Dec 3, 2006 10:38 am
Re: Basic question about voltage
John, you state,
"While you may be able to draw some inference about the state of a battery by measuring its voltage, remember that an almost dead battery will still measure about 12 volts."
Well, I realize that an almost dead battery might measure 12 volts, but in my experience with batteries -- no matter what kind you're talking about (car batteries, flashlight batteries, ni-cads, lead acid etc. etc.)-- when they're dead or almost so, and there is no charging being attempted, their voltage never measures anything close to what it would be if it was fresh and able to hold a charge.
While I understand what you're saying about a 200 amp hour battery being able to give 200 amps for one hour etc. , there is no practical way I know of to calculate (on an everyday basis) how much you're discharging a battery. The only exception would be if you always use the same load (same amp draw), and keep a fairly close track of the time. In real life, I use different loads at different times (as I would think most users do), and there is no way I am going to start carrying a stopwatch around and/or a clipboard with all the loads I've used (and how long I've used each of them).
While a voltmeter may not be as accurate, I would think it would give me a close enough estimate of the percentage of the discharge, so long as the battery(s) in question are not
being charged when the measurement is observed.
I've had my solar system for around two+ years, and I've always gone by what the voltmeter says. Furthermore, at night, the volt reading always drops predictably, depending on the amount of the draw from what I'm powering. If it is a 100 watt load, then obviously the volts drop slower than if the load is higher. If it is a 600 watt wet/dry vacuum, then the volts drop much quicker. So to me, the voltmeter is definitely telling me how much I am "withdrawing from my account," and how quickly. To say it is only a general inference would imply that sometimes it would drop significantly quicker or slower, but that this drop would have nothing to do with the load being put ont he battery, or for how long (which makes absolutely no sense). In other words, you seem to be saying that there is no correlation between the formula which you gave in your post for a 200 am p hour battery, and what a voltmeter would say. This I can't believe. I would think that if I discharged a "fresh, fully charged" 200 amp hour battery with a 200 watt load for 6 hours, that the voltmeter would consistently read near 12.00 volts, give or take a tiny amount. That would be more than a mere inference to my mind.
What I need is a practical answer to a practical question. And that question is still: "If one withdraws one's battery(s) down to 12.4 volts (measured with all loads off, and all charging off), has one discharged them (approximately) 20%? . . . or is the correct measurement taken with the loads on?"
If voltage is so irrelevant, then why do so many people use it, and why do chargers often have voltmeters built in?