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Old 03-23-2014, 08:49 AM   #29
caltexflanc
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I am not particularly good at articulating this, but at the risk of introducing more confusion, here goes.

The modern battery monitors take into account all the same factors, and more (such as using the Peukert formula to a certain degree), that one would use to judge a battery's "age" and condition. This is primarily done while evaluating the charge cycle, reevaluating the actual charge efficiency every time full charge is reached. Typically, they determine a full charge has been reached when all three parameters are met:

1. The charging voltage has stabilized over a period of time.
2. The charging current has decreased to a low percentage of the amp- hour capacity – normally less than 2%.
3. The amp-hours that were removed from the battery are within 1% of fully being returned.

They evaluate what it took to get to get to these thresholds and how long, what the voltage reading vs amp hours consumed, and rate of discharge. So if you tell it that it is looking at a 420 amp hour battery, but the thing isn't acting like one, it will with readjust the charge efficiency factor as the battery dies. I reached the point on my old 410AH inverter bank where SOC was 50% after 100 amp hours were removed, and it was taking forever to get them to full charge.

One factor that the thread has ignored, I think, and may or may not come into play with Tim's situation, is that the rate of discharge affects capacity fairly dramatically. A 100ah (using the 20 hour standard) rated battery's capacity is based on a 5 amps discharge rate. However, at a 25 amp discharge rate has about a 50ah capacity (2 hours) So it is for practical purposes "spent" at 1 hour.(as you can see it is not a linear relationship) So a good SOC meter also takes this into account over time.

The Magnum, Victron (just skimmed it) and Trimetric manuals all discuss these factors to varying degrees, and the different devices seem to address them in slightly different ways.

Anyway, some food for thought, flames and comments welcome!
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