Charge and discharge rates of a battery are governed by C-rates. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 1Ah should provide 1A for one hour. The same battery discharging at 0.5C should provide 500mA for two hours, and at 2C it delivers 2A for 30 minutes. Losses at fast discharges reduce the discharge time and these losses also affect charge times.
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A C-rate of 1C is also known as a one-hour discharge; 0.5C or C/2 is a two-hour discharge and 0.2C or C/5 is a 5-hour discharge. Some high-performance batteries can be charged and discharged above 1C with moderate stress. Table 1 illustrates typical times at various C-rates.
C-rateTime5C12 min2C30 min
1C1h0.5C or C/22h0.2C or C/55h0.1C or C/h0.05C or C/h
The battery capacity, or the amount of energy a battery can hold, can be measured with a battery analyzer. (See BU-909: Battery Test Equipment) The analyzer discharges the battery at a calibrated current while measuring the time until the end-of-discharge voltage is reached. For lead acid, the end-of-discharge is typically 1.75V/cell, for NiCd/NiMH 1.0V/cell and for Li-ion 3.0V/cell. If a 1Ah battery provides 1A for one hour, an analyzer displaying the results in percentage of the nominal rating will show 100 percent. If the discharge lasts 30 minutes before reaching the end-of-discharge cut-off voltage, then the battery has a capacity of 50 percent. A new battery is sometimes overrated and can produce more than 100 percent capacity; others are underrated and never reach 100 percent, even after priming.
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When discharging a battery with a battery analyzer capable of applying different C rates, a higher C rate will produce a lower capacity reading and vice versa. By discharging the 1Ah battery at the faster 2C-rate, or 2A, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same since the identical amount of energy is dispensed over a shorter time. In reality, internal losses turn some of the energy into heat and lower the resulting capacity to about 95 percent or less. Discharging the same battery at 0.5C, or 500mA over 2 hours, will likely increase the capacity to above 100 percent.
To obtain a reasonably good capacity reading, manufacturers commonly rate alkaline and lead acid batteries at a very low 0.05C, or a 20-hour discharge. Even at this slow discharge rate, lead acid seldom attains a 100 percent capacity as the batteries are overrated. Manufacturers provide capacity offsets to adjust for the discrepancies if discharged at a higher C rate than specified. (See also BU-503: How to Calculate Battery Runtime) Figure 2 illustrates the discharge times of a lead acid battery at various loads expressed in C-rate.
Smaller batteries are rated at a 1C discharge rate. Due to sluggish behavior, lead acid is rated at 0.2C (5h) and 0.05C (20h).
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This is about high C-rate charging of manufactured LFP batteries.
Contents:
- 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
- I comment on the context
- 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 AN. 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 AN 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
W 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 AN won’t care that I feed it with W nominal?
3c:
Should I set the AN
below the AN’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 AN’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 6WWater pump ~75W charger 10WRange hood fan 70WLights (maxed out) ~115W (
never happens)VHF charger 22Wtotal max load -300W / 19ARealistically I rarely exceed 12A and 19A may not have ever occurred.
Q? Am I being overly or excessively OCD/hesitant to use the AN 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
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.
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...
I am not sure if any BMS has more than 2 states, on and off. Besides… Isn’t this what charge controllers are designed to do (scale current): I am not sure I would want a more complex BMS as it would likely reduce reliability. And for the last line of defence, BMSs need more reliability if...