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Optimal DOD (Depth of Discharge) and SOC (State of Charge)

MaikaiLifeDIY

Solar Enthusiast
Joined
Nov 16, 2022
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North Las Vegas
Hi everyone,

I've been thinking recently about the topic of optimal battery life settings in an off-grid situation. Like everyone else here, I would like for my batteries to last as many years as possible without compromising "too much" on their convenience. In other words, it's probably more convenient to have fewer dollars spent on batteries, however, a constant load will discharge a single battery much faster than an array of batteries.

Having read through this article, it appears to me that if you could run your batteries between 25% DOD and 75% SOC that, (under optimal temperature) you would get the longest lifespan out of your LifeP04 battery(s). I'm thinking of them in use on an off-grid home, typically sun every day, with constant loads throughout the night and day.

What is everyone's thoughts on this? Once there is an agreed answer to what is the best DOD and SOC, I would like to translate that into actual settings to put into a Victron Solar Charge Controller and battery disconnect.

Cheers, and thank you in advance for your time and thoughts.
 
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Your thought process is sound. I would add the following.

SoC Range: 20% to 95%. IMHO your numbers are a little to conservative and you would be wasting available capacity.
Charge rate: Bulk Amperage limited to 0.20 to 0.25C & Absorption Voltage limited to 3.45 to 3.50V (per cell)
As a general rule LFP doesn't need Float charging unless you are using Grid Sell Back or Peak Load Shaving features. In that case keep Float voltage low, 3.35 to 3.4V per cell.
Discharge rate: Size your battery pack(s) so even when the inverter is at max capacity they don't discharged at more than 0.5 to 0.6C.
 
Having read through this article, it appears to me that if you could run your batteries between 25% DOD and 75% SOC that, (under optimal temperature) you would
This article is about LiPo and NMC lithium-Ion batteries. LFP cells are quite different.

In most Li-Ion chemistries, the cathode material is the weakest link. LFP cathode is the most rugged of all lithium-ion batteries. LFP strength is the iron which provides vertical lattice support after most of the lithium has been removed from cathode near full charging. LFP downside is the cathode produces less electrode voltage so cell has less overall voltage. Other Li-Ion cathode lattices become weak and vulnerable to collapse damage when cell is fully charged. This is why EV's avoid fully charging their batteries as it reduces their battery longevity.

LFP electrode potential is almost perfectly flat across its discharge. Most of the SoC voltage change in LFP vs SoC is due to graphite electrode potential change. Graphite electrode potential ranges from about 0.25v at full discharge to near zero volts at full charge. Negative anode potential subtracts from positive cathode electrode potential to yield battery terminal voltage.

The lower overall voltage of LFP also provides more margin to high voltage limit of electrolyte which has greater decomposition breakdown above about 4.3-4.4 vdc of cell voltage.

Most all lithium-ion battery electrolytes and graphite anode is pretty much the same. The improved fire resistance of LFP is due to LFP cathode releases less oxygen when a thermal runaway occurs, bursting the cell.

For LFP, the anode graphite is the weakest link. Graphite expands about 11% in volume when a cell is fully charged. This puts stress on the graphite lattice and fractures the Solid Electrolyte Layer (SEI) which coats the graphite to keep electrons in graphite from escaping into electrolyte which causes chemical decomposition of the electrolyte.

SEI protective layer is regrown during subsequent charging cycles but consumes some of free lithium and electrolyte to rebuild the layer reducing cell capacity over cycles. This is the normal inevitable cause of cell aging. The SEI repair also thickens the SEI layer which increases cell impedance over time causing greater terminal voltage slump for given cell current.

Only LTO cells have greater cycling lifetime on anode. LTO is the negative anode, replacing graphite, and has little expansion due to full charging. Downside is LTO anode has a large electrode voltage which subtracts from positive cathode voltage. LTO cells are usually made with NMC or LFP cathode material. LTO anode and LFP cathode gives the best longevity but lowest cell voltage.

