Okay you've heard me discuss how to safely charge these batteries when used as house banks at factional "C" usage, and here's a prime example of what I am talking about.
As we can see in this image the battery has hit full at just 13.88V (pack voltage) with a .4C charge rate. A .4C charge means a 40A charge on a 100Ah battery pack. Anything above this voltage point is technically over charging the battery. If you stopped at this 13.9V level, and tested this pack for Ah capacity, you would see approx 98% - 107% of the rated Ah capacity. I know this because I have conducted these tests NUMEROUS TIMES...
This over charge can easily be denoted by the abrupt hockey stick rise in voltage once the cells hit "full"..
FACT: Charging my own 400Ah Winston pack to 13.8V results in 425Ah's at a .25C load or a 100 amp continuous load for over 4 hours straight.
Just for grins how many Ah's would we get out of a "full" flooded lead acid bank when capacity tested at a .25C load? Let's do the math:
400Ah FLA bank with a Peukert's Constant of 1.3 driving a .25C load = 247Ah bank!!!
WOW!!! Test a flooded lead acid battery at the same rate I do a 400 LFP bank, .25C, which has only charged to 13.8V, and you get 178 Ah's LESS capacity out of the FLA bank than you do from the LFP bank...! The reality is that on boats we are NOT discharging at a .25C rate and we are discharging at fractional "C" rates.
PONDER THIS: The ONLY benefit to charging to voltages above 13.8V - 14.0V is a slightly shorter current taper at the top end of charge.
I allow the current to fall to 10A at 13.8V before deeming the bank "full". This equals 425Ah's at a .25C load. All LFP banks will have slightly varying capacities, and the factory rating is at a .5C load. The 425Ah @ .25C correlates to just about 397 - 400Ah @ 13.8V and a .5C load. Why charge higher, & run more risk, if we can EXCEED rated capacity at just 13.8V - 14.0V at fractional "C" loads??
MAX CHARGE VOLTAGE: Max safe charge voltage for Winston cells, as house bank use = 14.2V
BEST: Charge Voltage for Winston Cells, as house bank = 13.8V to 14.0V
If you choose CALB cells & a DIY build MAX should ideally be 14.0V.
Why? Take a look at what happens to the cell voltages as they get into the upper knee. We have the lowest cell at 3.69V and the top cell at 3.81V which is now into the danger zone! The pack voltage may still look okay but we are now cutting into the cycle life of a couple of the cells by over charging them..
While I have known lower charge voltages work fine for off-grid and fractional C use, and are arguably safer, research is finally coming out to back this up and to also show that higher voltages lead to shorter cycling life too.
Tesla and other Li battery research institutions have now been able to show that regularly pushing Li chemistry cells to high charging voltages results in the build up of electrolyte oxidation by-products which adhere to the negative plate. This eventually leads to a shut down of the cells. It has been shown that higher charge voltages, with all Li chemistries, results in shorter cycle life. Essentially higher charge voltages result in more electrolyte oxidation clogging the negative plate. While the capacity may look good for a period of time the cells eventually fall of the proverbial cliff. In NMC cells (LiNiMnCoO2) cycle life degradation was accelerated as charge voltages were pushed higher. Testing showed that a max charge voltage of 4.20VPC (these are not LFP) showed very little capacity fade where as a max charge voltage of 4.35VPC resulted in less than 200 cycles and a charge voltage of 4.45VPC resulted in less than 60 cycles. It is not however just max voltages that affect the cells it is time at voltage. In the tests above the cells were simply charge "TO" the upper voltage but with the lead acid chargers we use voltages are HELD at a steady voltage for a period of time. In the marine environment, in order to compensate and not over-stress the cells with CV charging, we can simply lower the max charge voltage. This serves to allow for the CV (constant voltage) stage to be safer for a longer duration.
As you push into the upper knee the cells can rapidly run out of balance as one cell becomes more full faster than another. As cells age Coulombic efficiency can change, especially when over-stressed by using high charge voltages and CC/CV charging equipment. The actual cell to cell capacity can also change. By staying out of both knee ranges, voltage wise, the cells tend to cycle up and down with very little voltage drift. Regularly pushing into the upper knee simply creates the need for more cell balancing and many of the BMS companies pray on this. IMHO for fractional C use cell balancing is not necessary until AFTER the HVC point so that it can be done SAFELY, MANUALLY/ATTENDED and SMARTLY, just like equalizing a lead acid battery..
The cells in this image are cells that are well within the ideal balance range when kept between 3.0VPC and 3.45VPC but that drift and begin separating over that voltage. If your pack is not perfectly top balanced, as this one was, you will begin drifting even sooner.
