High C-rate Charging questions

[12VoltInstalls]

This is about high C-rate charging of manufactured LFP batteries.

Contents:

1. I’m studying up before I do something stupid because I know enough to know what I don’t know; I snippet-quote from other threads
2. I comment on the context
3. I ask a few questions

1:

LFP while discharging has no clue about where the juice is going, it does not care. When charging, it can take a low amp charge to full 1C or better (depending on your cells type).

c rate will be the maximum that one bms can handle

@Substrate said:

posted one scientific paper that specifically says that DoD is a factor, but is much less significant than calendar time and temperature. He then posts a different scientific paper which doesn't even support his assertion.

These threads are all over the place. While I learned some things I still thought I’d confirm my assumptions.

2:
I have two 140Ah 12V btrpower batteries to be series 12V and likely to order a third. 125A Class T fuses on each pos(+), 2ga series cables, 2/0 to busbar. Btrpower website, “…battery is equipped with 100A BMS (Battery Management System) to protect it from overcharge, over-discharge, over-current, and short circuit…”

Just received a new 200VOC Epever 6420AN. I think I have 2ga charging cables but might be 4ga- (I bought whatever the largest gage the Epever specs indicated would fit in the terminal fittings.)
Will plumb six new-old-stock 315W RecSolar panels 3S2P to it. Panels are 45.5VOC, nominal 36.8V, short circuit 9.09A, nominal power 8.62A.
Have a 4215AN and want to connect existing 100W panels 4S2P to same batteries. 6ga cables to battery busbar.
This will provide an expected 90- 100A of charging to the battery bank, or .5C for 2S, .33 for 3S
2000W QZRELB/Reliable psw inverter fed with 2/0 cables

3a:
Is my assumption correct the .5C will not affect the overall lifespan in any significant manner?
3b:
Am I correct that the Epever 6420AN won’t care that I feed it with 1890W nominal?
3c:
Should I set the 6420AN below the 4215AN’s charging cutoff by .1V or ___V?
3d:
I am typically opposed to using the ‘load’ terminals on an SCC. However, I would like to use the 4215AN’s load terminals to take advantage of its native kWh record keeping. My 12V circuit rarely sees 15A. Max load would be winter with the furnace fan; summer load will not have furnace load.

Cell signal booster 6W​

Water pump ~75W​

Phone charger 10W​

Range hood fan 70W​

Lights (maxed out) ~115W (never happens)​

VHF charger 22W​

total max load -300W / 19A​

​

Realistically I rarely exceed 12A and 19A may not have ever occurred.
Q? Am I being overly or excessively OCD/hesitant to use the 4215AN load terminals?
The DC fusebox is rated at 100A but I used 8ga cable and fused the feed with a 30A ATC fuse (might be 20A) and it’s never blown.

While I believe all the above is acceptable use, input from individuals with more in-use experience and knowledge will be appreciated. Thank you

 

[RCinFLA]

Charging is more stressful on LFP cells for given cell current compared to discharging.

Charging is lithium ions moving into negative graphite electrode lattice. At full charge, the lithium ions stuff the graphite electrode causing graphite to expand in volume by about 10%. This can fracture graphite causing electrically isolated chips. It also fractures the Solid Electrolyte Interface (SEI) layer coating the graphite. SEI layer helps to retard electrons from escaping graphite into electrolyte. Electrons getting into electrolyte breaks down the electrolyte and chemically combine with free lithium ions making pure lithium.

Some fracturing of SEI layer is normal and is also repaired during charging, but this repair consumes some of the cell's available free lithium ions reducing cell capacity over use cycles. It also thickens the SEI layer which increase cell's internal impedance causing greater terminal voltage slump under discharge. It is the primary cell aging process.

Bottlenecking of lithium ions near the graphite electrode interface during charging due to excessive charge rate or charging at colder temperatures increases the chance of electrons from graphite to chemically bond with incoming lithium ions, turning them into pure lithium which can no longer contribute to cell capacity. Pure lithium metal is also bad to have around as it can grow conductive dendrite needles that can punch through cell's separator layer between neg and pos electrodes creating internal shorts.

The greater the charge rate, the greater the cell damage. Above 0.5C charge rate, for the typical blue cell electrode thickness, the degradation rate accelerates. At cold temperatures, the lithium-ion mobility slows down increasing the bottlenecking of lithium-ions entering graphite at a lower charge rate current. Cooler cell temperatures accelerate cell damage during charging.

 

[12VoltInstalls]

The greater the charge rate, the greater the cell damage

So adding one more ~1800Wh/140Ah battery for 3P config will creep me down in C-rate to an acceptably moderate level.

420Ah storage (three 140Ah batts) would tolerate 100A charging for ~.25 C-rate. While 280Ah into two 140Ah batts would still be a modest .35 C-rate.

Thanks.

 

[chrisski]

3a:
Is my assumption correct the .5C will not affect the overall lifespan in any significant manner?

In my case no, but I only use the batteries 90 days a year, so I would not see any difference. If you use these everyday and this is the set of batteries you want to hand down to the next gernaeration as an inheritance, it may.

On a side note, what type of VHF are you charging?

 

[SeaGal]

This is about high C-rate charging of manufactured LFP batteries...

Is my assumption correct the .5C will not affect the overall lifespan in any significant manner?..

As @RCinFLA noted, it's complex.

It will depend on temperature and SOC. EVE's recommended charge rates for their cells, which presumably will roughly apply to all LiFePO4 batteries, goes like this...

e.g.. below 15C you shouldn't be charging at more than 0.4C if SOC >70%

See also my posting here on a thread discussing more sophisticated charge vs. temp control...

Science based discussion on charging LiFePO4 below 32°F

It wouldn't be hard for an engineer designing a BMS to scale charge current with temperature and state of charge. Seems like it could be done in firmware. For example: 10A@-10°C, 15A@-5°C, 20A@0°C, 30A@5°C, 40A@10°C, 100A@20°C. This would allow them to better protect the battery from damage at...

diysolarforum.com

 

[12VoltInstalls]

Q? Am I being overly or excessively OCD/hesitant to use the 4215AN load terminals?

Has anyone actually used the load terminals for everyday loads 10-20A for period of time or perhaps longterm?
I’ve always advised against it and never have used SCC load terminals on any unit ever… but I’m considering it

 

[12VoltInstalls]

If you use these everyday and this is the set of batteries you want to hand down to the next gernaeration as an inheritance, it may.

Well the 4000 cycles life is like ten years and people say in 10-15 years lfp begin to die from ‘calendar aging’ so I don’t expect them to last forever ?
They will pay for themselves by the monthly expense reduction they enable in a month of use, for one thing. And compared to flooded lead acid life and cost they’ll make a ‘profit’ after 26/28 months of use as their balance sheet cost line will surpass lead acid by twofold in ~5years so? a penny saved is a penny earned; lead acid batteries enabled me to pay for these.

I get your point, though. I just don’t expect them to be worth very much in 10 years.

On a side note, what type of VHF are you charging?

Sorry to disappoint but it’s just a handheld Cobra marine radio I use fishing. I have an on-board, but it’s handy to have on other people’s boats or to monitor things.