Could one completely charge a LiFeP04 from float mode?

[-robw-]

I asked that of my CC manufacturer, and the rep said no.

But here I sit trying to get my CC config right, observing 20 amps being supplied in float mode (13.8v). I got here by setting absorb to 14.4v, and absorb time to 3 seconds (ending amps = 0). Absorb voltage was hit at 85% SOC, so now the remaining 15% can come while in float mode?

Inverter is disconnected.

Should add: 2x 24v, 200ah strings

[Ampster]

You have described Float mode as a Constant Voltage mode of 13.8 volts (3.45 volts per cell) not to exceed 20 Amps. Your batteries will accept that voltage until the Amperage begins to taper. presumably to zero or when the sun goes down. That voltage is somewhere in the range of 90-95% full and is the same voltage I use for my 42kWh LFP pack. The battery cannot distinguish between Absorb or Float since they are both Constant Voltage modes. You could accomplish the same result by making Absorb 13.8 and lengthening the time. Most Lithium users do not use a Float setting because it is not good to leave Lithium batteries at a high voltage for a long time. In your case since any charge would terminate when the sun goes down is is not as big an issue. After the charging stops the cells will settle at their resting voltage of about 3.32 - 3.35 volts per cell or about 13.3 volts for the battery.
I am assuming a 12 volt battery since you used typical 12 volt specs but you did mention two 24 volt strings. Can you clarify what those strings are?

[-robw-]

Yes, 4 12v, 200ah batteries. What do you mean by "not to exceed 20 amps"? Does my Midnite Classic 150 set that, because you're right in that is what I observed: 20 ~ amps for about 3 hours, then it dropped to < 1 amp.

[-robw-]

So I beat dusk by about 4 hours, soon to increase to 6+ hours.

[Ampster]

Yes, 4 12v, 200ah batteries. What do you mean by "not to exceed 20 amps"? Does my Midnite Classic 150 set that, because you're right in that is what I observed: 20 ~ amps for about 3 hours, then it dropped to &lt; 1 amp.

I did not mean anything. I was merely making an assumption that you set that on the Midnite Classic or that was the maximum or default out of the Midnite Classic. Charge controllers do not usually set an output level unless it is a default. If you indeed have four 12 volt batteries totaling 800 Ahrs you could charge at a higher current. Do you know what the current is during the Constant Current (Bulk) stage? The answer to your basic question about completely charging an LFP battery really depends how how much you have discharged the battery overnight. To state the obvious answer, the more you take out the more you will have to put in.

[-robw-]

One can set the max output amps lower on a Classic but doing so would remove benefits of absorb mode (Classic doesn't have a bulk voltage setting, it's absorb). I have mine set at 60 amps, with my panels able to hit 60-70.

So, I guess my battery bank could only handle that 20 amps after reaching absorb voltage. That makes sense. And since 20 amps was reliably supplied for 3 hours, that means 60/400 = bulk/absorb voltage of 28.8 gets me 85% charged. Logically, if I increase absorb time to 1 hour (from 3 seconds) I'll get to 90-95% charged and then, as many, including you, suggest I should kill float mode. But if I do that I would need to use re-bulk mode? Is there a suggestion for what voltage I re-bulk at, given something must displace float in case I have a heavy use day?

I have 2 strings of 24v, 200ah batteries. 4 batteries, 24v total, 400 ah total.

[Ampster]

One can set the max output amps lower on a Classic but doing so would remove benefits of absorb mode (Classic doesn't have a bulk voltage setting, it's absorb). I have mine set at 60 amps, with my panels able to hit 60-70.

Bulk is commonly referred to as Constant Current and in the case of a charge controller the constant current setting is often the amount of current the charge controller can send based on the output of the panels. That setting should not exceed 200 Amps which is .5C based on that capacity of your pack. The constant voltage (Absorb) setting is the voltage at which Constant Current (Bulk) stage ends and the battery transitions into Constant Voltage stage. You have 400 Amphours of batteries and the recommended maximum charging current for many Lithium chemistries is .5C which is 200 Amps. I doubt that your charge controller can put out that kind of Amperage so you just want to get as many Amps up to that limit that your panels can produce while they are in that stage of charging.

So, I guess my battery bank could only handle that 20 amps after reaching absorb voltage. That makes sense.

