TrailManor Owner's Forum - View Single Post - Charging the battery with a generator: AC or DC?

**Bill** · 12-12-2003, 03:16 PM

The plug-in digital voltmeter is a good idea. With experience, you will learn what it means with your particular setup, and that is the best lesson.

To the best of my knowledge, there is not a good hard-and-fast correlation between battery voltage and state of charge. This situation arises from several factors.

Battery voltage depends on battery temperature. A cold battery will register a lower voltage than a warm battery, even at the same state of charge.
Battery voltage depends on the amount of current that is being drawn at the moment.
Battery voltage depends on the amount of current that has recently been drawn - this is the recovery time that you referred to.
Battery voltage depends on what kind of battery you have - flooded vs gel-cell, etc.
Battery voltage is pretty meaningless if the battery is being charged.
The total range of battery voltage, from fully discharged to fully charged, is very small. Thus even a small change in voltage, from measurement error or from any of the above factors, may represent a large change in state of charge.
State of charge (empty, full, half, etc) is not a well defined term. Think of it this way. It is pretty clear when a water bucket is full, half full, or empty. But stuff that bucket with a big sponge, and it is not nearly as clear. If you turn the bucket over and pour out all the water, is the bucket empty? Probably not - you can always squeeze a bit more water out of the sponge. Now slosh in a bunch of water until the bucket overflows. Is the bucket full? Probably not - you can squeeze in a bit more if you do it slowly and let the sponge absorb it.

What you really want to know is stuff like "When the digital voltmeter reads 12.5 volts, my furnace will run for another hour, but not much longer." You learn this from experience, not from a chart or formula.

I suppose that, as a general rule, a resting battery may present something on the order of 13-13.5 volts when fully charged, and 12 volts or so as it dies. If you search around the web, you will find several rules of thumb like this - and they don't agree.

Bill

12-12-2003, 03:16 PM	#4
Bill Site Team Join Date: Feb 2003 Location: The mountains of Scottsdale, AZ, and the beaches of Maine Posts: 10,112	Re:Charging the battery with a generator: AC or DC? The plug-in digital voltmeter is a good idea. With experience, you will learn what it means with your particular setup, and that is the best lesson. To the best of my knowledge, there is not a good hard-and-fast correlation between battery voltage and state of charge. This situation arises from several factors. Battery voltage depends on battery temperature. A cold battery will register a lower voltage than a warm battery, even at the same state of charge. Battery voltage depends on the amount of current that is being drawn at the moment. Battery voltage depends on the amount of current that has recently been drawn - this is the recovery time that you referred to. Battery voltage depends on what kind of battery you have - flooded vs gel-cell, etc. Battery voltage is pretty meaningless if the battery is being charged. The total range of battery voltage, from fully discharged to fully charged, is very small. Thus even a small change in voltage, from measurement error or from any of the above factors, may represent a large change in state of charge. State of charge (empty, full, half, etc) is not a well defined term. Think of it this way. It is pretty clear when a water bucket is full, half full, or empty. But stuff that bucket with a big sponge, and it is not nearly as clear. If you turn the bucket over and pour out all the water, is the bucket empty? Probably not - you can always squeeze a bit more water out of the sponge. Now slosh in a bunch of water until the bucket overflows. Is the bucket full? Probably not - you can squeeze in a bit more if you do it slowly and let the sponge absorb it. What you really want to know is stuff like "When the digital voltmeter reads 12.5 volts, my furnace will run for another hour, but not much longer." You learn this from experience, not from a chart or formula. I suppose that, as a general rule, a resting battery may present something on the order of 13-13.5 volts when fully charged, and 12 volts or so as it dies. If you search around the web, you will find several rules of thumb like this - and they don't agree. Bill