Lead-Acid Batteries Connections

Correct!!!!!!!!!

I would cross-connect, so each parallel row is connected, and each series row as well.

They should stay equalized as long as they are not discharged too far.

This is a lot of power. Both voltage and amrperage. Be careful!!!!!!!!

Any working battery pack must be maintained appropriately, do your homework.

While it is certainly possible to series twenty 12 volt batteries to get 240 volts DC, you cannot parallel them to get extended amp-hours. 50 AH is what you will get no matter which way you wire the batteries. If you need 200 AH, you must use 200 AH rated batteries.

E. Wiz,

I am trying to understand the logic or rather illogic of your answer. It does not compute!

One series string of 20 X 12V @ 50AH gives 240 VDC at 50 AH. When one parallels three more series strings of 240 VDC @ 50 AH you get 150 AH more in any battery bank I have worked with. Ideally all of the batteries have to have the same charge/discharge characteristics to help keep them balanced but that is about it.

Please explain!

Happy New Year!
TT3

WRONG!!!

Please read #15 below instead of jumping to conclusions first. Without the proper definitions, an accurate response cannot be given.

WRONG!!! 4x50=200 (or did, when I went to school)

Stu.

PLEASE....read through the entire thread before commenting, we've had enough commenting on old parts of the thread now. Errors and misstatements have been already been acknowledged. I think that most of the data that Kilomark needs has already been given. Thank you all for caring enough to reply.

No illogic involved, gentleman. You cannot add amp-hour ratings together like voltage or current. Yes, if you parallel the batteries, you will increase the total amperage available but that does NOT INCREASE amp-hours. Those are not additive in any way, shape or form.

For example, if said 50 amp-hour batteries are discharged, from an initial fully charged condition, at the constant rate of 5 amps, the battery will reach it normal 'discharged' minimum terminal voltage in ten hours, 10 hours * 5 Amps = 50 amp-hours. If you pull 10 amps, then the battery will discharge in half the time, 5 hours and so on. Paralleling batteries will multiply the amount (assuming equally charged capacities) of available current by n - batteries but the amp-hours still remain the same. This is a very basic formula for calculating required battery capacity. Battery manufacturers provide curves to show the various discharge times at various loads in their data sheets.

As I stated, no matter how you series-parallel batteries, the amp-hours remain the same, only the current/voltage capacity changes.

So, You are telling us that if we put one fully charged 50 AH battery in parallel with another fully charged 50 AH battery of the same type, we will not be able to draw 50 Ah from each battery for a total of 100 Ah?

From my observation, if I put two identical batteries in parallel, they will share what ever load I put on them equally within the limits of internal, wiring and terminal resistances.

(Sometimes I just enjoy a good rolling in the mud tussle! See the screen name.)

Have FUN!
Turbotroll3

You are right turbotroll3,

I think the Wiz might have been at the wine gums over the xmas period.

I have used series parralel 12 V 180Ah batteries for welding purposes, they work fine with nothing but a diode and fuses on the end of each bank to limit them.

Hello Turbotroll3:

And all in this post.

Very interesting and maybe the only 'real' way to 'prove' this is to set up the 20 batteries and see what happens?

Anyway, my money is on the man in the red corner! Throw your money at me, come on now who do you think will 'win' red or blue?

I usually learn stuff on CR4. Not sure what to think about this slog?!

Take care and a greta new year to all..................

This and your first answer are utterly and totally wrong. Further, the correct answer can be found in perhaps 1000 or more locations on the internet, as well as in the most basic electronics and electrical texts.

Aside from the basic misconception you are promoting, your contention that "If you pull 10 amps then the battery will discharge in half the time..." is also incorrect. In lead acid batteries, Peukert's effect means that the actual AH delivered is not independent of rate. Therefore, if the battery is rated 50 AH at 10 hours, it will deliver less than 50AH at a 10 amp (5 hour) rate, and even less yet at a 20 amp (2.5 hour) rate.

To argue that your basic contention is true, you would have to argue that the law conservation of energy, a central tenet of physics, is incorrect. That would be a tough sell.

Alas, poor Youric, read on, you're behind the times and backwards as well. We've already hashed out Peukert's effect.

Albeit incorrectly, on your part.

