Electric Blanket

[Raz]

Originally Posted by David Tilston

But the point was that 0.1 VDC has to push through resistance, both internal, and external. What is being measured at the terminals is the battery voltage. The resistance of the battery is incorporated in the sense of voltage drop, (rise?) under current flow, but the remaining 0.1 VDC wont cause much current flow, and the internal resistance of the battery is still in the circuit.

As an exaggerated example, if the wire has a resistance of 5 ohms, and the battery also has a resistance of 5 ohms. With a voltage differential of 0.1 VDC, the current flow should be 0.01 ADC. VS 0.02 ADC if the battery has 0 ohms.

Am I incorrect in this?

The internal resistance is inside battery. All your voltage measurements are outside the battery. I= .1/ 5. There is a voltage drop across the internal resistance but it's part of this circuit.

[David Tilston]

So, in effect, are you are saying that the battery has no resistance? Is this because it is charging?

If I short out a 1.5 VDC carbon battery, (for a safe example) with a 4/0 wire, (very low resistance). I am certain that the internal resistance of the battery will limit the current flow to something that will not melt the wire. I suspect the voltage at the battery terminals would drop to near zero since the voltage drop is all internal. Why would that not apply when charging as well?

Why is the internal resistance not limiting the current when charging?

Not trying to be argumentative, just trying to understand.

[Raz]

Originally Posted by David Tilston

So, in effect, are you are saying that the battery has no resistance? Is this because it is charging?

If I short out a 1.5 VDC carbon battery, (for a safe example) with a 4/0 wire, (very low resistance). I am certain that the internal resistance of the battery will limit the current flow to something that will not melt the wire. I suspect the voltage at the battery terminals would drop to near zero since the voltage drop is all internal. Why would that not apply when charging as well?

Why is the internal resistance not limiting the current when charging?

Not trying to be argumentative, just trying to understand.

The battery does have an internal resistance. As current flows it reduces the voltage at the terminals. It's that reduced voltage that we use in our calculation. So the effect of the internal resistance is to make terminal voltage smaller. Making the terminal voltage smaller makes the total current smaller.

I think you are putting internal resistance after the terminals and not before. Don't confuse E terminal with E thevenin. Now there's a name you've not heard in years.

[blodn1]

"E thevenin" ... puts hands over ears and says "lalalala" very loudly.

Thought I last heard that term when my electrical courses were over. And no, I don't remember what it is - probably never really knew :-)

[Night Sailor]

Originally Posted by P. Raz

The battery does have an internal resistance. As current flows it reduces the voltage at the terminals. It's that reduced voltage that we use in our calculation. So the effect of the internal resistance is to make terminal voltage smaller. Making the terminal voltage smaller makes the total current smaller.

I think you are putting internal resistance after the terminals and not before. Don't confuse E terminal with E thevenin. Now there's a name you've not heard in years.

Now he is Raz'ing you. Ha!

I know my battery is charged by a number of factors, one of which is the battery temperature. The temp dropped after a while. It has had several days of charging. I used the internal charger at night and the solar panels during the day.

It was no longer accepting much of anything except a float voltage--the charging voltages met the spec for the battery. The temperature drop coincided with the moving to the fully charged state. I gave it a bit extra time since the last little bit of charge takes longer.

In any event, I should have my plumbing work done soon and I'll turn the heat and water back on, and take it off of shore power to see how it all works out.

[Night Sailor]

I am pulling about 1 amp per blanket level per side. So a setting of 5 pulls 5 amps, but it is a pulsed load, not a 100% duty cycle at that level.

[Night Sailor]

The electric blanket is working great. Now that it has warmed up some I can rely on the blanket almost entirely and turn off the furnace completely or use it to warm things up and then shut it down.

Using the blanket on 5 for one side I am seeing very little power consumption because it is offset by savings on the furnace and blower. Even on cold nights--it hit 25 recently, with the furnace on I can balance the usage of both and the extra power draw has been minimal.

[carlkeigley]

So glad it's working out.