Backup battery float voltage.

[Maitake]

Lfp battery primarily used as backup will be float charged indefinitely at 3.370v/cell Why not have the balancer active at this voltage? The charts I can find show this as a safe and fully charged condition for LFP. If it takes days or weeks to balance and fully charge at this voltage it's no big deal.

When in regular solar service, the End of charge voltage will be 3.45-3.55/cell because the batteries would not have extended charge times at 3.37v to charge and balance properly.

Change my mind...

 

[timselectric]

You can't balance LFP cells in the flat part of the curve. Balancing works on voltage difference.
There is no voltage difference in the flat part.
One cell could be at 30% and another at 70%.

 

[Steve_S]

Floating at 3.370Vpc will NOT stress the cells as that is well within the Working Voltage Range of 3.000-3.400 Vpc (Volts per cell). LFP does not stratify nor does it have a "memory" like some chemistries.

In general, you can simply charge at 3.425Vpc till you reach EndAmps/TailCurrent and then Float at 3.400 all day long without issues.

EndAmps/TailCurrent is calculated as shown here.
AmpHour Rating of the pack X 0.05 - 280AH X 0.05 = 14A ~or~ 100AH X 0.05 = 5A
Once the battery pack drops to taking only X EndAmps the SCC should be set to flip to Float Mode. Not all SCC's have this ability and it varies by brand & model. Victron, Midnite and similar do support this.

 

[Maitake]

You can't balance LFP cells in the flat part of the curve. Balancing works on voltage difference.
There is no voltage difference in the flat part.
One cell could be at 30% and another at 70%.

I thought this too.
But everywhere I look, lfp is 100% charged at 3.37v when the current is zero. If this is true, they must be balanced. It may take days for the cutrent to drop to zero, but I don't care.
I get that this is not appropriate for a system getting cycled daily, the cells can ALSO be imbalanced at 3.37. But if held there until the current is zero they will be balanced, no?

 

[timselectric]

If this is true, they must be balanced.

They may or may not be balanced. There is no way to know at that voltage. They stop taking amps, when the cumulative voltage matches the charging voltage.
It's the total voltage. Doesn't matter how the total is tallied from each cell.

 

[Supervstech]

I wouldn't do this for monthly storage LFP.

I would set float to 3.30 v
From what I have read here, LFP experiences plating and cell degradation if held close to 100% without use.

A cell at 3.37V is basically 100% SOC.

And cell imbalances could cause damage at this low a setting.

Reducing to 3.30 keeps you out of the 100% area, safely and keeps the cells ready to reach 100% with a load on them after you bump the float up to 3.45V when you put the cells into service again.