The easyest would naturally be to connect 20x 12v batteries in series......Very expensive! jurieSA

Wiz is correct if you look at it this way:The total wattage is greater with the series-parallel connection, but the total amp hours per battery is the same.20 hours times 50 amp hours equals 1000 amp hours.The total desired is 200 amp hours.Currents divide in parallel, and voltages divide in series connections. IF all batteries are the same voltage, then each battery will contribute an equal share of current,but on an individual basis, each one can only contribute 50 amp hours.Likewise with the voltage. If you connect a 10 amp load to a 50 amp hour battery, it will theoretically be dead in 5 hours.If you connect a 10 amp load to 2ea. 50 amp hour batteries in parallel,each battery will contribute 5 amps to the load(assuming equal voltages for each battery). And it will take twice as long to kill the battery. As the number of parallel batteries increases, the load on each decreases in proportion. A note of caution:Each battery in series must carry the full total current of the output of that string.Size all interconnect wires for this factor. HTRN

GA.

GA from me too.

The solution is: 4 parallel strings from 20 serial connected batteries in every string. The total number of 12V 50AH batteries are 80.

Good morning gentlemen.

Sorry, I misstated my position, I neglected to define what I was talking about.

First, the questioner did not state his actual discharge current which has some bearing on this discussion. His battery arrangement is of a dangerous nature.

I will try to clarify what I had intended to say in the first place.

The amp-hour rating of a typical battery, in this case, 12 volts at 50 AH, is at a discharge rate of .05C (2.5A), 'C' being the amp-hour rating. This is predicated on the final discharge cell voltage. This cell voltage varies depending on the discharge current (i.e. load). At the nominal 0.05C rate, this is 1.75 volts and is 100% of rated AH capacity and takes 20 hours to arrive at. At a discharge rate of 0.4C, the cell voltage would be ~1.725 volts and you'd get approximately 80% capacity.

However, at a 1C discharge rate, the final terminal voltage is 1.50 volts and the battery will have only produced 50% of rated capacity (50 A for 30 minutes). Continuing to discharge at this rate to zero volts will only produce a maximum of approximately 75% of rated capacity because the rapidly declining terminal voltage will reduce the load current to a trickle.

What I should have included is that unless you are discharging these batteries at the 0.05C rate (or close to it), which I doubt is the case here, you cannot get the requested AH capacity unless you provide even more paralleling of batteries. The battery curves must be consulted in order to determine how much actual amp-hour capacity at a given discharge rate is available.

This data is available in most battery manufacturer's data sheets or technical manuals.

The two gentlemen who initially took me to task were correct, in that I did not make myself clear, their statements are only correct under certain conditions, not all conditions. No, I had not imbibed, only tired from a long day, I'll give you a chuckle from me for it.

I would be most interested in the application that requires a 240 volt, 200AH battery stack. Theoretically, it is certainly possible but implementing it would be complicated, dangerous and quite expensive.

Me thinks thou doth protest too much!

The reason to parallel batteries is to increase the current capacity and to spread the discharge current across multiple sources. This will reduce the load per individual battery and when there are sufficient cells paralleled, allow one to achieve or better that magic 0.05C.

As to the danger of constructing such an assembly, I humbly submit that only ordinary care such as is required with any current/voltage source of similar capacity is necessary. This is not an atomic bomb.

If the batteries are AGM types, they may not even explode when given a dead short. Common lead/acid wet cells would likely give one sprays of steaming acid to dodge in addition to the flying bits of casing, separator and plate.

If the batteries are low series resistance lithium types, a short circuit would likely be very interesting to watch from a respectful distance, with hearing protection!

I have also done and said stupid things when I was tired. That is why I seldom give my opinion on important subjects when I am out of it for what ever reason. Besides, who ever listens to trolls anyway?

To Turbotroll3:

I doth not protest! I am quoting from tech manuals from the battery manufacturers themselves. Yes, you can parallel all you want BUT you only increase cost, complexity and the problems! The idea is to KISS (keep it simple stupid....as we say here in the states), the more you add to a 'solution', the more likely the solution is going to have more problems and fail.

If the Questioner has access to heavier duty batteries, such as telecom rated, then a 10 hour (0.1C) instead of 20 hour (0.05C) rating applies (so does a greater cost). There is nothing magical about these numbers, they are standard battery terms which applies no matter what.

Additionally, charging becomes a problem. A series string is limited to no more than ten 12 volt batteries (which is within this application's limits) but, per manufacturer's data, series charging requires equal length interconnect wires of the same gauge (size) and there is also the issue of imbalance in the cells, which will become greater with use. The batteries must be all of the same type and age. Further, it is recommended that no more than two batteries be series charged by one charger, to minimize differences in batteries, with a constant current source and zener diode regulation across each battery or group of batteries.

