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Batteries in Series or Parallel

12/18/2011 10:02 AM

I am building a 24VDC 1200AH battery bank. To obtain this voltage and amp hours I can place 12Volt or 6Volt or 4Volt or 2Volt batteries in series parallel groups. Any one of the voltages when connected for 24VDC will give me the identical 1200AH capacity.

Consulting with fellow engineers I hear different ideas about series/parallel arrangements. Some say that for my case if I use two sets of 12 2Volt batteries arranged in two series strings which are then paralleled I will have the best arrangement as far as cell equalization goes. They say that if I used 12Volt batteries, which implies 12 pairs with 2 in series in each pair and then all them in parallel there is a higher chance of unequal charging.

Then some argue in favor of the 12Volt batteries saying that with a single string of 2Volt batteries if 1 unit fails I would lose the whole string of 12. They also say that using 12V batteries I can remove and add units when the system is up and running whereas if I used 2 long series strings this would be nearly impossible and dangerous.

Then the argument continues with somebody saying that the hardware for connecting 2 series strings of 2Volt batteries is much simpler and economic than using 12 sets of 2 12Volt batteries.

Then along comes another opinion. One friend said he ordered a system and when the batteries arrived one of them was damaged. Since he was using 12V batteries he was able to complete the job with just 2 batteries missing. When the replacement battery arrived it was simple for him to add them to the up and running system. (He did an external recharge first to avoid and heavy current flow at the moment of connecting)

And so it goes, on and on.

I am more inclined to go with what is electrically better and that seems to be the 2 strings of 2V batteries.

So I am faced with making a decision. Do I buy 24 2V batteries at 600AH each or do I buy 24 12V batteries at 100AH each

What is this forums opinion regarding this subject. What experiences have you had with this question?

Thanks

John

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#1

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 10:25 AM

Be careful when paralleling batteries that enormous current does not flow from a fully charged into a fully discharged battery.

Also look into the Peukert effect for how many a-h you can reasonably expect.

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#4
In reply to #1

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 11:08 AM

Yes, I agree. I have built dozens of systems and when using many batteries in parallel its easier to remove one and add another as long as the one to be added is fully charged. When working on a live system I prefer to only do changes when the battery bank is neither charging or discharging. At my house we have a solar system with grid powered chargers to help if needed. At night time the batteries are powering the inverter which powers the house. In the day time when the solar panels have recharged the battery bank, usually by around 0200PM the system runs neutral, ie, no current flows into or out of the battery bank. This is when a hot swap is best done.

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#2

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 10:26 AM
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#6
In reply to #2

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 11:23 AM

I went to that site but nobody addresses my question which is written in my opening statement. Whats better 24 2V cells or 24 12volt batteries?

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#3

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 10:26 AM
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#5
In reply to #3

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 11:15 AM

Thanks. I forgot to mention that we require sealed AGM batteries or VRLA. I selected LIBERTY LS2-600 based on a history of excellent results with them. Also I have a previously built rack so sizing is critical.

The real issue I am asking help about is the series versus parallel question and charging equilibrium between cells.

So summing up should I buy 24 LS2-600, or 24 LS12-100. Both fit the rack and produce identical AH so which set up is better?

Thanks

John

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#8
In reply to #5

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 11:43 AM

24 LS2-600 is better.

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#10
In reply to #8

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 12:31 PM

Thank You, this is what I suspected.

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#16
In reply to #10

Re: BATTERIES IN SERIES OR PARALLEL

12/19/2011 12:41 PM

suspected or expected?

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#17
In reply to #16

Re: BATTERIES IN SERIES OR PARALLEL

12/19/2011 1:03 PM

Could be either.

I'd say suspected is proper in this context.

Where's Eugene the editor?

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#18
In reply to #17

Re: BATTERIES IN SERIES OR PARALLEL

12/19/2011 1:10 PM

Surely I am not!

Am an ESL student

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#21
In reply to #18

Re: BATTERIES IN SERIES OR PARALLEL

12/20/2011 8:21 AM

I don't understand what you mean. You are not the subject here, JOHN H VAN ALLEN is the subject.

Try again.

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#11
In reply to #5

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 11:13 PM

Presuming that the fork lift manufacturer engineers have already covered this ground, the newest fork lifts use 2 volt cells: 18 or 24 each, in series.

