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:
- the self-discharge rate is so low no offsetting current is required
- 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:
- charge to the “boost” setpoint
- and stop charging
- 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
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.