Parallel strings have less of a problem, as the charger current will divide equally among the cells by the time every one is fully charged. The series/parallel strings must be isolated during charging from each other.

This is all referenced to the battery manufacturers, as an example, you can go to www.Power-Sonic.com and check out their tech manual, you will find that it agrees completely with what I'm saying.

Since the 'solution', as put forward, requires a total of 80 batteries, we're talking a real mess with all of the associated chances for trouble. I disagree about the possible level of danger here, unless the correct fusing is put in place (which raises additional resistance imbalances and connection points), a cascade failure could result, particularly in the parallel strings but the series strings are not immune to it either. This much power is dangerous, period and is not to be minimized in any way. I've seen battery banks blow and it ain't pretty. Yes, you might get lucky and get away without anything serious happening beyond battery failure, but you're tempting fate by being too lax in the safety department.

The simplest solution would be to use twenty 12 volt 200 AH batteries but that was not his question. I could expound further but I hope it is not necessary.

The amp-hour rating of a typical battery, in this case, 12 volts at 50 AH, is at a discharge rate of .05C (2.5A), 'C' being the amp-hour rating.

Wrong. Batteries are rated by the manufacturer, and different manufacturers use different ratings. Many use a .1c rate (10 hour rate), because it can better approximate real usage.

However, at a 1C discharge rate, the final terminal voltage is 1.50 volts and the battery will have only produced 50% of rated capacity (50 A for 30 minutes). Continuing to discharge at this rate to zero volts will only produce a maximum of approximately 75% of rated capacity because the rapidly declining terminal voltage will reduce the load current to a trickle.

This is inaccurate too. The voltage of a "dead" lead acid battery is typically 10 - 11 volts, depending upon construction (AGM, gel, flooded, etc.) Tests at different discharge rates are cut off at a consistent voltage, not at different voltages for different discharge rates. Also, one never discharges an LA battery to 0 volts, and doing so (aside from nullifying your" warranty) does not deliver 75% of rated capacity if a 1C rate delivers 50% of rated capacity. And obviously, there is no reasonable or practical reason to do this: things designed to run on 12 volts do not run well at all on 8V or 6V or 4V or 2V or 1V.

A Genesis XE95 AGM battery is fairly typical: it is nominally rated 95 AH at a ten hour rate. It will deliver 9.6 amps for ten hours after which is will be "dead" at a voltage of 10.02 V at 25 degrees C. At a one hour rate, it will deliver 73.5 amps (77% of its ten hour rate performance -- quite a difference from the 50% you are suggesting.) If Genesis chose to rate this battery at a 20 hour rate, it would deliver 5.2 amps for 20 hours for a rating of 104 AH. Thus their use of a 10 hour rating would be considered a bit more conservative that the often used 10 hour rate.

I would be most interested in the application that requires a 240 volt, 200AH battery stack. Theoretically, it is certainly possible but implementing it would be complicated, dangerous and quite expensive.

It is not only theoretically possible but very common. 240 volts is obviously a dangerous voltage, but it is the norm that grid tie inverter systems are close to or equal in DC voltage to the house supply AC voltage. A 240V 200AH (48kWh) pack is a little smaller (in energy storage capacity) than the pack in the Tesla electric car. The original poster's system would be about an order of magnitude less complicated than the Tesla pack in which 6000 individual cells are used, an a series-parallel arrangement.

Using 200 AH batteries would be easier and slightly cheaper than using 50 AH batteries, but the principal of using parallelled series strings is extremely common and well accepted. In either case, a battery management system is strongly recommended (i.e., simply using a series only arrangement does not obviate the need for one).

My dear sir, you are arguing with statements which can be found in the technical manuals of battery manufacturers, not mine. Yes, some batteries are specified at 0.1C rates (I mentioned this already, please read more carefully) and others are specified at even greater discharge rates than 0.1C for their 'normal' rating, I covered both standard charge rates.

The 1.0C discharge rate statement you are arguing about is not inaccurate, it also is plainly stated in the tech manuals. The 50% rule pertains to a cell voltage of between 1.50 and 1.75 volts depending on the rate of discharge and is very near to the point of cell voltage dropping precipitously, as I stated. The Genesis battery you are referring to may have a different set of specifications but that does not apply to all lead-acid batteries by any means.