No it doesn not make sense. It is likely a default or a setting. If your battery can handle 200 Amps based on its capacity, It could easily start the constant voltage (Absorb) stage at 200 Amps and taper down as voltage stays the same during that stage.

And since 20 amps was reliably supplied for 3 hours, that means 60/400 = bulk/absorb voltage of 28.8 gets me 85% charged.

28.8 volts is 3.6 volts per cell which is more likely 99 percent of SOC. That is an ending voltage that most likely will shorten the number of cycles you get out of your pack.

Logically, if I increase absorb time to 1 hour (from 3 seconds) I'll get to 90-95% charged and then, as many, including you, suggest I should kill float mode. But if I do that I would need to use re-bulk mode? Is there a suggestion for what voltage I re-bulk at, given something must displace float in case I have a heavy use day?

I have 2 strings of 24v, 200ah batteries. 4 batteries, 24v total, 400 ah total.

You can get to 95 percent by stopping charging at 3.45 per cell with no absorb. The real value of constant voltage (absorb) is that it gives the BMS some time to balance the cells if any is needed. Most balancing does not start until 3.4 volts per cell. Do you have any data to know how well balanced your pack is when it is at the top?

[-robw-]

I charged then trickle charged each battery for 2 days when received. that's the best I could do for the top. "28.8 volts is 3.6 volts per cell". Sure, at rest, but not while being charged. Once 28.8 is hit with my setup it appears it's closer to 3.3v/cell. Question for myself is how to handle the 15% DOD remaining after 28.8 is hit. On the one hand, I've read that topping batteries each day decreases cycles. On the other, I need 300 ah out of my 400 ah bank each night.

[Ampster]

....... Question for myself is how to handle the 15% DOD remaining after 28.8 is hit. On the one hand, I've read that topping batteries each day decreases cycles. On the other, I need 300 ah out of my 400 ah bank each night.

Fundementally; whether you charge to 3.45 or 3.65 volt per cell, they are going to come to rest at 3.3 volts per cell. That does not mean there is any loss of capacity. Do a dicharge test to prove that to youself. Or measure the Amphours or Watthours that it takes to go from 3.45 to 3.65 volts per cell to see this for yourself.
There is no 15% DOD after you charge to 3.6 per cell. I think you would be advised to familiarize youself with the basics of Lithium batteries. You are making assumptions that are not consistent with how these batteries work. It is not difficult to use 75% of capacity of Lithium batteries. I do that often with my EVs. Topping batteries daily is not the issue with shortening their life. It is leaving them there by trickle charging them at high float voltages for long periods of time.

[-robw-]

Battery manufacturer specs indicate 13.8v for float. I wouldn't put it past them (China and all), but you're saying they're wrong? And if so, again, if sitting batteries in float for half the day is wrong how do I address daytime usage? Set float for, say, 13.3v and re-bulk (reinitiating bulk/absorb mode) at 13.2v? How does one handle that? I really need to sit at around 90% SOC by dusk.

[Ampster]

Battery manufacturer specs indicate 13.8v for float. I wouldn't put it past them (China and all), but you're saying they're wrong?

Earlier I said the Float and Absorb are Constant Voltage modes. If you think in terms of that it does not matter. Float is often used as a trickle charge mode your obversation support that it goes into a one Amp trickle charge. Float in that context can shorten the life of Lithium batteries. I just want to make sure other readers do not interpret Float as being good for Lithium batteries. I Using another mode of Constant Voltage stage such as the one identified as Absorb is probably better. The reasons are explained below.

And if so, again, if sitting batteries in float for half the day is wrong how do I address daytime usage? Set float for, say, 13.3v and re-bulk (reinitiating bulk/absorb mode) at 13.2v? How does one handle that? I really need to sit at around 90% SOC by dusk.

Earlier you used a voltage setting of 28.8 which translates to 3.6 volts per cell and that is a voltage that will shorten the life of your batteries if constantly maintained or kept at that voltage. How much is your daytime usage in Amps or Watts? That can have an important result on whether your batteries will stay at 95% State of Charge or will lose some capacity. The key component is what kind of current your controller can put out. You have not identified that and it would better inform this conversation.

