And in your opinion is a higher 1 voltage over 2 achieving a real benefit for LFP? A real difference in charging speed?
In the NMC world are higher 1 voltages common as well?
What happens if bulk, absorb, and float are all set to 54v?
Say on the average Lifepo4 server rack batteryDepends on the battery.
What happens if bulk/absorb, and float are all set to 54v?
Say on the average Lifepo4server rack battery
I see one chart. Is there a different chart for a higher absorption voltage I'm missing?Yes. My 13.6V chart demonstrates that a lower absorption voltage took about 7.5 hours starting at 20A vs. a little over 5 hours.
I see one chart. Is there a different chart for a higher absorption voltage I'm missing?
You are just repeating the same point I am questioning that different voltage targets are necessary to speed up charging, and it doesn't look that true for Lifepo4.
Don't get ahead of yourself here and weaken your argument.The battery will not reach full charge if it's only charged up to the float voltage.
Don't get ahead of yourself here and weaken your argument.
I know, and you weren't too interested in thoughtfully exploring it. So I'm kind of done having to try to rephrase the question to you.I'm not sure what your asking.
I'm entertaining the idea that what if we did, would the difference in charge performance be as great as this community's consensus proposes.Or are you arguing that we only need 1 voltage setting and that's "float"?
I'm entertaining the idea that what if we did, would the difference in charge performance be as great as this community's consensus proposes.
The counter arguments are:
1. You wouldn't be in good balancing voltage.
2. The charge would be too slow.
And I'm saying would it really? I think you can balance at float and that is now more or less the consensus. And would it be that slow? If we're talking about 5% SOC left on the table, is that 5% worth packing these higher voltages in every day?
As previously stated, you won't fully charge the battery.What happens if bulk, absorb, and float are all set to 54v?
I implore you to watch those two Andy at off-grid garage videos. Obviously you have not.@sunshine_eggo The 3.65 charge is very interesting to this discussion, thank you. And I appreciate the refutation of my hypothesis from someone who is familiar with another battery chemistry with one charging voltage. So I can be sure it's not that you're not getting my point.
I'm not quite understand your assumptive charting based on the crossing. In one example, the 3.65 chart, the original chart does go into CV right at the voltage crossing. I don't understand the mathematical mechanism, but it does. In the other chart though, the original chart does not go into CV at a crossing.
Also, these two tests are at different currents, which is going to affect the CC duration as well as the voltage. I'm not saying you said they were a 1 for 1, just thinking it through here.
Right but if you zoom in on sunshines graph, you will notice that at 13.6 volts it required over 5 hours of absorption to get full. So first it required time to charge to the voltage and then it required five more hours to fully saturate the cells at that voltage 13.6. the sun isn't up for that long. Most people here are cycling their batteries everyday so all of this charging and absorbing and floating has to happen within the context of one sunny day.I think the discussion here should be best taken to the optimal charging protocol for a single cell, not a series combination with a bms. If the 3.45 (or whatever) bulk limit, the 3.45 (or whatever) absorb, and 3.35 (or whatever) float is chosen because of balancing issues, so be it, but if it is because of electrochemical considerations of the electrodes, then that is more fundamental, and should be considered first.
I don't have a good answer, as I'm not knowledgeable with lifepos in particular, but I do understand electrodes, their thermodynamics, and kinetics pretty well, but more from a corrosion stance. I can imagine that modelling lifepo cathodes could be pretty complicated, as it involves solid state diffusion in the cathode. What I can say is that considering non-equilibrium kinetics (charging/discharging) is where a good deal of the complications arise. The most meaningful potential is that at open circuit, when there are no loads or charging, and after everything has relaxed, which can take some time. When you charge, there is an overpotential associated with ohmic loss in the electrolyte, an activation overpotential associated with driving the fundamental reaction a certain direction at a certain rate, and a diffusion overpotential, that results from the fact that some ions in solution have different diffusivities than others, and are essentially separated from one another (like a little capacitor) when they are pulled one way in an electric field, but at the same time are trying to even out concentrations due to diffusion. These overpotentials disappear when you stop charging and have no loads, after some time anyways. They are a strong function of current density, in varying and often complicated ways. What's determining the energy density of the system (roughly the soc) at this point is the electrochemical potential of the fundamental reactions, which is pretty straightforward, and the amount of 'stuff' at one electrode or the other.
All of the overpotentials result in efficiency losses, so you can see that the most efficient way to draw power from a battery, or charge it, is to draw infinitesimal currents, which is clearly not practical. It makes sense that you would try to hold a cell at its open circuit potential at full charge, which I understand is around 3.35V. It's entirely possible that some time is desirable at higher potentials for some reason related to the state of the electrodes - I just don't know.
I think what hwy17 is trying to get at is is there a point of ever being above 3.35 during a charge. The way I see it is yes, because for any meaningful charging current, there is an associated overpotential, so to snap back at the right open circuit potential of full-ish soc, you charge above it, but still below 3.65, and wait for current to taper down a bit. Outside of unknown (to me) electrode effects, you'd still get to the same soc by holding at 3.35 for infinite time, sort of like walking halfway to a wall each time you walk to it. The overpotentials diminish with decreasing current. So, in short, it is a 'time' reason. For a 16s battery, I can see there being reasons involved with the balancing dance, but these are separate.
Go watch those videos.I'm entertaining the idea that what if we did, would the difference in charge performance be as great as this community's consensus proposes.
The counter arguments are:
1. You wouldn't be in good balancing voltage.
2. The charge would be too slow.
And I'm saying would it really? I think you can balance at float and that is now more or less the consensus. And would it be that slow? If we're talking about 5% SOC left on the table, is that 5% worth packing these higher voltages in every day?