You are talking about a specific battery from Genesis, the rest of us are speaking in general battery terms. The 'Questioner' did not specify what battery type nor manufacturer he had access to, therefor you argument is not sanguine.

Tesla is using Lithium -ion cells, not part of this discussion. We are discussing general terms with the 'Questioner' and properly imparting the danger of working with such battery stacks when one has not the required experience. This has also been covered in other parts of this thread.

My dear sir, you are arguing with statements which can be found in the technical manuals of battery manufacturers, not mine.

Perhaps you should look at information from other manufacturers, then. No reputable manufacturer would say: "Continuing to discharge at this rate to zero volts will only produce a maximum of approximately 75% of rated capacity because the rapidly declining terminal voltage will reduce the load current to a trickle."

First, the sentence is circuitous: you can't "discharge at this rate" and also discharge at a declining rate at which you "reduce the load current to a trickle." The words "at this rate" would have to mean at the 1C rate, in this case 50 amps.

Battery manufacturers go to some length to maintain discharge rates in testing -- they don't simply hook up a light bulb and see how long it takes for the battery to go dead, for example.

You are talking about a specific battery from Genesis, the rest of us are speaking in general battery terms.

I suspect that the original poster understands that there are many types and manufacturers of lead acid batteries, and that I was using the Genesis as an illustration that there are loads of batteries that do not operate along the lines you describe. If you can present technical literature from a battery manufacturer that shows that a 1C discharge rate reduces capacity to 50% of nominal, then that would give the OP another perspective. I couldn't find a battery that would fit that curve in those for which I have technical data. More importantly, it would be helpful for the OP to provide specs from batteries with a range of Peukert's factors, so that he can see that effect in action. The Genesis data shows the effects. If you have manufacturer's test data that supports your contention, then the OP might find that useful, too.

In this thread, you have written: While it is certainly possible to series twenty 12 volt batteries to get 240 volts DC, you cannot parallel them to get extended amp-hours.

Then, after having been corrected, you again claimed: You cannot add amp-hour ratings together like voltage or current. Yes, if you parallel the batteries, you will increase the total amperage available but that does NOT INCREASE amp-hours. Those are not additive in any way, shape or form.

Apparently, you now understand that you were completely incorrect. To regain some credibility, perhaps you could provide a link to battery manufacturer's site which states: "Continuing to discharge at this rate to zero volts will only produce a maximum of approximately 75% of rated capacity because the rapidly declining terminal voltage will reduce the load current to a trickle."

Of course I am toying with you, partly because it is rare and funny to have someone present info so obviously wrong, be corrected, and than have the same individual present the same incorrect information. It's all the more funny when the individual in question considers himself a "wiz" in the subject matter. But I am sure I have made at least as many mistakes in my life as you have in yours, so take my ribbing with a grain of salt. Welcome to CR4.

For one thing, I provided a link to a "creditable' source twice, which for your lack of notice, I will provide here again, www.power-sonic.com. Not that they are the only source of this data, as you say, you can check with other manufacturers and find the same thing. Each manufacturer provides specific data based on their own line of batteries, since you like to quote Genesis, that is fine, just don't try to use it as the end all of data.

You are either misinterpreting or not understanding those statements which you have quoted again, here in your last link. Check out the tech manuals, you will find that what I said is correct. The 50% discharge capacity figure is correct and the 75% of rated capacity figure is exactly what is given.

Most real world loads do not discharge at a constant rate, they consist of an actual dynamic load which, if left connected to the battery will drain it until the load no longer functions at all. The 75% figure is based on how much capacity could actually be drained out of a battery by a load, such as a resistor, which will continue to drain power to zero volts. I am well aware that most loads, of which I presume you are speaking of, will not function to the point of completely draining a battery to zero volts nor did I say that, that was your presumption.

Are you telling me that you have never left the lights on in your car and completely drained the battery to zero? If you are, you are in a very small group.

I presumed nothing about Kilomark's requirements beyond what he initially said. You obviously are, arriving after he posted additional information.

I admitted to my initial flub, but you are still wrong about your claims in your last posting. Check out Power-Sonic and you will find the data to which I am referring. I certainly hope that you do not think that Power-Sonic is not a reputable battery supplier, since the facts otherwise bear that out.

I think this thread has been more than adequately answered quite well before you decided to post, so let's end this nonsense and quit wasting time Youric.

Good evening to all and to all a good night.

