Just remember that the oft-repeated saying that LFP is tolerant of over-voltage ONLY means that they don't immediately go kaboom like other lithium chemistries, but you ARE damaging the heck out of it if the over-voltage is reasonable. Example: if you forget that you don't take LFP above 3.6v per cell, and charge it accidentally to 4.2v - no drama, but rapid cell damage / aging occurs.
A simple 4S battery can be used, with the proviso that you aren't using CRAP. Each cell must be measured and vetted to be close to each other in both capacity and internal resistance. Wiring infrastructure must also be sound and not contribute to the resistance. (Ie, no steel washers, magnets, or other ho-key diy setups).
This matching procedure, and initial balance, is part of the routine for LFP batteries such as motorcycle starting like Shorai, Antigravity (starter only, not the other stuff like jumpstarters which DON'T use LFP!). Yes, some manufacturers use internal balancers etc, but that is to protect itself from the non battery-savvy average consumer - which we aren't.
LFP cells will charge to full if a cc/cv routine is used with a CV value of anywhere from 3.45 to 3.6v per cell. The amount of "absorb" time to full takes longer at 3.45v than it does 3.6v.
Ideally, start out with closely matched cells, charge each one to full. An initial individual charge of 3.6v, at less than 0.3C (to avoid phase delay) until current naturally drops to about 0.05C to perhaps 0.025C would be considered fully charged. Assemble into the 4S configuration with GOOD wiring.
Under NORMAL use, drop the overall voltage to no more than about 14.1v. If you have the luxury of time, then 13.8v will do the job - it will only take longer to absorb to full. STILL, keep an eye on it, and make sure no cell exceeds 3.6v at the end of charge. If it does, then you have problems - bad wiring, CRAP cells, etc.
No need to charge to full every time! Here, the simplest measure is to just stop charging once it reaches the CV voltage point, or perhaps pull it before the absorb stage finishes.
This is the issue with RV'ers who rely upon basically Pb chargers - those chargers typically want to finish an absorb cycle, which is not necessary if you don't want to be charging to full each and every time. This is also what many don't understand - that ANY CV voltage from 13.8 to 14.4v for a 4S pack will charge to full if absorb is given enough *time*.
From a smart battery enthusiast standpoint -if you are safe about it - well matched quality cells, individually charged to full at the outset, can be charged as a "two terminal" battery anywhere from 13.8 to 14.4v, preferable the lower end of the scale of no more than 14.0v with an attendant longer period of absorb if you really need to charge to full.
OF COURSE use some sort of low-voltage disconnect! 12.5v under a reasonable load (like less than 0.2C) can suffice, and still leave you with about 20% capacity remaining. Actual capacities depend upon the load current, but at reasonable draws like 0.2C or less, this is in the ballpark.
Don't use CRAP cells, or cells with an unknown history of storage / use!
If you research LFP threads going back to the origins of time (circa 2007 or so), many LFP projects go south due to this. They bought "used" cells. Or they did something stupid - let's bring it to our level:
Dumb project seen #1:
Assemble a 4S pack of LFP consisting of three 26650's, and one 14400 sized cells. Can you top-balance this with all the custom circuitry? You sure can. Will it be useful? Hardly - that 14400 will get hammered. This is the extreme end of why one needs to use closely matched cells in both capacity and internal resistance to be able to pull off a simple 4S setup.
TIME IS A KILLER:
Reasonable balance (no more than 0.070v [70 millivolt] per-cell difference) is fine. But some go nuts with bleeder/balance circuits that add 5-10 minutes each and every cycle trying to get the cell voltages to 0.00001v as if that actually meant something in overall capacity. Over time, each 5 minute balance cycle holding one or more cells at high voltages takes it's toll. Oh no, not right now - but down the road. Might as well have stayed with lead-acid's cycle life.
Gotcha!
Note that I am NOT anti-balance under the right conditions. Next door neighbor / grandma / kids etc prone to abusing batteries? By all means.
Just be on the lookout for unscrupulous vendors who use the bleeder/balance circuits as a way to stuff CRAP cells inside their boxes, rather than use quality.
