chrisski
Solar Boondocker
- Joined
- Aug 14, 2020
- Messages
- 5,189
What is your technique to finding a Float Voltage for your Lithium Iron Phosphate Cells?
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I'm finding there's a bit of art to setting Lithium voltage Bulk and float settings. The spec sheets for cells don't usually say what to set for Bulk and float. Just max and minimum voltages for cells. A pre-made battery makes it easy for you by giving you these values.
I have a 24 volt, 25 ah, 600 wh pack that can charge and discharge at 1C. These are the TopBand cells for sale on Battery plus. I want to discharge these at 200 watts, or .33C and find a float voltage for this that will keep these batteries level.
For 24 volts, you definitely don't want to set bulk to 29.2 (3.65 per cell), so I went with 27.5, and am pretty happy with that. Battery charged up, went into absorption and current tapered back to nothing nothing and then entered float.
So in float, I put a 200 watt load on it. With float set to 26.4, I got about 170 watts being pushed back into the batteries to make up for what was being lost. This is a rather small pack, 600 wh (25 ah), and after 3 hours this net 30 watt loss put the State of Charge at 83%. So more amps out than amps in.
Now I know you're not supposed constantly keep a charge on Lithium's when they're full, but you don't want to have the sun go down with the SOC being 50%, which this pack is headed to.
So, I've upped my Float to 26.7, .3 volts higher, which is now pushing amps back into the batteries to recharge. This may be my voltage I want.
I also expect this particular portable build to be worked pretty constantly when I have it hooked up at .33C. I also think that if I use this at a real reduced output, like .05 C or 1.2 amp, I would find that 26.4 is probably OK.
===================================================
I'm finding there's a bit of art to setting Lithium voltage Bulk and float settings. The spec sheets for cells don't usually say what to set for Bulk and float. Just max and minimum voltages for cells. A pre-made battery makes it easy for you by giving you these values.
I have a 24 volt, 25 ah, 600 wh pack that can charge and discharge at 1C. These are the TopBand cells for sale on Battery plus. I want to discharge these at 200 watts, or .33C and find a float voltage for this that will keep these batteries level.
For 24 volts, you definitely don't want to set bulk to 29.2 (3.65 per cell), so I went with 27.5, and am pretty happy with that. Battery charged up, went into absorption and current tapered back to nothing nothing and then entered float.
So in float, I put a 200 watt load on it. With float set to 26.4, I got about 170 watts being pushed back into the batteries to make up for what was being lost. This is a rather small pack, 600 wh (25 ah), and after 3 hours this net 30 watt loss put the State of Charge at 83%. So more amps out than amps in.
Now I know you're not supposed constantly keep a charge on Lithium's when they're full, but you don't want to have the sun go down with the SOC being 50%, which this pack is headed to.
So, I've upped my Float to 26.7, .3 volts higher, which is now pushing amps back into the batteries to recharge. This may be my voltage I want.
I also expect this particular portable build to be worked pretty constantly when I have it hooked up at .33C. I also think that if I use this at a real reduced output, like .05 C or 1.2 amp, I would find that 26.4 is probably OK.