Tracking State of Charge for a Lithium Battery

[raysun]

It had been over 9 days since my 450AH LiFePO4 battery was fully charged and the FNDC flagged Charged Parameters Met.

I took a look at the FNDC's reported state of charge and compared it to a Victron BMV700's reported state of charge and they differed by a full 10% with the FNDC reporting the higher of the two readings.

I decided to fully charge the battery to sync the two meters, and to see which was closer to the true SoC.

Using my generator to assist in charging, the FNDC reported 100% as seen in the chart above. Note the battery voltage, under charge, and with no load, is nowhere near the 56.4V charge termination target.

Charging continued, and an additional 64AH was taken from the chargers when charge was terminated. That's well more charging current than either meter indicated would be needed.

The takeaway here is more frequent full charge syncing is needed. I took a glance at the past 9 days to see if there was any anomalous meter event that might explain such a large discrepancy, and turned up a "smoking gun" the day before:

At the end of the 1400 hour, the SoC was reported at 89%. In the 1500 hour the SoC jumped to 100%, even though more current was drawn from the battery than was added.

IIRC, this is programmed behavior in the FNDC. It has something to do with "protecting the (lead acid) battery" if the monitor doesn't think 100% SoC will be reached. I never quite understood the logic of this, and its certainly vexing when trying to track SoC faithfully over a number of days.

[sbrownian]

I've seen that behavior quite a bit.

The SOC just putts along at 85 or so, then pow! 100%

[raysun]

Yes. Its mentioned in the manual, or an app note. I can't recall that I've ever seen a full explanation for the logic.

[JRHill]

At the end of the 1400 hour, the SoC was reported at 89%. In the 1500 hour the SoC jumped to 100%, even though more current was drawn from the battery than was added.

So did you hit the 'Days Since' and the FNDC reset to 100% at that point?

[raysun]

At the end of the 1400 hour, the SoC was reported at 89%. In the 1500 hour the SoC jumped to 100%, even though more current was drawn from the battery than was added.

So did you hit the 'Days Since' and the FNDC reset to 100% at that point?

No, the 100% "adjustment" point was during day 8. It was something I've seen many times, but ignored with by previous battery, since I basically charged it fully every day.

Yesterday was a "lazy" charging day with overcast all day, and the battery charge sort of ambled up into the 80 SoC range. I noticed later it hit 100% but didn't think much of it.

Today I happened to be in the equipment shed and noticed the two monitors were off by 10% in respect to each other.

Checking the FNDC stats showed 9.1 days since full. The previous checks during that past days showed the two monitors within 3% of each other so I suspected it was the FNDC "leaping" to 100% yesterday, and sure enough.