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LiFePO4 bulk Re-Charge voltage?

kampto

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Mar 6, 2022
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Im switching my 13yr old system over to LiFePO4 batterys. 2 Jita 12V 300ah in series to be exact. My charge controller is a Xantrex 150/60 MPPT and it has a setting for what voltage it will restart Bulk charge. My question is what should that voltage be? This is an off grid cabin which always has some amount of power draw but not much unless im there.

And here's the other settings I've researched and thinking about, please comment as Im new to LiFePO4:
Bulk = 29.2V (2x 14.6V)
Absorb = 28.8V (2x 14.4V). Instead of 29.2V because the min absorb time is 120min. I think 60min would be best.
Absorb time = 120min. This is the minimum the Xantrex CC can take
Float = Off
Equalize = Never
Restart Bulk charge = 26.4V (2x 13.2V) ??? This reasonable ???
Temp compensation = Off

Side question about float, is Off really the best? Why wouldn't I want to use the Solar wile sun is up to keep say above 90% SOC, ~26.6V
 
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Since you have a small "parasitic" load on the batteries all the time and as you pointed out, "Why waste the sun when its available". I would set the Recharge Voltage fairly high so the charge controller is activated almost every day to keep the battery topped up. The key is to keep the charge voltages at a modest level so the cells are not getting "blasted" everyday but are being maintained without unnecessary cycling.
The following is based on individual cell voltage which is how I think about it. Just multiply these values by the appropriate number of cells, 4, 8, 16 for your situation.
After a full recharge LFP cells seem to settle in at about 3.3V when in stand-by with only a couple hundred mA being drawn. So set your "Recharge Voltage" on the charge controller menu at around this value.
Bulk Charge Voltage: 3.45 to 3.55V (Since its a 60A charger the max it can output is only 0.2C so that's good)
Absorb Charge Voltage: 3.45 to 3.55V. As the battery charges up and "pushes back" the current will naturally fall.
Float Voltage: 3.3 to 3.4V. When the sun goes down charging will stop for the night.
These are very conservative values and should not stress your battery at all while keeping it topped up and ready for you when needed.
 
How many days battery power is in place? Off is fine if you have three+ days of reserve. Off is not fine if the battery gets low as the sun is going down and the system shuts down at night.

While in use I would probably be floating 26.6 to 26.8 and rebulk about 26.2 to 26.4 volts.

Maybe turn the float off for periods of non-use.

I would lower the bulk & absorb a bit also.
 
Great advice guys! Will put back in the float and lower the rest a bit. Im thinking my days without sun capacity will be about 2 to 3 if im not there. Most info I found online was for RV's which I suppose could sit long periods with zero use.

I will start with something like this and see how the cycle go:
Battery: LiFePO4 25.6V/300ah (2 in 12.8V series)
Bulk = 28.6V (2x 14.3V)
Absorb = 28.4V (2x 14.2V).
Absorb time = 120min. This is the minimum time the Xantrex CC can take. 60min would be best or monitor current drop.
Float = 26.6V (2x 13.3V) ~90% SOC
Restart Bulk charge = 26.4V (2x 13.2V) ~70% SOC
Equalize = Never
Temp compensation = Off
Inverter Low Battery Cut Off = 24.2V ~12% SOC. Will only get here if panels covered in snow for multiple days, ~12 times a year.
Inverter Low Battery Cut Back On = 25.0V ~15% SOC. I have no choice here as this is a fixed value on my Magnum 4024, If I could raises this I would raise the Low Bat Cut voltage also so maybe LowCut = 25.0V and LowCutOn = 25.8V
 
That is too aggressive and will cause the BMS' to trip. There is NO NEED to charge above 3.500 Volts per cell.
Here is more conservative settings that will help saturate (not just surface charge) the batteries.

Battery: LiFePO4 25.6V/300ah (7680Wh or 7.6kWh)
Bulk = 27.9V (3.4875Vpc) (Vpc = Volts per cell)
Absorb = 27.9V as above
Absorb time = 120min. This is the minimum time the Xantrex CC can take. THIS IS OK BUT !!! BMS tripping MAY occur, if it does, step ABSORB Voltage down by 0.1 increments.
Float = 27.7V (3.4625Vpc) (Simplest to keep it 0.2V below Absorb and YES use FLOAT !!! regardless)
Restart Bulk charge = 25.0V
Equalize = Never
Temp compensation = Off
** Inverter Low Battery Cut Off = 24.2V ~12% SOC. Will only get here if panels covered in snow for multiple days, ~12 times a year.
** Inverter Low Battery Cut Back On = 25.0V ~15% SOC. I have no choice here as this is a fixed value on my Magnum 4024, If I could raises this I would raise the Low Bat Cut voltage also so maybe LowCut = 25.0V and LowCutOn = 25.8V

* 12V LFP can be safely cutoff at 11.0V or 3.750V per cell / 22.0V for 24V setup.
From 2.500-2.950 and from 3.500-3.650 only represents 5-7% of Gross Capacity.
Gross Capacity is the Full Allowable Voltage Range of 2.500-3.650
Working Voltage Range is from 3.000-3.400 with Nominal being 3.200 Volts per cell. THIS IS WHERE the AH Rating comes from ! That is where the Juice Lives in that very tiny 0.4 Voltage Range per cell !

BTW: 25.0V = 3.125Vpc this is not 15%.
0% of Working Range = (24.0V - 3.000Vpc). With A+ Premium Cells that can be 2.900Vpc or 23.2V
0% of Allowable Voltage Range is (20.0V - 2.500Vpc)

IMPORTANT !
Do Correct for Voltage Loss ! Check terminals at SCC and at the Battery Terminals during a Normal Charge Cycle & during a Normal Discharge Cycle (normal regular loads, not when running a temp heavy load like a Microwave). Also do this with the Inveter & Battery Terminals in the same conditions.

You will find the drop is different when charging vs discharging, so you have to correct / adjust for that. Remember that with LFP unlike Brute Force Lead, they are MIllivolt & Milliamp sensitive, "just good enough" is NOT good enough.

Hope it helps, Good Luck
 
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