Setting Float Voltage

chrisski

Photon Sorcerer
What is your technique to finding a Float Voltage for your Lithium Iron Phosphate Cells?
===================================================
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.
 

smoothJoey

Ooga Booga!
What is your technique to finding a Float Voltage for your Lithium Iron Phosphate Cells?
===================================================
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.
According to @RCinFLA float above 3.35 volts per cell is unduly stressful.
My daily cycle is 3.41 volts per cell with a tail current of 12 amps to a soft landing at 3.3 volts float.
 

Supervstech

Administrator
Staff member
Moderator
I'm trying to follow what you are doing.
If the sun goes down and your pack is at 50%, the pack is too small, or the solar is...

I thought Load should be handled by the pv during the day, with the pack sitting at float below 3.35v per cell.
The overnight load shouldn't deplete the cells below 15%soc, and the solar should fill the pack before noon or so, and loads run from solar.
 

chrisski

Photon Sorcerer
My daily cycle is 3.41 volts per cell with a tail current of 12 amps to a soft landing at 3.3 volts float.
With this 3.3 volts float, what do you see for SOCs when the sun starts to go down?

According to @RCinFLA float above 3.35 volts per cell is unduly stressful.
This 3.35 is 26.8 volts with a 24 volt, so with this, I am am below that tolerance of 26.7. What you are using is 3.3, or 26.4 volts. I'd feel more comfortable with that, but seems my batteries are slowly getting drained with this .33 C usage. Once I've done this a few times, I may opt for that and just let the battery recharge after I'm done with the load.

=========================

Basically trying to cook in a four quart crockpot and seems like this will take 6 hours on high to make the meal. The draw from the inverter is 193 watts, but seems like with inverter losses, closer to 205 watts from the batteries.

=========================

I'm finding building my own battery pack out of cells is about every bit as complicated as the 1350 watt panel build I did last year on my RV with FLA batteries. I'm glad I did not try this all last year.

I'm trying to follow what you are doing.
If the sun goes down and your pack is at 50%, the pack is too small, or the solar is...

That could be true. This is a 25 ah "Starter" battery I make my mistakes on before I put 2 much larger 280 ah in my RV. This small pack I want a constant .33C load, but the RV will have a max brief output of less than .2C.

These are the components at the heart of this build: a 25 ah battery, a 300 watt constant inverter, with a 75 volt, 15 amp charger. There's 600 watts of panels hooked up, and the pic shows 300 watts. This will be quite different from the RV build.

1631994751300.png1631994772313.png1631994794041.png1631994883825.png
 

Short_Shot

Photon Sorcerer
I thought Load should be handled by the pv during the day
Would be nice. Sometimes though we've got enough power to make it through a weekend with the solar as more of an extension than anything.

For me, a 280ah 12v pack alone will last me through about 72 hours of my typical use, while my solar will extend that to about 5 days if its really sunny, but isn't enough to really keep up indefinitely.

But that's fine because it's rare I ever take a trip much beyond 4 days.

It's a side effect of not really having a place to put said solar on my wee baby teardrop. Even my 100w folding panel is annoyingly in the way lol
 

smoothJoey

Ooga Booga!
With this 3.3 volts float, what do you see for SOCs when the sun starts to go down?
My charger is ac2dc and starts charging each night at 1:00am, it finishes just before 7:00am.
The battery is popping into the high knee and pretty much fully charged.
The battery has discharged ~100 amp hours by the time(~10:00pm) the charger takes the full load.
This leaves ~180ah for contingency.
This 3.35 is 26.8 volts with a 24 volt, so with this, I am am below that tolerance of 26.7. What you are using is 3.3, or 26.4 volts. I'd feel more comfortable with that, but seems my batteries are slowly getting drained with this .33 C usage.
The charger should eventually service the whole load.
Is your charger able to deliver the equivalent of .33c?
 

chrisski

Photon Sorcerer
Is your charger able to deliver the equivalent of .33c?
I may have said charger, and I'll edit that in the original post, but I meant the Solar Charge controller. The SCC is a Victron and can deliver 15 amps at 24 volts, which is .6C and the TopBand cells are rated for a "Standard Charge Method of 1C CC/CV"

At 26.4 (3.3 volts per cell) float and a 200 watt load, I'm getting about 170 watts of from the SCC, so about a 30 watt loss. I push the float up to 26.7 (3.34 volts per cell) and I am now able to push 280 watts, which cover the 200 watt load and starts to charge the batteries again with about 110 watts.

Foods been cooking in the crockpot for 5 hours. One more hour to go.
 

smoothJoey

Ooga Booga!
@chrisski our use cases are different.
Since you are dependent on sunshine to charge I would think you want your batteries full ASAP but you want your float higher so that you save your battery capacity for the night.
 

chrisski

Photon Sorcerer
Thanks.

Right now the clouds have rolled in and is killing my production. 600 watts of panels is only making 100 watts. One of those rare cloudy Arizona days.
 

Short_Shot

Photon Sorcerer
The draw from the inverter is 193 watts, but seems like with inverter losses, closer to 205 watts from the batteries.
Have you actually measured this?

I don't know if it'll affect your figuring but 193w draw from your inverter and 89% efficiency you're at 217w.

