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Author Topic: Simple Electronics Question
MacManKrisK

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Icon 5 posted June 28, 2009 18:48      Profile for MacManKrisK     Send New Private Message       Edit/Delete Post   Reply With Quote 
How soon I forget...

I'm trying to use two 12V batteries and get 12V through one circuit and 24V though another circuit. I know there's a reason this won't work, but I can't remember why. Here's a simple diagram.

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Both batteries are 12V. A voltmeter on points 1 and 2 should give me a 24V potential difference. A voltmeter on points 3 and 4 should give me 12V potential difference. Right?

But I know that there's a reason this shouldn't work. My brain just can't remember why.

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Posts: 2331 | From: Southwest Michigan, USA | Registered: Oct 2001  |  IP: Logged
The Famous Druid

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Icon 1 posted June 28, 2009 20:11      Profile for The Famous Druid     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by MacManKrisK:
A voltmeter on points 3 and 4 should give me 12V potential difference. Right?

disclaimer: I'm afraid of electrickery, it doesn't like me.


A voltmeter at points 3 and 4 will 'see' the voltages from both batteries, the +12V from one, and the -12V from the other. They'll cancel out.

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Ashitaka

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Icon 1 posted June 28, 2009 23:29      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
buy a third batterie

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TheMoMan
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Icon 1 posted June 29, 2009 03:00      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
_____________________ MacManKrisK __

____ Actually this should work, provided small enough loads. If the load at 3/4 is too big it will draw down the one battery and thus cause the over all voltage to sag. Charging both batteries to cap. will be difficult because one will be discharged further than the other. However for small loads and with large enough Batteries this is a voltage divider.

____ What are you using this for?

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Benjamin Franklin,

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Ugurcan

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Icon 1 posted June 29, 2009 03:47      Profile for Ugurcan   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
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I'm no expert, but voltagewise this should work. The shortcoming is, the resistivity of the components should be exactly the same.

Note: Here's a cool Java based circuit simulator. http://www.falstad.com/circuit/

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MacManKrisK

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Icon 1 posted June 29, 2009 11:15      Profile for MacManKrisK     Send New Private Message       Edit/Delete Post   Reply With Quote 
The load that should be 12V (between 3 and 4) is the electrical system of my VW and would vary in current depending on if the radio or dome light, etc. was on. The load between 1 and 2 is a UPS that I would like to use as an inverter and would also vary depending on the load on the UPS.

Also, this is a "static" setup, something that I would connect once stationary and remove before starting the vehicle and going somewhere so charging it in this configuration is moot.

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"Buy low, sell high
get rich and you still die"


Posts: 2331 | From: Southwest Michigan, USA | Registered: Oct 2001  |  IP: Logged
TheMoMan
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Icon 1 posted June 29, 2009 13:42      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
________________ MacManKrisK _________________

____ MMKK this is the way I would approach this, get a cigarette lighter extension cord cut off the female end. Build a battery box for your second battery, You will also need a double pole double throw switch. Functionally you want to charge with the cord plugged in going down the road, when you need the 24V you would throw the switch, and the two output terminals would go high.

____ I am going to create a chart to wire your box with. Look at the back of the DPDT switch, you will see six terminals.

A---B
C---D
E---F

____ From the cigarette lighter cord Plus or Positive to both A and F. Minus or Negative to B and output Negative.

____ C goes to the portable Battery + Positive and D goes to the portable battery Negative

____ E will go to Positive out.

____ I would caution against having the switch in the charge position during engine start, as the portable battery will try to jump start the engine.

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Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
MacManKrisK

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Icon 1 posted June 30, 2009 18:46      Profile for MacManKrisK     Send New Private Message       Edit/Delete Post   Reply With Quote 
MoMan: I drew out the circuit you described.... genius... sheer genius!

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"Buy low, sell high
get rich and you still die"


Posts: 2331 | From: Southwest Michigan, USA | Registered: Oct 2001  |  IP: Logged
Cap'n Vic

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Icon 1 posted June 30, 2009 19:03      Profile for Cap'n Vic     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by MacManKrisK:
How soon I forget...

I'm trying to use two 12V batteries and get 12V through one circuit and 24V though another circuit. I know there's a reason this won't work, but I can't remember why.

The reason that won't work is because as soon as you hook your voltmeter across 1 and 2 you've completed a short circuit. *Bang!* You hook up ammeters (not voltmeters) in series.

Anywho, see below. With this setup you get two supplys, independently fused, the switch can be ganged or not depending on the application, as Mo indicated. Use an ammeter (in series in each branch) to determine the max current pulled, obviously that will happen as devices like starters, compressors or electric bongs are initialized. Figure out the max and double the fuse size, carry spares (of slightly higher ratings) until you've determined the maximum safe fuse. Of course, burning your bus would suck and any electrical Jerry rigging may void your insurance.


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TheMoMan
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Icon 1 posted July 01, 2009 04:19      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
__________________ MacManKrisK______________________

___ Not really, just dreamed that up to have bright dim on the tractor as the bulbs are single element. For the switch I use a DPDT center off.

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Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged


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