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jrcrilly
Refractor wienie no more

Reged: 04/30/03

Loc: NE Ohio
Re: Powering your mount with Pre-charged AAs. [Re: Jon Isaacs]
#6047099 - 08/26/13 02:24 PM

Quote:

Each battery is putting out 5.75 amps if it wired in series. The circuit diagram is simple, all the current passes through each battery.

Yes.

The root of this confusion is the inappropriate mixing of the terms "Amps" and "Amp-hours". If you want to compare energy density in differing voltage conditions it is necessary to use a measurement that includes voltage; in this case Watt-hours is appropriate. For round numbers, let's consider a stack of 2 Amp-hour, 1.5V cells. Each has a capacity of 3 Watt-hours. As Jon and others have explained, a series stack of ten 1.5V, 2 Amp-hour cells will yield 15 volts at 2 Amp-hours, or 30 Watt-hours. That's correct, and we could have gotten there by simply multiplying 3 Watt-hours X 10. 15 Volts at 10 Amp-hours would require 150 Watt-hours. You are only using 10 cells of 3 Watt-hours each so there's only enough energy there for 30.

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tecmage
Carpal Tunnel

Reged: 01/13/10

Loc: Glenview, IL
Re: Powering your mount with Pre-charged AAs. [Re: jrcrilly]
#6047114 - 08/26/13 02:36 PM

Agreed. By definition, elements in series have the same current flowing THROUGH them, and elements in parallel have the same voltage ACROSS them. If I put 10, 1.5V batteries in parallel, the voltage across them is 1.5V. If I put those same batteries in series, and measure voltage across them, I get 15V.

You have to be careful with multimeters. They have a little internal resistance to prevent the meter from changing the values you're trying to read. I usually get one student a semester that blows the fuse on a meter because they used it wrong.

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GeneT
Ely Kid

Reged: 11/07/08

Loc: South Texas
Re: Powering your mount with Pre-charged AAs. [Re: Eddgie]
#6047933 - 08/26/13 11:14 PM

Great information Eddgie!
GeneT

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John Carruthers
Skiprat

Reged: 02/02/07

Loc: Kent, UK
Re: Powering your mount with Pre-charged AAs. [Re: tecmage]
#6048164 - 08/27/13 03:17 AM

Best demo I've seen is the early electric submarines where batteries were switched between "Group up" and "Group down", short and fast vs long and slow.

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Midnight Dan
Postmaster

Reged: 01/23/08

Loc: Hilton, NY, Yellow Zone (Bortl...
Re: Powering your mount with Pre-charged AAs. [Re: Eddgie]
#6048276 - 08/27/13 07:23 AM

Quote:

Of course someone implied earlier that you could not get more than 2 amps out of the pack if the batteries were wired in series, so maybe my meter is incorrect or something.

No, you can easily get more than 2 amps out the batteries as your meter is correctly showing.

Amps is a measure of how much current is flowing, not the capacity of the batteries. The capacity is measured in amp-hours, at a particular voltage, and is found by measuring how long the batteries can continue to deliver a certain amount of current.

If you set up your batteries with a resistor so they are delivering say 5 amps, and they last for 1/2 hour, then the capacity is 5 times 1/2, or 2.5 amp hours.

And Jon is correct. If you put any number of 2 amp hour cells together in series, you still only have 2 amp hours, you just have it at a higher voltage.

-Dan

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Powering your mount with Pre-charged AAs. [Re: John Carruthers]
#6048311 - 08/27/13 07:58 AM

Quote:

Best demo I've seen is the early electric submarines where batteries were switched between "Group up" and "Group down", short and fast vs long and slow.

John:

Were they controlling the speed of the motors with this series/parallel approach?

In the old days, diesel trucks sometimes had 24 volt starters and four 6 volt batteries that were charged as 2 pairs of 6 volt wired in series. To start they were all in series.

Another example of a series/parallel device is the Marx Bank, these are used to generate high voltage pulses, they are charged in parallel and discharged in series. We have some at work, 150 KiloVolt and 450 KiloVolt units used to produce flash X-rays.

Jon

Edited by Jon Isaacs (08/27/13 08:02 AM)

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