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Author
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Topic: Simple Electronics Question
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MacManKrisK
Gold Hearted SuperFan!
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posted June 28, 2009 18:48
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.
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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"Buy low, sell high
get rich and you still die"
Posts: 2325 | From: Southwest Michigan, USA | Registered: Oct 2001
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The Famous Druid
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posted June 28, 2009 20:11
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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If you watch 'The History Of NASA' backwards, it's about a space agency that has no manned spaceflight capability, then does low-orbit flights, then lands on the Moon.
Posts: 10311 | From: Melbourne, Australia | Registered: Oct 2002
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Ashitaka
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posted June 28, 2009 23:29
buy a third batterie
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TheMoMan
BlabberMouth, a Blabber Odyssey
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posted June 29, 2009 03:00
_____________________ 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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If it don't glow it ain't Ham Radio
Posts: 5071 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002
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Ugurcan
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posted June 29, 2009 03:47
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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"Constants aren't, Variables won't..."
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MacManKrisK
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posted June 29, 2009 11:15
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: 2325 | From: Southwest Michigan, USA | Registered: Oct 2001
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TheMoMan
BlabberMouth, a Blabber Odyssey
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posted June 29, 2009 13:42
________________ 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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If it don't glow it ain't Ham Radio
Posts: 5071 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002
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Cap'n Vic
Member # 1477
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posted June 30, 2009 19:03
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
BlabberMouth, a Blabber Odyssey
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posted July 01, 2009 04:19
__________________ 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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If it don't glow it ain't Ham Radio
Posts: 5071 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002
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