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resting SoC in Battery Backup system and battery health

callmeburton

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If this has been discussed before I would love a link to the discussion ... I couldn't find it with several searches I tried.

With LiFePo4 batteries what is the preferred resting SoC for a battery backup system ?
Should I be cycling the battery every X days down to a certain SoC and then allowing it to charge back up?


I have a battery backup system in place, the SoC is currently 100% (as discovered / set by my BMS bypassing at 3.5vdc ... the actual voltage at the cell is likely closer to 3.475-3.48)

When I had my electric motorcycle the preferred SoC for a resting bike was 80% and we were only suppose to peak above 90% if we were going to instantly start riding the bike but most of us charged to 90% because it took too long to get the last 10%

This battery will be migrated into an off grid house we are building in the future but right now it is being used as an always on battery backup system (general diagram at first post here https://diysolarforum.com/threads/i...or-battery-bank-and-sma-si-load-center.52698/ )

It will likely be used as a battery backup for at least another year before it is installed in the new off grid home.

Thoughts? Feelings? Mystical voodoo? All welcome.
 
I am in a similar situation - working with EG4 Lifepower batteries and the EG4 18kpv inverter, where my main aim is to provide battery backup to some critical circuits in case of power outage. I have an older solar array with Enphase microinverters and have been selling excess power to PGE here in California for the last 10 years.
 
I ran the SI with DIY LiFePo4 battery and Batrium BMS for about a year ... this system has now been moved to our new home we are building which is offgrid and reconfigured with another SI for split phase. The battery backup was nice BUT I couldn't use the SI built in relays well to auto disconnect loads if the batteries were low. And occationally it would loose the CAN signal from the BMS (hasn't happened on the new build yet) and restart the system >__<
 
At a rested open circuit voltage, a LFP cell is fully charged at 3.43 vdc. Doing an absorb charge above this level but below 3.65 vdc will speed up recharging and will create a surface charge, that will take hours to a few days to bleed off on its own. The surface charge has no significant capacity. Above 3.65v there is an accelerated degradation of electrolyte, really getting bad above about 4.3vdc causing cell bloating due to electrolyte solvent decomposition. Beside bloating gases it leaves behind tars that clog electrode pores restricting lithium-ion migration between positive and negative electrodes.

You should not apply a continuous float charge above 3.43v per cell.

LFP cathode is very rugged and can take full charging. Higher float voltage does accelerate electrolyte degradation a slight amount.

Unless you need full capacity, a long-term float voltage of 3.35v per cell is a good compromise. It provides over 90% cell capacity.

For continuous float maintenance on LFP battery you also have to think about individual cell self-leakage. This will degrade balancing of cell's state of charge matching over time as cell leakage rate will not be identical. BMS's typically do not balance cells until a cell exceeds 3.4v so you should do a higher absorb voltage charge every several months to give BMS time to balance cells.

Normal life degradation on LFP cells is dominated by expansion and contraction of negative anode graphite electrode over SoC. From zero to full state of charge the graphite expands about 11%.

Lithium-Ion batteries with graphite negative electrodes grow a protective Solid Electrolyte Interface around the graphite granules. This SEI layer helps prevent electron leakage out of graphite into electrolyte when cell is charged. When electrons get into electrolyte it decomposes some of the electrolyte. It is normally a minor about of degradation, unless you overcharge battery.

Aging process is the charge/discharge cycling with the associated expansion and contraction of graphite volume. This creates cracks in the SEI protective shells around graphite allowing more electron leakage into electrolyte. The cracks in SEI shells are repaired by regrowth during subsequent charging but each time some free lithium and electrolyte is consumed to make the SEI repairs.

These repairs consume some of the available free lithium reducing the capacity of cell a slight amount for each cycle and repairs on SEI layer consumes a little electrolyte to rebuild the SEI layer. The SEI layer repairs also thicken SEI layer over cycle life restricting lithium ion pass through which increases cell impedance over aging cycles.
 
If I understand the answer given by @RCinFLA, they are saying that it is best to keep the LFP batteries at 90% SOC and not cycle them if you don't need to. Is that right?
The wider you run the discharge range, the more expansion and contraction of negative anode graphite which eventually degrades cell.

LFP is fairly rugged for cathode strength. Electrolyte is slightly more vulnerable to sitting at higher state of charge voltage, but it is not a big factor.

Many folks have heard not to fully charge Li-Ion batteries for maximum longevity. This does not really apply to LFP lithium-ion batteries. It applied to nickel-based cathode lithium-ion batteries like found in most EV's and cell phones (4.2v cells). It has got to do with the cathode lattice vertical support when fully charged where most of the lithium has left the cathode and transferred to graphite anode. In LFP cells, the iron provides vertical support for cathode lattice when fully charged and cathode loses the lattice support provided by lithium. This gives LFP strong, long lasting cathodes. On most other types of Li-Ion chemistries, the cathode is the weak link, determining the longevity of battery.
 
The wider you run the discharge range, the more expansion and contraction of negative anode graphite which eventually degrades cell.

LFP is fairly rugged for cathode strength. Electrolyte is slightly more vulnerable to sitting at higher state of charge voltage, but it is not a big factor.

Many folks have heard not to fully charge Li-Ion batteries for maximum longevity. This does not really apply to LFP lithium-ion batteries. It applied to nickel-based cathode lithium-ion batteries like found in most EV's and cell phones (4.2v cells). It has got to do with the cathode lattice vertical support when fully charged where most of the lithium has left the cathode and transferred to graphite anode. In LFP cells, the iron provides vertical support for cathode lattice when fully charged and cathode loses the lattice support provided by lithium. This gives LFP strong, long lasting cathodes. On most other types of Li-Ion chemistries, the cathode is the weak link, determining the longevity of battery.
Hello sir, your comment totally contradicts with what I searched on the internet. Everyone says storing LFP at high SOC damages the cells. Some even blames sellers for not discharging after matching and balancing cells before shipping out.

May I ask whether if your statement is true for all kinds of LFP currently available on the market? (Not including new stuffs like LMFP)
And all grades? Like Grade B and Used Cells as well?
 
The difference in LFP cathode is the iron provides cathode vertical lattice support as the lithium is extracted from the cathode lattice structure when cell is fully charged. LFP cycle life is limited by graphite anode which is about an order of magnitude more cycles life than ternary cathode based Li-Ion batteries.

Ternary lithium-ion batteries (like NCA, NMC) batteries lose cathode vertical lattice support when fully charged where most of the lithium, providing the only vertical lattice support, is extracted from cathode. This makes the cathode more vulnerable to structural collapse permanently damaging sections of the cathode. It is like a multi-level parking garage with many of the vertical support pillars removed. This type of cells' cycle life it limited by the longevity of the cathode, not the graphite anode.
 
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