# Senior Moment



## 107558 (Oct 10, 2007)

OK I'll admit it, I'm having a "Senior Moment" and I can't remember the answer.

If you connect two 12v batteries in parallel it doubles the current but the voltage remains the same.

If you connect two 12v batteries in series the doubles the voltage but the current remains the same.

It sounds correct and makes sense (to me anyway) but I just can't remember if this is correct.

I suppose I better start eating boiled fish in preparation


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## DABurleigh (May 9, 2005)

Well, it depends on the load. I get the sentiment. Strictly true for voltage; not strictly true for current. Maybe if you give the context of the question.

Dave


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## geraldandannie (Jun 4, 2006)

JQL said:


> If you connect two 12v batteries in parallel it doubles the current but the voltage remains the same.


Hmm. In practice, the current is restricted by the size of the cables, and trying to take too much current from a battery will damage it. What it will do is double (more or less) the capacity of the battery.

Gerald


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## 107558 (Oct 10, 2007)

DABurleigh said:


> Well, it depends on the load. I get the sentiment. Strictly true for voltage; not strictly true for current. Maybe if you give the context of the question.
> 
> Dave


I'm thinking (not very successfully  ) of changing my batteries for 6v ones...


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## DABurleigh (May 9, 2005)

Then get the biggest capacity 6V deep cycle ones that will fit, stick two in series for 12V systems, and kiss any battery matching problems goodbye.

Dave


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## rayc (Jun 3, 2008)

John, If you change "current" for "capacity" {ampere hours} in your question then for practical purposes you are correct {assuming both batteries are the same type}.

If you connect two 12v batteries in parallel it doubles the *capacity* but the voltage remains the same. ( as Gerald said).

If you connect two 12v batteries in series it doubles the voltage but the *capacity* remains the same.

Ray


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## DABurleigh (May 9, 2005)

I'm afraid I don't see that. If you connect two batteries, whether in series or in parallel, the capacity is the sum of both batteries - it is all usable energy 

Dave


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## Penquin (Oct 15, 2007)

Might as well get involved - although not a physicist or electronics expert (I teach biology - its much more fun), as far as I am aware the reason why 2 x 12v batteries in series doubles the voltage is that effectively you have twice as many cells pushing out the electrons, the reason why the capacity stays the same is that you are now delivering it at twice the voltage and it might have something to do with Ohm's Law (real memories of A level Physics now from 295 years ago - or so it seems),

I seem to remember that Ohm's Law was V=IR where V = voltage, I = current and R = resistance.

Not sure any of that is correct or relevant but it has impressed me and my wife - so now soemone can shatter my illusions of a brain that still has some capacity (its connected in series with hers - twice the get up and go power but the same capacity [as hers])


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## rayc (Jun 3, 2008)

Bear with me as I have been down Poole Quay for a drink. Penguin is correct that if you join batteries in series you have doubled the cells in a serial fashion i.e. each cell is approx 2 volts and six per battery - join two batteries in series and you have 12 x 2v = 24v.The AH capacity is still that of a single battery but double the voltage. 

If you join them in parallel the voltage is still 12v but you have provided two cells for each of the 2 volts thus doubling the AH capacity.

Ray


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## DABurleigh (May 9, 2005)

Patent it as a perpetual motion machine and you'll be rich.


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## geraldandannie (Jun 4, 2006)

Same energy values in both cases (e.g. volts X amp hr), but double the voltage reduces your amp hr capacity at that voltage.

If a piece of equipment needs 60 watts to run, using W = V X I means that at 12V, it would consume 5 A, and a capacity of 100 AHr of batteries would give you 20 hrs. At 24V, it would consume 2.5A, but in that configuration, you would have only 50 AHr of capacity, giving you .... 20 hrs of use. Which is what DAB is saying (I think).

In practice, an operating voltage of 12V is best for a wider choice of equipment.

Gerald


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## Penquin (Oct 15, 2007)

Wow I'm impressed!

Whern I posted earlier I fully expected to be proved wrong (physics was never an easy subject for a non-mathematician) since there are lots of forum members with excellent knowledge bases much more recently accessed than mine.

So it appears I might have been correct in my understanding, thanks for the positive reinforcement - we all have senior moments but it is nice to be able to get such helpful advice so readily.

On those high notes I had better stoke up the zimmer frame and warm up the cocoa - its time for the bedsox to be donned for us (Question Time is on and its great to see basic points being debated with such opposing views by such "important"`people (Jerry Springer, David Davies, Nigel Farage, Hilary Benn, et al).


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## stendec (Jan 26, 2007)

geraldandannie said:


> If a piece of equipment needs 60 watts to run, using W = V X I means that at 12V, it would consume 5 A, and a capacity of 100 AHr of batteries would give you 20 hrs. At 24V, it would consume 2.5A, but in that configuration, you would have only 50 AHr of capacity, giving you .... 20 hrs of use.


