alferz avatar image
alferz asked

Multiplus-II State of Charge (SOC) Calculation (Undercharging)

I like to keep my LiFePO4 batteries in a comfortable range of 60%-90% state of charge and the SOC calculation combined with AC disconnect settings are a great way to maintain this with the MP-II (24v/3000/2x120). For my batteries the optimum charge settings to achieve 90% are 27.40v absorption voltage, 1h absorption time and a 26.70v float voltage. Right when the bulk charge hits 27.40v and enters absorption phase, the pack is about 73% charged, so I set "State of charge when bulk finished" to 73.0% in General settings, and it works as expected. The absorption charge tapers off the current a bit for 1 hour, and at the end of the hour the battery is exactly at 90% according to my BMS and verified with a final capacity check using a full charge.

The problem is the SOC calculated by the MP-II jumps from 90% as it should be, straight to 100% right at the end of that absorption phase as it switches over to the float cycle, even though at that lower float voltage of 26.7v, the current immediately drops to 0a. It seems the SOC calculation wants to hit 100% when entering float despite the "State of charge when bulk finished" setting.

Just wondering if there are any workarounds.

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Alexandra avatar image Alexandra ♦ commented ·

The VE Bus SOC wants to synchronize to 100%. It is not shunt based so relies on calculations to work out SOC, needs a good start point.

You will need a BMV if you are trying to hover your SOC. Or cap charged voltage in DVCC. i have not actually tried but use the BMV to send a charge do not charge signal with the bms assistant. (Half baked idea there, like i said have not tried it.)

PS if you have can managed batteries then it will be ignored and the battery will do what it wants to anyway.

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nickdb avatar image
nickdb answered ·

At 60-90% you're never going to allow the cells to properly balance and will be doing more harm than good.

If this is a commercial (supported) battery, it will likely void your warranty also. If it is DIY, well then it is your business what you do to them.

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