So you want an *at rest* SOC chart for LFP?
Believe it or not, here you go. From Shorai. LFP motorcycle battery manufacturer.
See the chart in this FAQ, in the "Can I use a lead-acid battery charger or tender" section for the chart. Uh, yeah, don't use a wall-wart motorcycle charger on your 400ah bank - but the voltages are right anyway:
Divide the 12v chart by 4 for individual cell voltages, and apply to your 48v bank. (the 12v batts have 4 cells inside, right so divide by 4...)
As valid for a 2.5ah LFP motorcycle batt as it is for a 400ah bank.
So - if an individual cell is sitting at rest (4-12 hours no load) 3.585v, then it is 100% full. Don't let it sit like that for long!
And, we can see that once a cell has reached 3.21v, *at rest*, it has reached the 20% knee you don't want to slide past.
So if you want to go by *RESTING* voltage alone, and take your own monitoring into account, this is handy rough estimation for SOC.
Yes, the Shorai's have a slightly different LFP chemical makeup than yours, but at least this is *something*.
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Hi openplanet!
Well, the deed is done I suppose - it is questions like these that make me always recommend a "practice" battery first, something more down to earth like a simple 40ah / 12v setup before going big. Nevertheless ..
Voltages are deceiving as an SOC indicator. It is the *at rest* voltages (which take about 4-12 hours) where it is *slightly* more accurate. Not practical if you are doing daily solar cycling!
In other words, if *after a charge and a rest / no load*, your cells are sitting close to 3.45v each, then you are fully charged. Not that you want to go there often in reality, but this is just discussion.
If after a rest / no load condition, your individual cells are sitting close to 3.0v, then you are approaching the bottom 20% knee.
WAIT! You mean there is only about a 0.45v difference between 80-100% and say 20% at the low end - *when measured at rest*? Yep. Better have an accurate metering system in place, with VERY good low-loss wiring infrastructure. That means good FLUKE multimeters for you diy types.
This is why for a large system like yours, you'll need to be watching coulombs, (aka a/h in out) in addition to voltage extremes. Doing experiments based on voltage solely with solar garden AA's, and hobbiest cells are a whole different ballgame from a practical aspect of a large bank with solar.
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A way to accurately measure LiFePO4 SOC?
I'll be taking delivery of 400ah of 48 volt lifepo4 in a couple months, so I've been studying as much as I can in advance. Based on what the consensus seems to be regarding maximizing cycles, my plan is to charge to no higher than 80% SOC and discharge to no lower than 20% SOC, I understand that deducing SOC from voltage is a challenge due to the extremely flat charging curve.
So here is my question: Given that charge current drops as the battery voltage approaches the charge voltage, doesn't this provide a good way to deduce SOC? For a given charge voltage, that is, doesn't the current (the battery's "acceptance") reflect the battery's SOC pretty accurately? I've seen specs that say, e.g. "for a 90% SOC, terminate charging when current falls to .005 C." For my 400ah bank, that would mean halting charging at 2a (with charge voltage set at manufacturer's recommendation). So doesn't it follow that stopping at 80% SOC would mean stopping at some higher amperage, e.g. 4a? If so, I wonder why there aren't tables that let you estimate SOC based on current at given charge voltages and for given Ah capacities.
I'm not an EE, so I realize this mightn't make sense. Happy to be educated! Thank you.
BTW--I found PNjunction's 07-12-2019, 05:08 PM post on bulk/absorb/CC/CV extremely enlightening, and I hope he weighs in on my post.
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