However the efficiency curve may not be flat and it could be a fair bit more. At 85% you're looking at ~227.

Might affect what you're counting on from your float?
 

time2roll

Photon Sorcerer
I plan to set Bulk at 14.0 and float at 13.0 volts. Once charged, my perception is to let the battery cycle (discharge) until the next morning.
 

chrisski

Photon Sorcerer
Have you actually measured this?

I don't know if it'll affect your figuring but 193w draw from your inverter and 89% efficiency you're at 217w.

However the efficiency curve may not be flat and it could be a fair bit more. At 85% you're looking at ~227.
You’re right. I have a shunt that does not provide watts, so I’m doing math in my head and basing it off what I see the SCC providing.

This inverter is 89% efficient so the load so the draw probably is closer to 217 watts
 

Short_Shot

Photon Sorcerer
You’re right. I have a shunt that does not provide watts, so I’m doing math in my head and basing it off what I see the SCC providing.

This inverter is 89% efficient so the load so the draw probably is closer to 217 watts
That's peak efficiency though. I didn't check if there's a curve for it, but I did look up that model.
 

chrisski

Photon Sorcerer
That's peak efficiency though. I didn't check if there's a curve for it, but I did look up that model.
Of course I forgot to check the actual pull from the inverter in amps and volts on the shunt before I shut it down. I'll try that tomorrow.

What I did like about that inverter is it has a .3 amp idle draw, but there's an eco mode where it goes to sleep after 50 seconds. That will save a lot of energy.

========================

I found my 15 amp SCC tripped the 15 amp Charge limit on the BMS. I unplugged the solar panels and the BMS reset itself. The BMS Shut off again when I dialed the SCC back to 14, and then to 13. I Set the SCC at 10 amps, and its charging steady there without cutting off.
 

chrisski

Photon Sorcerer
I found out that for when charging off solar power, you want to Set your float voltage at your Battery Resting Voltage. I'm sure this has been recommended somewhere before.

My Battery Resting Voltage after 24 hours fully charged when the inverter was off was at 26.7. When I set my Float to 26.7, the SCC pushed at least as many watts into the system as was being used, and charged the battery if needed. When I set my Float Voltage to 26.4, the SCC did not deliver as many watts into the system as being used.

Float ChargeWatts NeededDeliveredSurplus / Deficit% Delivered
26.7​
Volts
219.1579​
Watts
260​
Watts
40.8421​
Watts
118.64%​
Watts
26.4​
Volts
219.1579​
Watts
164​
Watts
-55.1579​
Watts
74.83%​
Watts

For the chart above, Watts needed is the Crockpot setting on high wattage, plus an idle draw of .03 amps as measured from the battery. This is a constant number. Delivered is how much wattage the Solar Charge Controller was delivering. This is really a slice in time.

For the 26.4 volt setting, before floating, the battery was charging for a few minutes, and once charged the wattage slowly dropped below the 220 watts needed as the Battery SOC dropped. After four hours, I was close to 80% SOC, and that's when I got the 164 watts with the SCC making 55 watts less than I needed.

I set the 26.7 after I'd been running the 26.4 float for a while. As time went on, the 260 watts delivered slowly dropped closer to the 219 watts needed. Before it could even out, actually sprinkled for the last hour, and the panels could only push 50 watts, so without a controlled setting, I'm left with what Mother nature sent me.

As for how efficient my inverter, taking into account an idle draw of .03 amps:
Crockpot SetingMeasured at BatteryMeasured At Inverter% Inverter Efficiency
Bat VoltAmp DrawAdj Amp Draw*PowerPower
Warm
26.5​
Volts
2.312​
Amps
2.282​
Amps
60.473​
Watts
50.8​
Watts
84.0%​
Low
26.2​
Volts
6.137​
Amps
6.107​
Amps
160.0034​
Watts
142.9​
Watts
89.3%​
High
26.3​
Volts
8.333​
Amps
8.303​
Amps
218.3689​
Watts
192.5​
Watts
88.2%​
The adjusted amp draw is the parasitic draw on my system. I think most of this .03 amps comes from the Victron 75/15 Solar Charge Controller.

Also, the different Idle Draws of the inverter:
Inverter Draws at 26.7 Volts
Mode: Off
0.03​
Amps
0.801​
Watts
Mode: On
0.32​
Amps
8.544​
Watts
Mode: Eco
0.08​
Amps
2.136​
Watts
Shunt Only Draws at 26.7 Volts
0.001​
Amps
0.0267​
Watts
Above is the idle draw of the Victron 370 VA Inverter in the three modes. With the way I have the shunt wired, I can measure a .001 amp draw when I hit the Master Power Switch. This has to be the draw from Voltage lead on the shunt. There is nothing else connected.
 

smoothJoey

Ooga Booga!
If the float voltage is lower than the battery voltage, the battery will service the load until the battery is drawn down to converge with float voltage.
 

Supervstech

Administrator
Staff member
Moderator
If the float voltage is lower than the battery voltage, the battery will service the load until the battery is drawn down to converge with float voltage.
This is a good point. So. If you use the battery infrequently, set the float around the storage value. If you use the battery daily, set at, or slightly above resting voltage.
 
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