winny
    Commented Oct 25, 2023 at 11:15
  • \$\begingroup\$ How are you achieving balancing with the cells in series? \$\endgroup\$
    – Finbarr
    Commented Oct 25, 2023 at 12:07
  • \$\begingroup\$ An mppt controller will certainly need to change (lower) its output voltage, otherwise there's no adjusting/throttling the current pulled by the battery. You can't track the MPP if you can't control current. you can add more caps to the output to lower ripple, though it doesn't really matter since the BMS will absorb most of it and the cells don't mind. \$\endgroup\$
    – dandavis
    Commented Oct 26, 2023 at 1:02

2 Answers 2

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The datasheet for the battery tells you to use constant current in section 4.1: The "Standard Charge" means charging with constant current 0.5C. So that's what you should do. Charging batteries is dangerous, so follow the instructions for the batteries you are charging. If you are making a MPPT solar but controller that’s nice, but don’t expect to just plug it in. You need to convert from your MPPT constant voltage into a constant current you can use to charge the batteries.

As you are using constant current, the voltage ripple is largely irrelevant. However section 10.1.2 of the datasheet states that it is strictly prohibited to expose the batteries to over voltages, suggesting that you are risking explosion. So don’t expose it to over voltage. This can be done by making the maximum voltage ripple still being less than 3.65V per cell.

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Once you connect the charger to the battery, the battery sets the voltage, not the charger. Therefore, your question becomes moot.

Instead of asking "How much charging ripple voltage is allowed while charging a LiFePO4 battery?" you should ask "How much charging ripple current is allowed while charging a LiFePO4 battery?".

Answer: 100 %. The cells do not care about ripple current, as long as the peak current remains within the maximum specified charging current. E.g., if the maximum charging current is 0.5 C, and the charger produces 0.1 C, then 100 % ripple current is a peak of 0.2 C, with is still fine.

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