End amps VS SOC

[Barry Fields]

Is it generally accepted that an end amps of 2% of C/20 is roughly equivalent to 95% SOC?
Have I over simplified? Please feel free to expand.

[Vic]

Is it generally accepted that an end amps of 2% of C/20 is roughly equivalent to 95% SOC?
Have I over simplified? Please feel free to expand.

Barry,  IMO,  things are not that simple.

My answer, is, "it all depends".

In my experience, (with Lead-Antinomy FLAs) for solar-charged off-grid systems (where the Bulk charge rate is not constant), that the ideal EA setting depends, in large measure,  on the DOD that the battery experienced in the prior discharge.   The deeper was that discharge the larger the EA setting would need to be  --  would need a larger curreent value.

With one typical system here,  the nominal EA runs between about 1.3, and 1.5% of the nominal 20 hour battery Capacity.

If the DOD, and solar charge rates are fairly repeatable,  then one can probably get EA to work acceptably-well.

Other battery charge formulas, are similarly inaccurate,  such as the one which tries to allow the calculation of needed Absorb time,  is in error for solar-charged off-grid systems.  This is because (as in the expmple above),  the solar Bulk charge rate (current) is NOT a fixed value, (and therefore, unpredictable), and yields erronious results.

All IMO,   FWIW, YMMV. and so on,  Vic

[Barry Fields]

With one typical system here,  the nominal EA runs between about 1.3, and 1.5% of the nominal 20 hour battery Capacity.

does that 1.3-1.5% get you to 95% or closer to 100%.
I am looking for an End Amps @ a stable absorb setpoint that would equate to an SOC of about 95%.
(note that my system starts the day around 80% SOC and I am charging with currents in the 5-15% of the c20 rate)
Hopefully I am not dreaming.
Thanks for your input

[boB]

Barry, if you are using lead acid batteries, you will want to complete the absorb cycle time completely (2 hours ?)  so that you get that full de-sulfation to do its job...  At least periodically.
That is what Absorb and lead acid is for.

So once in a while you will want to defeat the ending amps in that case.

If lithium, you will need to make sure it is in Absorb at least as long as the manufacturer's specifications say.  In that case, the Absorb time can be used to let the BMS in the battery system do that balancing properly.

Those are my additions to Vic's recommendations.

boB

[Vic]

With one typical system here,  the nominal EA runs between about 1.3, and 1.5% of the nominal 20 hour battery Capacity.

does that 1.3-1.5% get you to 95% or closer to 100%.
I am looking for an End Amps @ a stable absorb setpoint that would equate to an SOC of about 95%.
(note that my system starts the day around 80% SOC and I am charging with currents in the 5-15% of the c20 rate)
Hopefully I am not dreaming.
Thanks for your input

Hi Barry,
The noted 1.3-1.5% of C20 is as full as the battery can get.  This is the point where, at a reasonable Absorb voltage, the battery charge current (measured by the WbJr), has stopped diminishing.  This is. by my definition,  SOC = 100%. (this  SOC has been with SG measurements).

FLA batteries are quite rugged, and generally forgiving,  so they can tolerate imperfection  --  nothing needs to be perfect, with them  ...

We try to charge to 100% SOC on every Summer day,  as this one system, here,  does not have enough PV power,  and I cannot afford to avoid doing the maximum use of available battery charge power on any day, because the system (generally), will be farther behind the following day.  This is a Summer thing, as the PVs are on racks, that set the elevation angle of 52 degrees (Latitude +15 degrees).  This elevation angle,  plus the elevated Summer temperatures take a large toll on daily PV production.  THis off-grid system was designed and built 18 years ago, when PVs were very expensive, and batteries, not so much so. The high elevation angle of the PVs, maximizes Fall/Winter/Spring PV power production,  allowing fewer PVs, to generally produce adequate power, generally, year-round ...  (just for a little context).

I DO need quite a lot  more PV power on this system,  but, I am not going to hand excavate the holes for piers, in our clay dirt,  and am not going to site-mix concrete to fill the holes for a new ground-mount PV array.  Also,  have not been able to find any competent contractor who is willing to come out into the Center Of Nowhere (my place), to do this work,  and so on.

FWIW, Vic