Absorb, or the lack of it until 100% SOC was the reason for the thread since when cruising it never or seldom gets absorb yet we are told LFP does not need to be fully charged each time. without a full charge I am not getting to the absorb stage. That is my confusion.
Those voltages sound high.
Just some additional thinking out loud on the topic overall.... Please take everything I say on the topic with a grain of salt.
It would seem, for absorb, you cant really just say that there is a standard absorption time. A need to absorb, or not can be based on a few things:
1) do you have a need for balancing
2) absorption voltage setting
It really is amazing how Lifepo4 cells respond to various voltages, and differences of just a few tenths of a volt can make all the difference in the world...and then the next few tenths of a volt hardly matter.
Take a look at two extremes. Use the Epoch 460 Marine as an example. Balancing begins at 13.35v and .030 cell deviation. That is just a piece of info to consider in the background when talking about absorption since poor practices could eventually affect balancing.
See Ben Steins excellent testing here:
Charging LiFePO4, what's the impact of lower voltages?
In these tests you can see a graph of actual measured capacity after charging to various voltages. As you can see 100% measured capacity can be achieved with voltages as low as 13.5 volts. The chart lower down the page has additional details like charged time. As you can see...you can in fact get to 100% actual rated capacity at 13.5 volts but it takes forever. 6 hours and 15 minutes for this 12volt 100ah batt! At an absorption voltage of 13.5 volts that battery will spend a VERY long time in absorption to reach 100% rated capacity. In the case of the Epoch example, we will be over the 13.35v threshold to begin balancing. We probably will be over the .030v deviation between cells at some point in that charge range so balancing will take place. Certainly since the absorption took so long to reach 100% SOC (as measured by discharge cap checking) the current during that time would be very very low so the low current passive balancer will have plenty of time to act. However the split between cells at such a low voltage and low current will not present a large disparity in cell deviation. It is this cell deviation that can be acted on by the balance mechanism that actually allows for effective balancing. So the additional time works to compensate.
Then look at a charge voltage of 14.6 v. The very same actual measured capacity (SOC) is reached using 14.6 as we reached with 13.5. But it took 3 hours and 30 minutes LESS time at 14.6 volts! For a battery charged to this high of an absorption voltage there will be pretty much zero need for any additional absorption time for the battery pack to charge to rated capacity. During a charge to 14.6 absorption, the cell deviation will be pushed much farther and result in a much higher cell delta when in these higher voltages. But it also results in some balancing via the Epochs passive balancer. But there will be more deviation remaining at the end of this charge. As these voltages get higher and higher things happen faster and faster. Once you hit absorption point at 14.6 the current will take a very rapid nose dive. It will do this because there are simply "no more seats at the table" at this voltage level. Where as reaching an absorption voltage at 13.5 there is a tremendous amount of "seats at the table" remaining as you hit that 13.5 and it takes hours of absorption to fill those remaining seats to achieve full capacity.. Between the 13.5 example and the 14.6 example there is a continuum in absorption time requirements to reach actual rated capacity. Probably not a linear continuum. Probably anything over 14.0 or 14.1 will not require any additional absorption time or at least very minimal. I use 13.9 with just a bit of absorption
So slight differences in voltage make huge differences in behavior and affect absorption requirements to reach rated capacity and can affect balancing. At the very very high voltage you are charging , there is no need to absorb further. And if for some reason your balancing delta was growing over time then a reduction in charge current and a reduction in absorption voltage followed by a longer absorption would probably be warranted. Some chargers or BMS's shut off charging based on tail current once at absorption voltage for this reason. Others prefer the timed approach possibly for additional balance time. Active balancers may require some additional time.
Your SOC meters are secondary to this voltage and amperage information that could be gleaned from just watching the voltage and current at the upper charge range of 13.5 to 14.6. It is this voltage and amperage info that will actually tell you if your batteries are charged to a particular "rated capacity". With just about any absorption voltage over 13.8 and charge tail current falling to 1 to 2%, you are fully charged regardless of what your SOC meter states.
Too much stock has been placed on the SOC gauge. They aren't actually anchored to anything except a known start point, which is full full full. Then count backwards from there. While it is somewhat useful...IMO get back in the habit of monitoring voltages and amperages at the tops of charge cycles to determine full. At least on occasion. SOC meters can drift, especially the ones that dont have settable charge detection parameters like the ones contained in drop in batteries or basic off the shelf BMS's. I have asked Epoch to add a button in the app to "Reset to 100% SOC". I would have no hesitation hitting such a button if I have observed the voltages and amperages at the top of a charge cycle and be confidant that I have correctly determined full capacity better than many SOC meters that do not have settable charged detection parameters. That is if I was actually watching at the top of the charge cycle..lol.
