Conservative Cycling Charge Range

[jnrhome]

In conservatively cycling lifepo4 batteries, would cycle life be higher, same or lower if you cycled between 100% and 50% SOC, as opposed to 75% and 25% SOC?

 

[Hedges]

Cycling to 100% would accelerate degradation.

Detecting 25%, 50%, or 75% is difficult because voltage curve is so flat. IR drop depending on charge/discharge current will dominate, making SoC determination from voltage more difficult.

I would suggest setting a maximum charge voltage just slightly up the knee. That might be something in the range of 85% to 95%.
Similarly, set a discharge cut-off slightly into the lower knee.

And of course, have a BMS that monitors individual cells and disconnects if they get out of limit.

 

[jnrhome]

ok, so you're saying 100-50 would have a lower cycle life than 75-25 because charging to 100 accelerates degradation.
Say i can detect SOC exactly despite the flat curve. In that case, 90-40 versus 75-25 versus 60-10. Would there be any cycle life improvement by using the center most median value, rather than the higher or lower median?
Put another way, if i wanted to use no more than 50% of my battery, which of the three above value sets would provide the greatest cycle life? Which would be worst?

 

[Hedges]

"Detecting" could be by counting charge in and out. Then periodically re-zeroing by charging to start of knee, at which point balancing could be performed.

I'm not sure. You'd have to search for some studies.

Something I have observed is that max charge rate is reduced as temperature approaches 0 degrees C or 55 degrees C, and as SoC approaches 80% to 90%.
You should have charging disabled outside certain temperatures, but if trying to charge at a rate of 0.5C or 1.0C (2 hours or 1 hour), that could need to be reduced under some conditions. If charge rate well below maximum, then just temperature cut-off and max voltage setting should be sufficient.

 

[MisterSandals]

Put another way, if i wanted to use no more than 50% of my battery, which of the three above value sets would provide the greatest cycle life? Which would be worst?

Not to be snarky (ok, mildly snarky but maybe thought provoking):

Why not buy 50% as much battery, beat the crap out of it daily for 10 years and then buy another (which would presumably be better and/or cheaper)?

 

[RStone13]

In conservatively cycling lifepo4 batteries, would cycle life be higher, same or lower if you cycled between 100% and 50% SOC, as opposed to 75% and 25% SOC?

lower

 

[RStone13]

ok, so you're saying 100-50 would have a lower cycle life than 75-25 because charging to 100 accelerates degradation.
Say i can detect SOC exactly despite the flat curve. In that case, 90-40 versus 75-25 versus 60-10. Would there be any cycle life improvement by using the center most median value, rather than the higher or lower median?
Put another way, if i wanted to use no more than 50% of my battery, which of the three above value sets would provide the greatest cycle life? Which would be worst?

90-30% would give you 60% use. & be gentle on cycle life, worst 60-10

 

[Steve_S]

The general recommendation is to use 90% capacity to maximize lifespan & number of cycles.
LFP Voltage Range is 2.500-3.650.
LFP Voltage Curve is 3.000-3.450.
The "cliffs" are 2.90-2.50 and 3.500-3.650. This is the Edges of the Voltage Span but only collectively represents about 8%.
LFP cells charged to 3.650V will all "Naturally" settle to about 3.500 +/- a little within an hour after charging. This is normal for LFP.
LFP cells do NOT like to be stored at 100% (stored meaning inactive and unconnected) They should be stored at 50% or 3.200V per cell which is the Nominal Power Point for the cells.

I run a 24V system. Charge to a max of 27.9V or 3.487V per cell (VPC) Low Volt Cutoff @ 2.65V per cell or 21.2V for the pack.

PS: Forget most of what you learned related to Lead Acid batteries, 90% of it does not apply and the terms are not entirely compatible with LFP as they are in a different context. We do not use DOD for example, we use SOC. The only way to know Lithium Status is by "Cell Voltage & the combined pack voltage". Accurate Coulomb Counting & Measurement is essential for determining SOC.

See links in my signature, there is a detailed LFP Voltage chart and more which would be useful to you.
Hope it helps, Good Luck.

 

[jnrhome]

First, I live in the virgin islands which has intermittent, dirty grid power. I regularly see voltages as low as 75 and as high as 160 (sometimes back and forth multiple times daily) with fluctuations in hertz from 58-62 as well. Grid power damages and destroys electronics, appliances and anything you plug straight into it. Getting off the grid was AWESOME as I haven't had one outage since I assumed control of my own electric generation and storage, nor have I lost one electric device to crappy power.

On occasion, we get a couple days of overcast weather which limits charge. On rarer occasions, we get hammered by a hurricane after which we are all truly off grid for extended periods of time. Running my system using 50% capacity and nearer to full charge gives me cloudy day peace of mind.

So then...
for greatest lifespan (number of cycles) 95-5% SOC. Running at 50% use isn't any better. If I ran it at 50% SOC as the median, I get the better lifespan versus running it at 90-40% SOC. Leaving an unused lithium bank at full charge is bad so no having two strings with one in service and the second in full standby.

PS- my lifepo4 pack to 80 days to arrive so the idea of "beat the crap out of it daily for 10 years and then buy another" doesn't excite me. I'd rather maximize the lifespan, saving the extra capacity for short term overcast days and extended outages post storm.

 

[Steve_S]

So then...
for greatest lifespan (number of cycles) 95-5% SOC. Running at 50% use isn't any better. If I ran it at 50% SOC as the median, I get the better lifespan versus running it at 90-40% SOC. Leaving an unused lithium bank at full charge is bad so no having two strings with one in service and the second in full standby.

I see where you're going BUT some of that is leftover "Lead-ism" With Lithium Based batteries 50%-75%-85% makes little to no difference. It's the top & bottom 10% that crosses the threshold. With Lead if you go below 50% DOD you are causing harm and that is part of "battery vernacular" because we have known this for 100+ years since FLA came to being.

I started with Big Heavy Rolls Lead and had to unlearn & relearn for Lithium Based systems. It is quite different.
I am presently running Bank-1 910AH LFP and Bank-2 856AH Heavy Lead. Within 2 weeks Bank-1 will be 1190AH/30.4kWh. and finally completed.

Please have a good look at this document from Victron on setting up battery packs in Parallel, starting at Page17 for LFP packs I believe.

https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

Do get the LFP Voltage Chart from my signature, it will clarify things for you. It is in our Resources Section.