I gave you a GA, so you are back to 0.....

Good post(s)

I do not know why you would want to do this and I think recharging will quickly become a nightmare for you........its also very dangerous to your health in several ways :-

1) Possible explosion if ventilation not correctly carried out

2) Electrocution

I wish you well, but be very very careful.

Best way to be careful is NOT to ever build such a construction.....

the problem will not be the charging it will be the sulfate of the plates as most 50 AH batteries have small plates you want to use deep cycle batteries so as there are deep plates to allow for a better charge.

we use packs like this for big jobs and charge different then most on the pack.

every 2 bateries have a feed as this way there will be no dead cells in the middle. you also may want to look at the charger that can output high freq as it is better for the batteries.

as per your dc will ill you yes it will most 60 hurtz power will

Richard

Capblanc:

Mind your manners! I didn't know you presumed to know more about this subject than the manufacturers, what I said is fact, backed up by the manufacturers. Yes amp-hours add but that doesn't mean you're going to get the 'rated' AH in your application.

No matter how many batteries you parallel, you will only get the specified AH percentage capacities, that I noted, of the total parallel AH ratings. Only the rated battery discharge rate of either .05C or 0.1C (depending on battery type) will produce the rated AH discharge capacity, any faster discharge rate will decrease the total AH available, period! At the rated AH capacity (1C), you will get 50% of the total no matter how many batteries you hang on it. Two parallel 50 AH (100 AH total) batteries discharged at 1C rate will only give you 50 AH to the discharged state. Three paralleled will only give you half of 150 AH or 75 AH available. Now tell me that this doesn't matter! If you need 200 AH capacity at a 1C discharge rate, then you will need a total of 400 AH in parallel to get there. The one thing we don't know in all of this, is just what the actual discharge rate is in this application but I rather doubt it will be .05C!

I don't recall seeing a 240 volt stack being built from 80 12 volt batteries to begin with. Yes, I've seen huge battery stacks that would very likely dwarf anything you come up with but they weren't made up of dozens of batteries. It is inefficient, costly and lousy engineering practice. Nor do I recall any one of us explicitly saying that such large battery stacks do not exist, they certainly do exist here in fairly large quantities.

Every remark I made was backed up by manufacturer recommendations and engineering practices, sorry you think that was rambling but it does pertain to the matter!

Batteries are not perfect nor identical, any balance of cell voltage/current will change over the life of the battery, nothing you do will change that, but the recommendations will help to insure the longest possible life span under the given conditions.

Unless our 'Questioner' will volunteer more information beyond the little that was given, I do not see the need for any further chit-chat.

Thank you all for adding to the conversation.

The questioner never mentioned exceeding the discharge rate, so 200Ah still holds true.

In fact, nothing beyond 240 volts, 200 AH and 12 v, 50 AH batteries were mentioned, therefore no one can 'assume' what the actual discharge rate is. Given the question, it would appear that the potential user of this stack does not know too much about building multiple battery stacks or or correct specification of their needs. 12v, 50AH batteries would be a very poor solution from an engineering point of view. Even IF we may assume that he has a quantity of these batteries available, a stack built with 80 batteries would still be a poor solution.

I think that Capblanc would agree with this assessment.

Thanks for the vast information you guys have given me. I have learned a lot from you. Thanks to CR4.

I'm trying to build a power backup that should provide 240 volts at 200 Amps. I was thing about connecting 80 12volt/50AH batteries in series/parallel connection. I now understand all the problems and cost that this might course me.

I will have a steam engine that will power a generator to produce electricity. When the steam engines stops for any reasons, them the backup should come on line. The generator will produce 10,000 watts of power. I will be pulling about 2500 watts of power to run electrical equipment

With all the information I gathered from you guys, I have come to these conclusions.

They are:

1) I'm going to limit my power to 240 volts @100 AH By using 20 12 volt 100 AH batteries. In doing so, I will be avoiding connecting the batteries in parallel (Although I will be reducing the Amp-Hours by half as before).

2) The batteries will be kept in a well-ventilated room where fresh air will enter the room from flood level and there will be an exhaust fan to pull out the air from the room through the roof. The room will be kept clean. I know that the exhaust air will contain enough hydrogen and oxygen to make an explosion.

3) The switching unit will be in another room that is isolated from the room that is housing the batteries. Only qualified technicians will operate this system. No animal or unauthorized person will be allowed in both rooms.

