0.5% X 20C for 100% SOC - myth?

[a64pilot]

Originally Posted by Oceansailor

Every time I read “my lead acid are full by noon with solar” I wonder how that is possible?

Very shallow discharge, good large Solar array and very long days as in being in high latitudes in July where the day is at least 14 hours long.
To also helps if you decide to define full as being 3% of end amps if the manufacturer says 1% to 3%.

But most think they are fully charged because the controller dropped to float, and it does so prematurely.

[kenbo]

Originally Posted by a64pilot

I believe the only time you really need to use end amps is with an AGM or Gel battery, and that’s because of course you can’t check specific gravity of the acid.
I would assume for batteries that you can check specific gravity, that method maybe out to be primary, with end amps as a back up as who checks SG of every battery at every charge?

Or said another way if your SG is correct and your at 3%, fine, but if it’s a little low, keep charging until either end amps stop dropping or SG is correct.
If end amps stop dropping and SG is low, then maybe an equalization ought to be done.
However I don’t see how anyone can equalize on Solar, most of the world has a hard time getting to full charge on Solar, much less holding equalization voltage for hours after the full charge.

Specific gravity is not an accurate measure of SOC. There are several variables that can give you an inaccurate reading. Please read this SmartGauge Electronics - Specific Gravity and State of Charge he explains why.

[sailorboy1]

I can set any absorption end current i want on my solar controller. So not very hard to set to a % plus a normal afternoon house load.

And i see above that some can talk themselves into following or not whichever part of manufacturer specs they wish to and explain as basically because they know best.

[john61ct]

Yes if your charge source can terminate based on a BM reporting endAmps directly, that is ideal.

But even without that, tweaking AHT until transition to Float frequently happens "in the ballpark" is fantastic

and even if SG were as accurate it certainly is not as convenient as checking a readout.

[ramblinrod]

Originally Posted by a64pilot

Very shallow discharge, good large Solar array and very long days as in being in high latitudes in July where the day is at least 14 hours long.
To also helps if you decide to define full as being 3% of end amps if the manufacturer says 1% to 3%.

But most think they are fully charged because the controller dropped to float, and it does so prematurely.

Agreed!

If consumption, storage, and generation is properly balanced, there’s no reason one cannot equalize on solar in mid summer (or mid winter for that matter, if the house loads are reduced) e.g. toss the perishables in a cooler with ice and shut the fridge down for a day, combine this with the defrost requirement and nothing is lost.

[rgleason]

Measuring State of Charge problems near 100%
6vdc Trojan T105 (225ah) Equalization

[Bycrick]

The Smartgauge article referred to in this thread is a good one on the futility of using specific gravity as proxy for state of charge. I once helped a cruiser who had brutally undercharged 3 Rolls 8D batteries to the point that they wouldn’t light a 200ma test light without dropping to 10.8 volts.

The recommended procedure was to charge them at an increasing voltage (up to the recommended equalization voltage) until the specific gravity stopped increasing for 3 hours. The explanation was that UNTIL the specific gravity quits going up, you haven’t really fully charged the battery.

I suspect that the various tail-current numbers are all just a guess about charging becoming less efficient as the SOC approaches 100%. Some balancing of running the charger for days, the amount of outgassing and temperature rise as the battery approaches full charge.

[john61ct]

No, the endAmps specification is the canonical definition of 100% Full by the maker, based on the design and chemistry of that model.

The other ways different chargers actually implement their stop-charge algorithm is usually just an approximation to enable reaching that state (or a bit past it on average), and requires testing and calibration of the charger settings,

often needing adjustment for changing usage patterns or other conditions.

Guessing should not be a big part of that process any more than a sight-in session zeroing a scope.

[Bycrick]

I’m not speaking of charging algorithms used by chargers, I’m speaking of chemistry. When you continue applying a charge current, when the specific gravity stops increasing, the battery has accepted as much charge as it is capable of. Any additional energy is then being expended as heat and outgassing. Since it’s impractical for a charger to actually measure the SG over time, the charger needs to use a "reasonable" proxy. The definition of "reasonable" appears to be different among the various manufacturers.

