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Old 13-10-2018, 20:37   #31
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Re: LiFePO4 and FLA: a match made in heaven?

Witch....where is the Fire and stake?!?
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Old 14-10-2018, 01:23   #32
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Re: LiFePO4 and FLA: a match made in heaven?

Quote:
Originally Posted by rgleason View Post
Has your thinking changed a little and what are your current "not above" and "not below"?
Your "summary" is not my words, never heard of a knee wall, where did that come from?

Best to just ask a specific question, in context, one at a time.

This is mine
>I define my 0% SoC as 11.9V (2.975Vpc) in a low-amp discharge context.
Yes, at higher amps, going a point or two lower on the LVD is equivalent, help reduce early cutoffs due to sag.
Setpoints should be easily adjusted to account for changing circumstances.
Look at any OTS BMS LVD cutoff, and you will see they are way way lower.
So lots of leeway there, I just choose to be more conservative.
And there is no significant usable AH capacity left between those two zones.


> Also if MaineSail has previously identified

Again, quote his actual words, with enough relevant context.

Giving your "summary" with different numbers from different use cases is misrepresenting others' recommendations.

> What voltages are generally accepted as the Manufacturer's range for full capacity 100% SoC?
Check with each manufacturer directly, to me that is not relevant to actual usage patterns.

If a vendor says the batt holds 80AH, and I find that charging to my 100%

3.45Vpc (holding Absorb tapering to .03C only when precise calibration required, not as in daily use where I "charge **to** and stop)

then discharging 80AH down from there

leaves voltage way above my 0%,

say 3.15Vpc after resting

then I'm happy.

Some arbitrarily higher / lower voltages that IMO may be pushing destructively into the shoulders, and any few AH capacity gained that way is just theoretical, have no utility IRL and thus are not of interest to me.

Or rather the areas above and below that vendor-defined 80AH are only of interest as no-go zones, to be avoided in daily use.

But say I found a batch of cells that required "a BMS" to do active cell-level balancing, and that functionality only kicked in at 3.5Vpc or greater.

I may then have periodic maintenance sessions

as one does equalizing FLA

where I increase my top-end setpoint to 3.55Vpc, and hold Absorb until each pack is re-balanced to spec

This higher-than-100% SoC has nothing to do with capacity IMO, and in fact is not in the least significant wrt usable AH, just surface charge and dissipated heat.

In doing a capacity load test down from my calibrated 100% above

if discharging to 2.975Vpc only used 75AH rather than 80, I would use that to re-calibrate my BM setting for bank capacity, and log the lost capacity with an eye to learning from that in order to try to avoid the same in other bank purchases or sub-optimal usage patterns, perhaps adjusting to a shallower DoD if possible in that install's context.
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Old 14-10-2018, 01:47   #33
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Re: LiFePO4 and FLA: a match made in heaven?

Quote:
Originally Posted by Cpt Pat View Post
LFP can be overcharged at 13.65 volts when the charge current is very low.
I assume you mean pushing with a held Absorb time, or even Floating for a long time at that voltage?

If just charging **to** 3.42 and then isolating fron the charge source, hard to see that being harmful.

> a SOC range regime of 20% to 80% SOC

See, to me that implies you are sacrificing 40% of rated capacity, leaving 50AH out of your nominal 125AH never used?

I can understand oversizing a bank a bit, using an on-average shallower draw in order to have a reserve pool for exigencies, and also getting maybe double the lifetime cycles as a bonus.

But the difference in practice of pushing **higher into the top** shoulder and stopping with no Absorb at 13.8V, is very low single digits of actual AH capacity.
http://www.cruisersforum.com/forums/....php?p=2731306If your "usage zero" is actually 12.7V, I'd bet,

compared to a usage zero of say 11.9V, which is ~10%

Your 75AH usage is really ranging something like 36% - 96% (arbitrary WAG of course)
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Old 14-10-2018, 19:32   #34
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Re: LiFePO4 and FLA: a match made in heaven?

Thank you Cpt Pat and JohnCt,
So John, for your batteries you are using
0% SoC as 11.9V (2.975Vpc) in a low-amp discharge context
100% SoC 13.8v (3.45Vpc)

Generally, do you then find that your measured amphours in that range are close to the manufacturers rated ah?

Yes, I was reading Cpt Pat's 80% - 20% figures and got concerned that LiFePo4 use range at 60% rather than 100% was too close to the way LA work and not worthwhile doing. Cpt Pat's writeup for how he uses his batteries is what got me trying to really start to understand how it works. His writeup in the other thread was very helpful.
in

Also, since we are being specific about use here, perhaps it would help if we knew what the battery manuf, cells and size were?

Thank you both for being explicit in the face of in-explicit questions.
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Old 14-10-2018, 19:45   #35
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Re: LiFePO4 and FLA: a match made in heaven?

I have stated many times, yes, with banks bought new, rated AH are right at or a bit lower than what I measure within the usable range.

Plenty of lower quality no-name and second-hand sets that is not the case, but that just means - unless returned for that reason - just adjust the BM capacity setting.

Same if there is a decline in SoH over time, but have seen none so far.
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