If one cell goes bad, it is cheaper and easier to replace than a multi cell unit.

Of course these are huge batteries, each weighing several hundred pounds, but I would think the same logic would apply.

They also have a balancing circuit on each cell to ensure equal charge of each cell.When each cell reaches the designed voltage, it bypasses that cell.

I have seen the equalizing circuit on line, but cannot remember where.

Good luck!

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#19
In reply to #5

Re: BATTERIES IN SERIES OR PARALLEL

12/19/2011 5:55 PM

Please note that all lead-acid batteries produce 2 volts per cell. The LS12-100 has six cells in series contained inside one jar, producing 12 volts from one end of the jar to the other. The LS2-600 has one cell in the same size jar and therefore has, per cell, 6 times the electrolyte and 6 times the plate area, which produces 6 times the current at 1/6 of the voltage compared to the LS12. With either design you will have the twelve cells in series to produce 24 volts. Those twelve cells will either be in two 6-cell LS12 jars, or twelve 1-cell LS2 jars.

Since you prefer VRLA batteries, consider that you will be unable to test the electrolyte in each cell. The only indicator you will have on the health of individual cells is the voltage across each cell. The LS2, having one cell per jar, gives access to the terminals for each cell, which allows you to check the voltage on each cell. The LS12 only has terminals on the first and last cell in the jar, so you are limited to testing across the 6-cell jar.

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#7

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 11:33 AM

Your choice of going with 2 strings of 12 2volt cells is the best arrangement. You have the advantage of monitoring each cell and replacement if needed. Also it is the preferred method employed on board marine vessels including submarines and ashore on railroad coaches.

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#9

Re: BATTERIES IN SERIES OR PARALLEL

12/18/2011 12:17 PM

I'd advice depending on maximum discharge level you expect from your system at normal duty. If it's expected to get discharged near 50% or worse, specially if you can't apply a topping charge every 5 to 10 discharges for balancing purposes, I'd definitely go for the 2V shells (even if it will be more expensive) The main reason is that with this (or worse) pattern one or more cells WILL fail long before others and it's less cost effective to replace a 12V pack than a single cell. S.M.

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#12

Re: Batteries in Series or Parallel

12/18/2011 11:29 PM

John,

I would support the previous posters who recommend using 2V cells. You have better control of battery maintenance and monitoring with individual cells.

If you are interested, there is at least one company offering a battery monitoring system that looks at cell voltages and compares it to a baseline voltage taken when the cell was new. It seems to me that some also do other checks such as impressing high voltage pulses across each battery to check internal resistance and the like, but these checks are aimed more at the huge battery banks that reside on uninterruptilble power supplies for data centers and the like. Those batteries spend most of their life fully charged, waiting to go under load and need to be able to supply current when called upon.

Jon.

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#13

Re: Batteries in Series or Parallel

12/19/2011 2:05 AM

Most "electrically correct" as you put it:

1 string of qty12 1200Ah 2V flooded cells connected in series.

However if one cell fails you lose the lot.

What do you require this autonomy for?

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#14
In reply to #13

Re: Batteries in Series or Parallel

12/19/2011 2:23 AM

Talking to myself....

I should have read the thread more carefully.

Go with #8's advice.

24x600/2

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#15

Re: Batteries in Series or Parallel

12/19/2011 12:38 PM

Parallel connections of any power source is not recommended as sophisticated circuitry is needed for load-sharing. And in battery case Charging will also be an additional problem.

If you cannot get the the single source of end requirement then make banks of required voltage of max AH batteries of max voltage available.

As minimum external connections, so min connecting points voltage drop and maintenance. The internal connections are more reliable due to professional techo used.

Series connection is recommended but if you can get the required capacity sources.

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#20
In reply to #15

Re: Batteries in Series or Parallel

12/20/2011 12:01 AM

Properly implemented parallel connection of batteries does not require any sophisticated circuitry or elaborate charging protocols.

Adequate conductor dimensioning and reliable, robust connection are the key requirements for interconnection of batteries either in parallel (to achieve desired autonomy) or series (to achieve desired voltage)

Monobloc batteries have shorter lives than batteries of discreet cells. The failure mode of monoblocs is most often an internal open circuit ie a failure of the internal series connections. Monoblocs are not easily or reliably repairable. Discreet cells with external connections are maintainable and the connections are easily maintained and reliably repairable.