4) Proper wire size, fuses and other protective devices will be used to avoid damage to the batteries and electrocution to the technicians. All connections will be kept proper.

5) I will keep the discharge rate at 0.1C. There might be some surges but I believe the rate will around 0.1C most of the time. I believe this will be enough for what I want to use the backup for.

6) To have a constant current going through each battery when charging, I will need to have zener diode across each battery.

Are my conclusions a good one? Are there other things I need to know?

get the catalytic battery caps. they use a tiny piece of platinized screen to make the H2 combine with O2 to make water in a gassing situation. They reduce boiloff by 90% or more

Hi Kilomark,

You don' say how you will use this power and what will be your load. Would I be correct in guessing it is for a domestic supply not connected to the national grid? If so your approach is probably not the best unless you have some ideas which you haven't told us about. What equipment will you be running on this reserve bank?

From your conclusions it is clear that you have put a great deal of thought into the proposed system but it is also clear you still have some gaps in your knowledge. Taking your conclusions in order:-

1)

a. There is no need to avoid paralleling, it is perfectly normal and safe.

b. As noted by EWiz your bank of 100Ah will only give you 50% of that figure unless you over discharge the batteries which will cause premature failure.

2) Your batteries should not be gassing so heavily as to have an explosive atmosphere in the battery room. If they are then your charging system is wrong. If the room is adequately sized and well vented then forced air ventilation should not be necessary. How are expecting to charge your bank of batteries? Do you have a 240V charger?

3) Good practice

4) Ditto

5) There is no problem in discharging faster than 0.1C but be aware that the discharge is not linearly proportional (100Ah_C20 =20hrs @ 5A but it does NOT = 5hrs @ 20A ! At C5 the same battery would probably be only 75~80Ah.) You will need to be careful not to over discharge your batteries as battery replacement is an expensive pastime.

6) You do NOT need any zener diodes. The laws of physics (Kirchhoff's 1^st law) dictate that all cells will receive the same current.

We need more information to be more specific with advice.

Regards

Chas

Excellent reply, I'm in full agreement. A little more information would be of great help in giving you the best and safest system.

If you do need more power than you can get from 100 AH batteries, you certainly can parallel another stack or use higher AH rated batteries (which may be cheaper).

Do you have access to sealed lead-acid batteries? They do not out gas unless something goes wrong, such as gross overcharging.

0.5C or 0.6C is quite safe and will afford you a reasonable backup time if the load needs extra power, you just have to pay attention to the discharge rates and time.

I would suggest going to the www.power-sonic.com website and download their tech manual on lead batteries, it is well written and should be understandable to a layman. If you need further answers, we're here for you.

Chas, we're in-sync, a good thing.

EWiz

1) I'm going to limit my power to 240 volts @100 AH By using 20 12 volt 100 AH batteries. In doing so, I will be avoiding connecting the batteries in parallel (Although I will be reducing the Amp-Hours by half as before).

I hope you are aware that limiting your battery pack size does not necessarily limit your power. A 100AH 240V pack (24 kWh) can deliver power at a 10C rate briefly: 240,000 watts. (See the Genesis link I provided: the XE 95 can deliver 5000A very briefly -- you can imagine that a tool dropped across certain terminals could cause an explosion you might not live through.)

Also I hope you are aware that 240 V at 100 AH is not a measure of power, but of energy.

Also be aware that (despite what another post seems to suggest) there is nothing dangerous or unusual about paralleling batteries. The dangers to humans are largely those of electrocution, fires and explosions, and a 240 V 25AH pack (let alone a bigger one) can cause all these. For electrocution, 240 volts is a real and present danger, even if the pack is only 1 AH. (A few milliamps across the chest is all you need, and high voltage is what gives you the required milliamps, given that your skin is often of fairly high resistance.)

Finally, be aware that 200AH batteries are available. If you need to run your 2500 watts for 19.2 hours, per your original plan, this could be simply accomplished with 240V worth of 200 AH batteries. (Note also that 12V is not necessarily the best format for these batteries: look into batteries from 2-8 V too -- these are often seen in installations like you are planning.)

2) The batteries will be kept in a well-ventilated room where fresh air will enter the room from flood level and there will be an exhaust fan to pull out the air from the room through the roof. The room will be kept clean. I know that the exhaust air will contain enough hydrogen and oxygen to make an explosion.

You might want to evaluate the need for explosion proof components in this room.