If you can get 98-99.9% of the capacity out of the bank at a 13.9V - 14.0V maximum charge voltage why go any higher? The answer is DON'T and your batteries will stay in good balance for many hundreds of cycles. My own 400 Ah bank has now well exceeded 750 cycles and is still in excellent balance and has lost virtually zero capacity. This is nearly 3X the average cycle life of any lead acid battery I tend to see in the real world of the marine environment.
Please, I ask anyone out there to present me any credible data that shows why going above 14.0V or 3.45VPC to 3.500V per cell is either good for the batteries or to give me just one solid reason why it is ever necessary to do so. I have nearly 100 research/white papers in my data base and not a single one of them gives any good reason to push these cells into the upper knee with regularity. Not a single ONE.
What do we know about higher voltages and LiFePO4 battery longevity?
# Just letting these batteries sit idly at full charge degrades a Li batteries life. The manufacturers want them stored at approx 50% SOC for the longest life.
JUST SAY NO TO FLOAT CHARGING!!!!
Please, please, please stop asking if it is okay to float your LiFePO4 batteries. The answer is still going to be NO!! If you want to continually float your LFP bank you bought them for the wrong reason. These batteries LOVE TO CYCLE. They hate being at 100% SOC and even the mere act of storing them at 100% SOC, as we need to do with lead acid, is damaging to their life span. Adding a slight over voltage, above cell resting voltage, is even WORSE for them. DO NOT FLOAT LFP!
If you are in a situation where charge equipment can't be turned off and would necessitate floating the LFP bank, you need to wire in a cross-over lead acid battery to handle shore-side or unattended duration's of alternative energy charging. For dock side or unattended uses you need to be able to discharge the LFP to 50-60% SOC and TAKE IT OFF-LINE while allowing the small lead acid bank to run DC system loads along with the shore side charger or alternative energy systems.
STORAGE SOC EXPERIMENT
I recently ended a very expensive experiment regarding storage SOC. The test duration wound up being 12 1/2 months using four 100Ah CALB cells where they were charged to 100% SOC and then left to sit idle with no connections to a BMS or other parasitic loads. The low temp recorded over the 12 1/2 months was 46F and the high temp was 87F and was meant to be a representation of the real wold. A min/max capture thermometer was used to record the peaks. The cells, prior to letting them sit at 100% SOC for 12 1/2 months, were regularly testing at 101.2 to 101.3 Ah's of capacity (previous 6 Ah capacity tests) as a 12V nominal bank. After 12 1/2 months the cells were discharged to a cut off voltage of 2.9V for the lowest cell. After 12 1/2 months of doing nothing but sitting there, at 100% SOC, the cells had lost 11.6% of their previous rigorously confirmed Ah capacity. Now imagine if you additionally stressed the cells by continually float charging them. Ouch!!!!
How can these manufacturers suggest that the mere act of storage, at 100% SOC, is bad for the cells, which I have confirmed is, and then tell you "sure charge them to 14.6V"......? How can tehy say "tore at 50-60% SOC yet then give you a "float" voltage? Really? Come on, let's use some common sense. FLOATING LFP BATTERIES IS NOT GOOD FOR THEM! Sorry I don't know how to be any clearer on this point.
I can sum up my feelings on the Chinese, and their often ridiculous charge voltage guidance, like this:
They figured out a great recipe, they can repeatably make the recipe, but they have NO IDEA WHY IT TASTES GOOD!
I would argue that pushing these batteries above 3.5VPC during regular charging is actually detrimental to longevity and not at all beneficial. Once again I challenge any and all Li battery researchers or scientists (I know many of you are reading this because I have your emails) to bring me any credible data to suggest a "need" for such insane charging guidance for the proposed use as a marine house bank.
At the very least pushing to these voltages causes a need for a balancing BMS. Pushing these cells beyond 3.5VPC / 14.0V/28V etc. can lead to nothing but problems in a fractional C system and holds your batteries in the upper knee, a range that can be detrimental to cycle life. IMHO if folks charged these batteries at sane voltages, and stayed out of the upper knee, there would likely be no need for a balancing BMS just HVC and LVC protections.
Like anything, the Chinese want to appease us and thus they tell us lead acid charging voltages are okay to use or they leave out critical points of how to charge these cells. Oh but wait "Don't store these batteries at more than 50% - 60% SOC because these voltages are damaging".... ? Huh? Really?
LFP batteries do not need to get back to 100% SOC, ever. Even if you only got to 95% and drained to 20% SOC you still get 75% usable capacity which is FAR greater than the 30-35% real world cycling you get from lead acid.
We also can't forget what we are looking at here. A .4C charge rate on a boat with a 400Ah LFP bank would be a charge rate of 160A continuous. This means an alternator with a 200A rating current limited to 160A. Most boats will struggle just to get to a .3C to .4C charge rate and these batteries can take much more..
Image Courtesy: Terry