Two 100 AHr batteries in series would give 24 volts with the same available current therefore if you draw half the current you double the time.
100 divided by 2.5 equals 40 hours.


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## 107558 (Oct 10, 2007)

Many thanks to everyone.

My "Senior Moment" has past helped by the consumption of a quantity of Vin de Plonk.

I was looking at installing a number of 6v 220ah traction batteries and was trying to work out how many I would need to last about a week, theoretically without sun and off EHU, without giving up my creature comforts.

The answer - about 6 8O 

I know I don't have room for 6 so back to the drawing board.


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## hilldweller (Mar 8, 2008)

JQL said:


> I know I don't have room for 6 so back to the drawing board.


That is a strange answer. With 6 you can get 36V, 18V or 6V none of which is any use to you.


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## geraldandannie (Jun 4, 2006)

stendec said:


> Two 100 AHr batteries in series would give 24 volts with the same available current therefore if you draw half the current you double the time.


But the two batteries in series would only have the same _capacity_ as a single battery.

I think my example was a bit misleading.

Say you have 2, 120AHr batteries.

In parallel, you have 12V output, so a piece of equipment drawing 12W (i.e. 1 Amp) would run for 240 hours.

In series, you have 24V output, so the same equipment would draw 0.5 Amp. However, you are drawing 0.5 Amp from both batteries simultaneously, so your equipment would run for 240 hours (120 AHr for each / 0.5).

Picture 2 packs of paper, 120 sheets each, one on top of the other. They need to be shredded, so an office worker puts each one into the shredder at a time, which takes a second per sheet, total 240 sheets X 1 second = 240 second.

Another office worker has another 2 packs to shred, but they feed 2 sheets at a time. Unfortunately, the shredder takes twice as long to shred the 2 sheets, so although there are only 120 'shreddings', each takes twice as long (2 seconds), which equals 240 seconds.

This is the 3rd analogy I've tried to think of  And it's still not that clear 

Gerald


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## DABurleigh (May 9, 2005)

The source of confusion is simply because ampere-hours is used as a convenient surrogate for energy. This downside of this convenience is that it is only valid at one assumed voltage.

Energy = current * time * voltage

(Joules = Amps * Seconds * Volts)

Hence my reference to perpetual motion, as liberties were being taken with the law on the conservation of energy 

If JQL has the volume and mass for 4, then a nice configuration would be 2 pairs of 6V in series, each pair connected in parallel. Because a parallel connection would be required, the batteries should be matched.

Dave


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## stendec (Jan 26, 2007)

Gerald, I think we agree but are looking at it from different angles. Two identical batteries produce twice the power of one whether in series or parallel. In series 100 amps at 24 volts is the same power as in parallel 200 amps at 12 volts, ie. 100 x 24 or 200 x 12 = 2400 watts. (in theory)

John's best bet would be to use 4 of his 6 volt batteries in series/parallel, two parallel pairs in series which will deliver 12 volts at 440 amps. This assuming that all batteries are in the same (good) condition. (more or less)


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## geraldandannie (Jun 4, 2006)

stendec said:


> Gerald, I think we agree but are looking at it from different angles.


I think so too :wink:

I agree - a configuration delivering 12V would be best, by far.

Gerald


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## rayc (Jun 3, 2008)

6 x 6v batteries can be connected in a serial / parallel configuration to give 12v. Make 3 banks of 2 batteries per bank in series and then parallel them together.


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## DABurleigh (May 9, 2005)

I never ceased to be surprised when so many successive posts in an MHF thread say the same thing. Is it just me? Do people simply transmit without first receiving?

Dave


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## stendec (Jan 26, 2007)

DABurleigh said:


> I never ceased to be surprised when so many successive posts in an MHF thread say the same thing. Is it just me? Do people simply transmit without first receiving?
> 
> Dave


Sorry Dave, I started my reply to Gerald before yours came up and with various interruptions this end yours got there first. I did check beforehand.


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## eddied (May 9, 2005)

Ciao tutti, all very interesting, but I don't need a senior moment to get confused. I have understood that linking 2 x 6v. batteries in seies will in effect give me 12v. output, but what about the recharging gear such as alternator/EHU/solar panels. etc. Does that still work OK to put juice back into the 6v. batteries thus linked? thanks and saluti, eddied


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## DABurleigh (May 9, 2005)

Yes.

A 6V battery is simply a 12V one divided into two, with 3 cells each. 

Dave


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## sallytrafic (Jan 17, 2006)

glad I stayed out of this one


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## 107558 (Oct 10, 2007)

sallytrafic said:


> glad I stayed out of this one


Cop out! :lol:

The idea, as has been expounded by DAB and others, is to connect them in series and parallel to produce 12v and 660amps - ish.


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