LiFePO4 float voltage experiments

I have generally been floating my 4S LiFePO4 bank at 13.4v since I got them. Ok, not really floating since Lithium doesn’t need to float:

1. the self-discharge rate is so low no offsetting current is required
2. Lithium doesn’t like being held at 100% anyhow

But using Float as a voltage floor allows us to back off from charging voltage while still letting solar shoulder daytime loads.

new factors to consider

higher temps, high SoC, and degradation

A few months ago I read some academic articles suggesting high SoC + ambients >80F can lead to degradation, so I decided to drop to 13.35v for the summer.

Renogy users in distress

I’ve been talking a bunch of Renogy users down from the ledge; they are confused about Renogy chargers’ default behavior in Li profiles:

1. charge to the “boost” setpoint
2. and stop charging
3. until Vbatt falls to teh “boost reconnect” setpoint (13.2v)

I’ve suspected this results in SoC ping-ponging between ~80% (?) and 100%, but I haven’t tested it or seen it tested. I have sunny days on tap so let’s see what happens

testing

Charge to ~100% SoC then allow the system to settle back to the quasi-float voltage

Final measurements taken when the sun had gone down enough that solar could no longer hold Vfloat.

Note: these measurements and behaviors are peculiar to my system; I expect them to be different for yours.

• at 13.4v my bank has typically rested at 98-99%
• I backed off to 13.35v for the summer, and observed about the same as above, sometimes dipping to 97%
• at 13.3v the system settled to ~90% and was still showing a -20w deficit.
• at 13.2v the system settled at 80% and -60w by sundown.

Smelling blood, I tried 13.31v. This settled at 93% and no deficit (0w in or out). It’s only two days’ data, but it seems promising. I suppose loads and solar charging will push the SoC% around a bit.

about those deficits

It’s not clear to me how the charger could hold 13.30v, the battery show 13.30v, and still be draining. Balancing between batteries? Batt trying to fall to a resting setpoint I don’t understand? Next time I get near the bank I’ll put a clamp meter on the paralleling wires to see if there’s any current on them.

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