5) I will keep the discharge rate at 0.1C. There might be some surges but I believe the rate will around 0.1C most of the time. I believe this will be enough for what I want to use the backup for.

Sounds good. Be aware that most batteries will handle far more current than this routinely. For instance, the typical 50 AH car battery will have a cold cranking amps rating of 600 amps or so. A deep cycle (more appropriate for your usage) battery might have about 1/2 or 2/3 this peak rating.

6) To have a constant current going through each battery when charging, I will need to have zener diode across each battery.

I don't think this will give you what you want. In practice, you either need to check batteries individually, and balance them manually, or let a battery management system do this for you. The current through each battery in series is the same, naturally. What you want is for the voltage to be the same.

The problem is that the internal resistance of each battery can vary over time, and therefore the voltage can vary, with some cells becoming overcharged and others undercharged. Once they are out of balance, they tend to become even more out of balance. But people have managed batteries manually for a long time. Often periodic "balancing charges" (to perhaps as high as 14.7 volts or so) will rebalance a battery pack. Zeners set to just above this voltage would not work to keep batteries balanced most of the time (because they'd never come into play), and if set for less than the recommended balancing voltage, they'd all heat up pretty dramatically during balancing, I'd think. But, in any case, read up on balancing.

Finally, be aware that inverters that step up voltage are common. If you can keep your generator downtime short, you could use a much smaller, lower voltage battery pack, and use the inverter to step the battery pack voltage up to the correct AC voltage. It's worth poking around for inverters: you may find that a commonly used spec is far cheaper than one less commonly used, and that perhaps by changing battery pack voltage you can use a cheaper inverter... or not.

EWiz,

I design, sell and install battery systems. I have a 4,000Ah bank running my "off grid" home. We are working at this time on a client's system of 5 x 1500Ah (total on bus 7,500Ah).

The biggest battery I have come across was in my aprenticship (40 years ago) which was in the internationl exchange at the foot of the Post Office Tower in London. Each cell was approx 1m x 1m x 0.5m which would probably give it around 15,000Ah at C10. Being telecomms it was of course 50V.

I hope this makes it clearer where I'm coming from.

Capblanc,

I've seen some big banks in telecom and other backup services. I have been responsible for a couple of fair size banks but nothing like that!

I yield my warm spot in the mud pit to you sir!

Happy New Year!

TT3

I recall a scrap yard friend of mine who scrapped out a huge uninterruptible power supply for a mainframe Cray computer. It was a 240 Volt three phase UPS and had 3 battery banks full of 6 volt 160 AH deep cycle cells that weighed about 100 pounds each and it was online 24-7.

I cannot recall the exact number of these batteries, but each stack had it's own cart that was 6 feet high and about 3 feet by 5 feet and had about 8 tiers of cells with each tier being about 20 cells, so each bank.

I think these people supplied it to Cray

http://www.riello-ups.co.uk/

each bank was on a wheeled cart in a large doored rack and must have weighed many tons. Each one was single series bank

Capblanc:

Good evening, we appear to be arguing over semantics, approaching the 'question' from two different prospects, there is no problem with that. Responding to the thread within the bounds of what was known from the 'Questioner' and responses. In my opinion, there is not enough information from the 'Questioner' for a definitive answer. If he was familiar enough with batteries, he would have realized that using 12 volt 50 AH batteries to build a 240 v, 200 AH stack would not have been good engineering. I believe you agree with that assessment to a fair degree. I don't think that you built your 4,000AH battery stack in a similar manner. Let's just say that we agree to disagree over some details and leave it at that.

The simplest solution to his problem would be twenty 12 volt or ten 24 volt, 200 AH batteries, staying strictly within what he said, with the recommendation that 12v, 50 AH batteries was not a good idea, given the parameters of such a stack.

The biggest batteries I have seen (dozens of them) were 2.5m high, 1m deep and 1.25m wide, this being a single battery, 6 volts. Needless to say, the plates were massive.

Good luck on your current installation.

Battery powered industrial fork lifts have some pretty hefty batteries in them.I have seen some with individual cells ~ (2.1volts) that were approx.1Mx1Mx1M,and these were series-paralleled to give either 24 or 36 volts,(nominal).The "straps" were heavy guage lead or lead alloy bus-bars. These would probably be cheaper and less trouble than monkying around with all of the smaller batteries.I am not sure of the amp hour ratings, but they should easily be obtained from various fork lift manufacturers or battery companies, like Douglas or Exide.Good luck and be careful. HTRN