Well, now I am thoroughly confused. I was hoping for a cookbook-type solution for someone who is NOT already an expert.
So let me propose another approach, which would involve a little extrapolation: I hook up my inverter and my 100 watt electric light. Those two items have a known draw. My little inverter draws 0.4 amps, or almost 5 watts.
Now, I let them run for 12 hours.
I unplug. Wait an hour till the battery settles down. Read the voltage to get the state of charge. Let's say it is down to 12.4, which is a 70 percent state of charge (looking at the standard table of voltages). That means that that wattage for that amount of time has used 30 percent of the total capacity, or 60 percent of the desired capacity (since I never want to bring it down below a 50 percent state of charge).
So then, after gathering that data and using a powerful mathematical aid (my wife), I can then calculate what that total real world capacity is.
I have read that using specific gravity to measure state of charge is more accurate than a multimeter. Maybe that is true with a decent instrument, but not with my little hydrometer -- it sort of points to various areas -- "good/fair/bad." It is ok for what it is, but it seems little better than a magic 8 ball.
Niner, I know that an equalizing charge will top it off. But in real-world, everyday usage, I don't do that. My wonderful little 120w portable solar panel brings it up to about 14 volts, almost every day. (That thing has changed my life.) So I want to see how the battery will perform under "battlefield" conditions, not in a perfect world.
And if I wanted to get a little more scientific (which I probably do), I would stop the experiment at three, six, nine, and twelve hours, to get a better set of results.
I am sure this is all too simplistic, but I am looking for something that an average RVer can easily do, rather than a very technical person.
Is this "plug and play and measure" method adequate? Or is there an even simpler method that I am overlooking?
