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bons asked

Smart Shunt underestimates remaining charge

I have a Smart Shunt and a paralleled pair of 300 ah LiFePo batteries with Bluetooth BMS built in. I often run a load of about 100 amps with about 50 amps simultaneously charging from solar. After 6-8 hours of operation my Smart Shunt estimates that I have about 10% charge remaining but my Bluetooth BMS indicates that I have about 30% remaining. At the start of each day I'm certain that I have 99-100% charge and both the Smart Shunt and BMS agree. I have my Peukert factor set to 1.05 and the amp hour settings are correct @ 600 ah. I'm trying to figure out why there is such a large disagreement. A 5% error is reasonable, I guess, but 20% seems too large.

SmartShunt
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Alexandra answered ·

@BonS

Check your discharge floor setting. It will count own to that, not to empty, unless you set it.

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bons answered ·

My discharge floor is set to 10%. When I reach the floor my battery says I still have about 30% remaining. It seems to me that the Smart Shunt is in error. I haven't set the Zero current calibration yet, so I'll do that and see, but it's hard to imagine that a little offset will create such a large discreppancy at high current draws of over 1kw.

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Alexandra avatar image Alexandra ♦ commented ·

@BonS

With a disagreement of two items, it is hard to know who is correct. Does the bms have a shunt in it? How does it calculate soc?

A zero current calibration may help.

It most likely you will need to change the peukerts a bit. Not all lithiums are 1.05

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bons avatar image bons Alexandra ♦ commented ·

I presume that the each battery BMS does, but the manufacturer, LifeBlue, doesn't say how they arrive at SOC. I've always trusted the battery for their SOC as I didn't have an external shunt. I will do the zero current calibration. After the next heavy cycle, I will also measure the zero-load battery voltage to get another estimate of their SOC. So, my Fluke, will be the 3rd party arbitrator and should point the finger at the source of the error.

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