LFP can be run over a wider SoC range than most other Li-Ion chemistries. Stressing of graphite at full charge, and lithium metal creation near negative anode at very deep discharge are the two most damaging abuse factors. High charge and discharge current on LFP cells is more degrading than SoC range but this can be mitigated by electrode thickness of given cell design.

Most of the DIY'er 'blue' prismatic cells are thick electrode design to give highest capacity for weight and volume to the detriment of peak discharge and charging current capability. On these cells it is better not to regularly exceed 0.5C cell current than worry about SoC range.

Many DIY'ers do more damage to their LFP cells by reducing peak charging voltage thinking it helps extend battery longevity which results in cells not getting sufficient cell balancing by BMS causing them to age unevenly. It is okay to do this, but they need balancing periodically, like once every month, to avoid getting too out of balance on cells. Greater discharge and charge current usage accelerated misbalance of cells by amplifying any mismatch between cells.

Match cells are not just cells with same capacity. Matched cells also have similar cell voltage slump (overpotential voltage) for same cell current. Series cells also need to be kept at similar temperature as cooler temps have a big effect on cell overpotential voltage versus cell current.
 
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Lots of great input, ideally what is the optimum DOD and SOC for LifePO4 batteries?

What would the charge controller and battery disconnect values be to obtain this for the following voltages?

12V
24V
48V

Specifically, what would you set the Absorption and Float voltages to in your charge controller to get the ideal SOC, and what would the settings be in the battery disconnect for maximum DOD in a battery disconnect? Also, how would you control the C rates?

Thank you,
 
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Stop discharge close to 3.0v per cell. Don't absorb charge above 3.55vdc per cell. Don't continuously float above 3.40v per cell.

It you don't need to have full capacity don't float above 3.35v per cell. That will give you about 80% useable capacity with 3.0v low voltage limit.

If you have heavy loads, like air conditioner, make sure you have enough battery capacity to avoid long discharging periods above 0.5 C(A).

Do make sure you get enough cell balancing. BMS's require greater than 3.4v per cell to balance. Know your balancing current so you can judge time necessary to balance out 0.5% of cell AH capacity SoC difference. Don't set balancing to start below 3.40v as you are likely to degrade balancing.
 
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Stop discharge close to 3.0v per cell. Don't absorb charge above 3.55vdc per cell. Don't continuously float above 3.40v per cell.

It you don't need to have full capacity don't float above 3.35v per cell. That will give you about 80% useable capacity with 3.0v low voltage limit.

If you have heavy loads, like air conditioner, make sure you have enough battery capacity to avoid long discharging periods above 0.5 C(A).

Do make sure you get enough cell balancing. BMS's require greater than 3.4v per cell to balance. Know your balancing current so you can judge time necessary to balance out 0.5% of cell AH capacity SoC difference. Don't set balancing to start below 3.40v as you are likely to degrade balancing.
Hi @RCinFLA, thank you for your response, would you be able to break that down into 12, 24, 48 volts, I'm not using raw cells and instead have an array of batteries in a 24V configuration.

Thanks again in advance!
 
There is a problem with all the charging advise when it comes to solar power. Solar power creates a variable charging situation due to how much sun you are getting. You also want to maximize charging when you can and also have power to supply loads. I suspect one persons best charging suggestions (in line with battery chemistry) is probably as good as another unless they have had their LiFePO4 battery for many years and it is still working.

My choice is to to charge to 28.6v boost, 130 minutes constant voltage time, Battery fully charged recovery voltage is 26.6v and discharge down to 25v (under load) before transfer to utility, for my 8S 24v LiFePO4. Float is not valid for my AIO when lithium batteries are selected. So that makes for 100% charged down to 20% charged approximately. But in reality charging begins again every time it drops below 26.6v if PV available.

I will get back to you in 3,5,10 years and relate if it was a good idea.
 
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