A 24V 1200Ah monobloc battery would be awesome to behold.

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#22
In reply to #20

Re: Batteries in Series or Parallel

12/20/2011 9:10 AM

When I build it I will send you a foto. Thank you and all the rest for your opinions. I had suspected that the 2V cells were the way to go but your opinions have reinforced this. My reasoning was that using 24 12volt 100Ah batteries I would have 24 x 6 = 144 cells to manage where as with 24 2Volt cells I just have 24 cells to manage. The chances of getting 144 cells all at identical voltages is much lower than just 24 cells. When I worked with the Telephone company we had 2volt cells the size of refrigerators set up for 48VDC which was why the phones never died even during black outs.

By the way the LS2-600 actually have 2 cells per jar and they are also sold as LS4-300 which are identical. You just reconfigure the bus bars for 4 volts instead of 2. So if I go with the LIBERTY´s I will actually have 48 cells not 24 but still good. I am also looking at the LIFELINE GPL-31T-2V which are individual cells and so with them I would indeed have only 24 cells. I have a lot of experience with LIBERTY and dont know anything about the LIFELINE´s. Have any of you tried them.

My system uses an EXELTECH MX inverter 5KW which runs a portion of a luxury home here in the Dominican Republic. It will soon have a solar array of about 5KW capacity controlled by an OutBack MPPT charge controller. This controller will start and stop a 40KW generator using its programmable auxiliary outputs. I set them up for no lower than 50% discharge which prolongs the life of the battery bank. Then the battery bank will get recharged via 5 IOTA DLS27-40 chargers. This runs the residence like a giant UPS so they never see any black outs or even transfer glitches since the inverter runs all the time. In this country we see black outs every day!

I custom build these systems on a special rack made in my shop. I will try to attach a picture of the one at my house which runs partly on solar power. In the picture I have 6 LS12-100 on the lower tier and 8 AC Delco on the upper tier. The LIBERTY´s lasted 12 years. The AC Delco were very bad. At another location I had one explode and since then I have a hearing loss at around 4000Hz, hehehe. You can also see in the picture the MPPT charge controller which gets power from my 1KW solar array. The EXELTECH inverter on the left can hold 5 cards at 1KW per card. I have 3 cards for our house and run everything except air conditioners. We use 12V SUNDANZER refrigerators and freezers.

Thanks to All of you

John

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#23
In reply to #22

Re: Batteries in Series or Parallel

12/20/2011 9:22 AM

Very nice.

Where's all the clutter? You must have tarted it up just for the photo, right?

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#26
In reply to #23

Re: Batteries in Series or Parallel

12/20/2011 3:06 PM

Actyally theres more clutter now, hehehe

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#24
In reply to #22

Re: Batteries in Series or Parallel

12/20/2011 10:04 AM

Sweet.

When will you cover those busbars?

If you short those bars while standing at the bench you may well be singing at a pitch that you can't hear anymore as well.

Safety mate......

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#27
In reply to #24

Re: Batteries in Series or Parallel

12/20/2011 3:09 PM

thats why my power room is just for me. But you are absolutely right and in the jobs I do for offices they are covered with clear plexiglass.

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#25
In reply to #22

Why 24V?

12/20/2011 12:24 PM

John,

Why did you choose 24V for the system voltage? Wouldn't 32 or 48V be a little nicer to keep the amps and "IsquaredR" losses down?

This is asked not knowing the availability of MPPT chargers and inverters in those higher voltages....

Jon.

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#28
In reply to #25

Re: Why 24V?

12/20/2011 3:11 PM

A lot of times I come across system that started small, like mine, at 12V or 24V then gradually grew. so you get stuck at the starting voltage.

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#29
In reply to #22

Re: Batteries in Series or Parallel

12/22/2011 5:09 PM

The most interesting item in the photo is your Tektronix 453/454 (lol) S.M.

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#30
In reply to #29

Re: Batteries in Series or Parallel

12/22/2011 5:58 PM

I call that scope ¨the tank¨ Its a real beauty and built to last forever. Too bad its so big and heavy.

Again, thanks to all of you and have a great Xmas.

John

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