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 10th May 2004, 18:48 #1 (permalink) Thread Starter   Join Date: Feb 2004 Location: East of Mars Posts: 32 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.
 10th May 2004, 19:13 #2 (permalink) Join Date: Aug 2000 Location: UK Posts: 1,909 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!
 11th May 2004, 02:58 #3 (permalink) Cunning Artificer   Join Date: Jun 2001 Location: The spiritual home of DeHavilland Age: 70 Posts: 3,044 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 06:09.
 11th May 2004, 03:44 #4 (permalink) Moderator   Join Date: Mar 2003 Location: Orlando, Florida Age: 62 Posts: 2,587 I go with the Black Sheep. Parallel = same volts, double current. Series = double volts, same current. Both = double power (watts)
11th May 2004, 07:21   #5 (permalink)

Join Date: May 2001
Location: Frankfurt, Germany
Posts: 143
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 &%\$\$+~"!

 11th May 2004, 08:41 #6 (permalink) Join Date: Apr 2002 Location: Bohol, Philippines Posts: 134 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.
 11th May 2004, 12:21 #7 (permalink) Join Date: Aug 2000 Location: UK Posts: 1,909 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)?
 11th May 2004, 12:53 #8 (permalink) Moderator   Join Date: Mar 2003 Location: Orlando, Florida Age: 62 Posts: 2,587 Yep - badly phrased (and old names for units). "Capacity" doubled - but still the same Voltage.
 11th May 2004, 13:07 #9 (permalink) Join Date: Aug 2000 Location: UK Posts: 1,909 .........you mean current (amps)?
 11th May 2004, 13:07 #10 (permalink) Join Date: Oct 2000 Location: N. Europe Posts: 436 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
 12th May 2004, 03:50 #11 (permalink) Thread Starter   Join Date: Feb 2004 Location: East of Mars Posts: 32 Thanks you all for helping me out on this one, it was just what I was looking for, medium salsa
 11th Jun 2004, 00:38 #12 (permalink) Props are for boats!   Join Date: Oct 2000 Location: An Asian Hub Age: 49 Posts: 989 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
 11th Jun 2004, 15:11 #13 (permalink) Join Date: Mar 2000 Location: Arizona USA Posts: 8,601 A battery cart for a KingAir using two 15amp hour 12v batteries in series would be just slightly ah...deficient.
 12th Jun 2004, 23:11 #14 (permalink) Props are for boats!   Join Date: Oct 2000 Location: An Asian Hub Age: 49 Posts: 989 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