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As I said in my previous posting, there are several variables that come into play. Perhaps a little additional info. To clarify my example, my refer is a standard 120v apartment size fridge. It runs off my inverter. The observed current draws I quoted were the DC #'s when the fridge was running. Therefore the inverter behaviour comes into play (ie it will draw more current to compensate for lower input voltage to try to maintain as close as it can to 110/120VAC output), as well as the fact that the fridge motor is a 120VAC motor. Refrigeration likely makes up the single biggest draw most people have, so it serves as a good example. But notwithstanding the actual current use from a technical perspective, James' comments are quite correct.....if I might interpret them loosely.
For a purely resistive load, and ignoring any potential resistance change with temperature for the sake of simplicity, you are quite correct John, in that for fixed resistance, reduced voltage will result in lower current flow. This would be the end of it if we were only discussing, say, DC lights (lower voltage just means dimmer light) and other 12v applications. But the two big gobblers of power are the refer (as mentioned above) and an inverter (and any/all that hangs off it). If you step back from the batteries for a second (figuratively speaking) and look at the load from the "accomplishment of work" perspective, James' comments re the fridge are right on (this is the loose interpretation part of James' "smart load"). ie Power required is that which is required to keep the inside of a fridge at a certain temp. If the cooling ability is degraded by the input voltage degradation, regardless of whether the current use is less or not, the fridge will have to run longer to mai!
ntain the temperature. Another example of this is our own experience of boiling water in the electric kettle to make coffee in the morning (I know, I know, all you non-"massive battery bank with inverter" owners are horrified at the thought of honking 145A out of your batteries for several minutes to boil water, or running a 1500W hair drier to dry the dog who fell into the chuck and who sleeps in bed with you.....but I assure you, it is quite common and acceptable). To boil x amount of water (or dry x amount of dog) is going to take a y amount of energy (Joules or watts or calories, take your pick). The water doesn't care if it is high voltage low current, low voltage high current, or low voltage low current (but for longer) making up the watts, but your batteries do. The higher the voltage you can maintain, the less TOTAL current (eg Amp-hours) you will need to accomplish some arbitrary amount of "work", and it is amp-hours that are managed in your battery usage. This is tru!
e in a non-converted use of battery power, but much more so when u
in all this discussion that throws the wrench into the works?).
I hope that helped rather than further confused...
Regards.....Randy
mv Island Time
----- Original Message -----
From: <>
To: <>; liveaboard <>
Sent: August 22, 1999 16:40
Subject: Re: lv-ab: Battery Charging: How Charge Large House Bank?
> Randy,
>
> I am at a loss to understand your experience. If the values you cite for your
> reefer are correct, it may have a microprocessor which controls the current
> to the motor so that the machine operates consistently over the typical range
> of voltage which cruisers experience.
>
> The fact remains that if you reduce voltage to a lamp, it will dim. A good
> example is the dimming of headlights when the starter is engaged and pulls
> the battery voltage down.The light output is reduced by virtue of less energy
> (watts) consumed. You'll recall that a load doesn't really "draw" current;
> it's the EMF that forces the electrons to flow through the load. If you
> reduce the EMF, the current will decrease.
>
> I spent years aboard ships which had 120/240VDC systems, and if we increased
> the generator voltage, the current increased, the motors speeded up and the
> lights got brighter; with decreased voltage, the ammeters declined, the
> motors slowed and the lights dimmed. I don't see how anyone can argue with
> that.
>
> Kind regards,
>
> John
> "Truelove"
>
> In a message dated 8/20/1999 5:46:07 PM Eastern Daylight Time,
> writes:
>
> > This is visible to me as my batteries discharge and the fridge, say, is
> running. With a full charge showing something in
> > the neighbourhood of 13V, it draws 9.2A. When my voltage has dropped to 12
> volts (at around 50-60% discharged), its
> > drawing over 10.
>
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