[john61ct]

The definition of 100% Full varies not just by company but by product line.

I don't think any of the various definitions can be called unreasonable?

I agree that the relative spec of "stop when X is no longer changing" is useful,

when absolute specs no longer apply to a worn bank.

And that hold true whether using SG or trailing current.

With sealed batteries, it is important not to keep pushing current in after amps acceptance has dropped to the endAmps spec, **or** current stops declining.

Continuing until **all current acceptance has stopped** is very harmful to their longevity.

And of course SG is inaccessible.

[Bycrick]

"Reasonable" isn’t a binary choice. The amount of heat or outgassing generated as you approach "full charge" will vary depending on several factors. The manufacturer will decide how much of these are "acceptable," not based on some hard rule. But at some point, you will not be able to move sulfate from the plates to the electrolyte without raising the voltage. At that point, the battery is, by definition, fully charged, as more charging energy will not result in more energy being stored.

So all the the different values of what the tail current "ought to be" are, at best, a reasonable engineering trade-off of a number of factors weighed by the manufacturer.

But none of them imply that the battery is "fully charged," only that it’s charged enough to stop wasting time and energy to get the last little bit.

[john61ct]

No, fully charged, or 100% SoC, is when you've met the maker's definition, and are not causing harm to the battery.

Otherwise the term overcharge would have no meaning.

Just like 0% SoC is not zero volts dead flat, but a resting voltage that industry or the maker has defined as such.

For 12V lead the standard is 10.5V under the prescribed load used for capacity testing, usually the 20hr rate, or 0.05C.

With lithium cells, the vendor min/max definitions are extremely stressful, and should not be used if longevity is important, so for normal daily cycling the customers' system designers use their own judgments.

[Bycrick]

Lithium cells have nothing to do with this discussion.

When a battery is at “zero" also has nothing to do with this discussion, but as you point out is an arbitrary definition agreed upon by the the manufacturers.

Nowhere have I even hinted at the idea that you can keep charging batteries forever. If you keep charging LA batteries, eventually you will get to a point where ALL of the supplied energy is e xpended as heat or electrolysis of the electrolyte. Where that point occurs will depend on the construction and aging of the battery. At that point, by chemical definition, the specific gravity will stop increasing and the battery can be considered fully charged. If you keep charging long enough past this point, you will either run out of electrolyte or overheat the battery.

Manufacturer recommendations are not developed by going up a mountain and bringing back stone tablets. Look at the number of chargers where the default values are far from optimal. They are based on the trade-offs that the manufacturer thinks are "reasonable." How much outgassing is acceptable? If I’m watching the batteries while they’re charging, I can accept more outgassing without running out of water. If I’m measuring the temperature constantly, I don’t have to automatically reduce the voltage after some fixed time period to prevent over-heating.

In the real world, we don’t have a full-time charging source. We can't.measure all the various parameters full-time. So we accept reasonable approximations.

The tail current required to be considered fully charged is the manufacturer's best guess of the values for a sample of their batteries when tested under a standard set of conditions. If you accept the generally accepted wisdom that ANY PSOC will gradually degrade a battery, then getting closer to fully charged will increase the available cycles. if they spec it good for 1000 cycles, they can probably tolerate a higher tail current than if they spec it for 2000 cycles.

[ranger58sb]

Originally Posted by sailorboy1

Batteries aren't really 100% charged till they will only accept 0.5% of the 20C capacity at absorption voltage, measured as amps.

Who can provide a link to a battery manufacturer or similar that says this?

Lifeline doesn't say that, exactly, but sorta kinda close. Their tech manual says "The battery is considered to be fully charged when the current drops below 0.5% of the battery's rated capacity (0.5A for a 100Ah battery)."

Odyssey's manual says "The graph shows that a healthy, fully charged ODYSSEY battery will have an OCV of 12.84V or higher at 25°C." I suppose that doesn't offer a compensation for capacity loss over time. Anyway, no mention of acceptance current.

-Chris

[john61ct]

Again, unless you call and ask their engineers.