3.45 volts is less of an issue and the capacity difference between 3.45 and 3.6 volts is negligible. Have you measured the Amphours or Watthours it takes your battery to go from 3.45 to 3.6 volts per cell?
You can use a rebulk or Float at 3.3 volts (13.2) as a fallback and it really depends on what happens with the mode described as Absorb. What is the maximum current you see during a sunny day when in bulk mode? That is likely the current that the Constant Voltage (Absorb) stage will start at when the charge controller goes through the transition. To accomplish your goal it is better to see if you can understand the current that goes into your battery. All I know is that the maximum current is 20 Amps using the Float setting. If you can get more than that earlier in the day by configuring the setting called Absorb you will have a better chance to maintain your capacity until the sun goes down.

[-robw-]

Earlier you used a voltage setting of 28.8 which translates to 3.6 volts per cell and that is a voltage that will shorten the life of your batteries if constantly maintained or kept at that voltage. How much is your daytime usage in Amps or Watts? That can have an important result on whether your batteries will stay at 95% State of Charge or will lose some capacity. The key component is what kind of current your controller can put out. You have not identified that and it would better inform this conversation.

Yes, my bad. I was trying to simplify things by referring in 12 volts. In actuality, my bulk/absorb is now set for 28.8v, ending amps is at 0, and absorb timer is at 30 minutes (changed that today). Float is at 27.60v.

Very little daytime usage, maybe 50 amps. At night that jumps to 300~ amps, including AC.

3.45 volts is less of an issue and the capacity difference between 3.45 and 3.6 volts is negligible. Have you measured the Amphours or Watthours it takes your battery to go from 3.45 to 3.6 volts per cell?

No, because I don't trust voltages that aren't at rest and getting them at rest is too long a process. I've had to do that a few times and it's a pain.

You can use a rebulk or Float at 3.3 volts (13.2) as a fallback and it really depends on what happens with the mode described as Absorb. What is the maximum current you see during a sunny day when in bulk mode? That is likely the current that the Constant Voltage (Absorb) stage will start at when the charge controller goes through the transition. To accomplish your goal it is better to see if you can understand the current that goes into your battery. All I know is that the maximum current is 20 Amps using the Float setting. If you can get more than that earlier in the day by configuring the setting called Absorb you will have a better chance to maintain your capacity until the sun goes down.

Max current from solar array is around 70ah. The 20 amp max in float makes sense because that's what I observed, except when some appliance kicked in and current matches draw, or tries to anyway.

[Ampster]

Yes, my bad. I was trying to simplify things by referring in 12 volts. In actuality, my bulk/absorb is now set for 28.8v, ending amps is at 0, and absorb timer is at 30 minutes (changed that today). Float is at 27.60v.

No worries, since I have been trying to use individual cell voltages, since at that level cell voltage matters. I would suggest setting the absorb timer for longer and seeing how many Amphours you get. Most likely it will be more than the 20 Amps per hour you were getting with the old float setting. The new float setting of 3.45 per cell should kick in later in the day to cover your daytime usage so you end the day at 95%.

Very little daytime usage, maybe 50 amps. At night that jumps to 300~ amps, including AC.

Is that Amps per hour? It is easier if you use Amphours or Watthours so you can see how much is coming out of your battery whose capacity is measured in Amphours or Watthours.

No, because I don't trust voltages that aren't at rest and getting them at rest is too long a process. I've had to do that a few times and it's a pain.

Voltage is only useful at the upper and lower knees of the curve to set charging level. Voltage on Lithium batteries is a poor indicator of capacity. The best indicator is a Coulomb counter which can be found in $30 Amp meters on Amazon.

Max current from solar array is around 70ah. The 20 amp max in float makes sense because that's what I observed, except when some appliance kicked in and current matches draw, or tries to anyway.

If you are saying the output of the solar array is 70 Amps then that is a good reason to lengthen the absorb time so you get more Amphours into the battery during the peak of the day. Again your use of Ah and Amps is making it hard for me to give you advice based on the confusing units of measure. How can I help you clarify? Do you understand that Amps and Watts are instantaneous measures and Ahs and Watthours are measures over time that relate to the capacity of your battery. They also relate the the generation and usage over time. Using the correct terms is the only way to know how much of your battery capacity you have used and how much you have left.