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Old 10th May 2004, 17:48   #1 (permalink)
 
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Connecting 12volt batteries together

I have a question about connecting batteries,

If you have say two 12volt 15amp/hr batteries does,

Batt 1 pos ¡V> Batt 2 pos
Batt 2 neg --> Batt 1 pos
= 12v 30amp/hr is this called parallel or series?

What about,

Batt 1 pos „³ Batt 2 neg
Batt 2 pos „³ Batt 1 neg
= 24 volt how many amps?

Seems to me every one is an expert on this one and no one really knows !!!

Hope to clear it up here.

just a quick note, for some reason iV and ,,3 should be little connecting arrows -- >.

must have been a glitch when I cut/paste.
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Old 10th May 2004, 18:13   #2 (permalink)
 
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Pos>Pos, Neg>Neg is called 'in parallel'. It's the way you should 'jump' start cars. It will maintain the voltage at 12v no matter how many batteries you connect this way 'in parallel'.
If you connect Batt 1 Neg> Batt 2 Pos, then across the other terminals, you will have the sum of the voltages of each battery. Starts to become a very powerful effect. Therefore I would expect the current (ie amps) to be the sum of each battery.
School physics was back in the 60s!
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Old 11th May 2004, 01:58   #3 (permalink)
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Not so for the series condition Notso.

Pos-Pos and Neg-Neg does connect the batteries in parallel, where the terminal voltage is the same as the individual battery terminal voltages and the output current is the sum of the currents in each battery. Thus two 12 volt 15 ampre-hour batteries connected in parallel produce a 12 volt 30 ampere hour power pack - delivering 30 amperes for one hour at a terminal voltage of 12 volts.

Connect the same two batteries in series by connecting one battery's positive terminal to the other's negative terminal and connecting the remaining positive and negative terminals to the load, produces a 24 volt power pack. The same current flows in both batteries, hence the capacity remains at 15 ampere hours - delivering 15 amperes for one hour at a terminal voltage of 24 volts.

Note that output power is doubled in both cases as 2V x A or V x 2A both equal 2(V x A)

A battery generally consists of a number of cells connected in series thus there are 6 cells in a 12 volt lead-acid car battery and 12 cells in a 24 volt aircraft battery; so a 24 volt battery is effectively the same as two 12 volt batteries connected in series within the same case.

Finally- and please, don't try this at home. Its very, very dangerous!
______________________

Batt 1 pos --> Batt 2 neg
Batt 2 pos --> Batt 1 neg
= 24 volt how many amps?"
______________________

The answer is lots and lots - until we reach the final melt down!!!

Last edited by Blacksheep; 11th May 2004 at 05:09.
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Old 11th May 2004, 02:44   #4 (permalink)


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I go with the Black Sheep.

Parallel = same volts, double current.

Series = double volts, same current.

Both = double power (watts)
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Old 11th May 2004, 06:21   #5 (permalink)
 
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Yes - full marks to Blacksheep from a former physics and electronics lecturer.

Another little note on the second condition:

Quote:
Batt 1 pos --> Batt 2 neg
Batt 2 pos --> Batt 1 neg
This is a short circuit - although the batteries are rated at 12v 30Ah - (They can supply 30A at 12V for an hour) the cranking power of the battery will be much higher (the current the battery can supply for a short duration). This could well be in the order of 300A here.

In a short circuit a phenominal amount of current will flow which will cause things to get extremely hot - you might even have an explosion on your hands, quicker than you can say "Oh &%$$+~"!
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Old 11th May 2004, 07:41   #6 (permalink)
 
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I think that keygrip has clouded the issue somewhat by his comment 'same volts....'.
Surely by putting the same volts across a resistor you must get the same current?
Isn't it the 'capacity' i.e. the ampere-hours and not the amps that are doubled?
Or perhaps Mr. Ohm was wrong after all.
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Old 11th May 2004, 11:21   #7 (permalink)
 
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Keygrip, I know a battery is in effect 6 x 2v cells in series, so connecting 2 batteries in series is like simply making one longer battery twice the size. But, if you have just one 2v cell, are you saying the current (amps) is the same as if you had 2 whole batteries connected in series (if according to the logic, connecting 2 batteries in series still only keeps the current the same)?
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Old 11th May 2004, 11:53   #8 (permalink)


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Yep - badly phrased (and old names for units).

"Capacity" doubled - but still the same Voltage.
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Old 11th May 2004, 12:07   #9 (permalink)
 
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.........you mean current (amps)?
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Old 11th May 2004, 12:07   #10 (permalink)
ft
 
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Lesse... as I had important things to do I thought I’d spell it out more in detail.

If you have a 10V (U=10 V) battery capable of providing 10 mAh, it will give you 1 mA (I = 1/1000 A) for 10 hours. This will happen if you short it with a resistance of U/I = 10 / (1/1000) ohms = 10 kohms. The power developed will be P = U * I = 10 * 1/1000 W = 10 mW.

Now add another battery. If you disconnect the +ve lead from bat 1 and connect it to the +ve side of bat 2, then add another lead from the +ve side of bat 1 to the -ve side of bat 2, you have them vired in series.

|--------------[ 10 kohm ] -------------|
|----[ - bat 1 + ] ----- [ - bat 2 + ] ----|

You’ll get U = 20 V over the resistance, with twice the current through the resistance and the batteries (I = 20/10 000 A = 2 mA), meaning you’ll use your batteries at twice the rate, giving you (10/1000)/(2/1000) = 5 hours of use. The power turned into heat in the resistor will be P = U * I = 20*2/1000 = 40 mW. Each battery will provide 20 mW of power (P=10 * 2/1000 W).

Connect the +ve poles of the batteries together and the -ve sides of the batteries together and add the resistor over the poles of bat 1. Now you have the batteries wired in parallell.

|----[ 10 kohm ]------|
|----[ - bat 1 + ] -----|
|----[ - bat 2 + ] -----|

You’ll have U = 10 V over the resistance, again giving you a current of I = U/R = 10/10000 = 1 mA. This current is split between the batteries, drawing 0.5 mA from each battery. At 0.5 mA, the batteries will last for (10/1000) / (0.5/1000) = 20 hours. The power generated will again be P = 10*1/1000 W = 10 mW.

Clear as mud?

Cheers,
Fred
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Old 12th May 2004, 02:50   #11 (permalink)
 
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Thanks you all for helping me out on this one, it was just what I was looking for,

medium salsa
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Old 10th Jun 2004, 23:38   #12 (permalink)
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Are you making a battery start cart for your King Air Salsa?
If so pm me I can send a diagram you can use.


Sheep
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Old 11th Jun 2004, 14:11   #13 (permalink)
 
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A battery cart for a KingAir using two 15amp hour 12v batteries in series would be just slightly ah...deficient.
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Old 12th Jun 2004, 22:11   #14 (permalink)
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411A,
Yes indeed that would be hopeless. We use 4 light truck batteries ie. N70s 2 pairs in series plus 4volt section on a forklift battery in series. 12V + 12V +4V= 28VOLTS WALA!Works very well, just have to keep servicing it meaning check electrolyte level and SG of the cells regualary especially when in tropic environs.
Also another trap for new players is get a good charger that cuts off when fully charged otherwise you may boil your cart dry.
Sheep
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