I am building a large water tank. I'm thinking to convert it into a big lead-acid battery. I am interested if exists a formula that could calculate how much will be the capacity of that battery, voltage, energy density, specific power, specific energy and other parameters ...

The volume of the finished tank will be 5 cubic meters. If we assume that the tank will be filled with an exact mixture of sulphuric acid and water like in the small one battery what will be the battery properties.

Does the dimensions of the battery determines the capacity of one single cell? Does the dimensions of the battery determines the voltage of one single cell?

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    \$\begingroup\$ The voltage per cell depends on the battery chemistry and is not affected by the geometry of the plates (see en.wikipedia.org/wiki/…). Also: Building a large tank of acid in your back yard sounds liek a bad idea to me. \$\endgroup\$ – 0x6d64 Nov 14 '11 at 8:41
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    \$\begingroup\$ You'll probably need city authorisation to have an hazardous construction this big in your house/backyard. Which you as probably won't get, since it's a DIY project, and design quality can't be guaranteed. \$\endgroup\$ – stevenvh Nov 14 '11 at 9:38
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    \$\begingroup\$ @Patrik - If you made a mistake, then fix it! There's a little "Edit" button that allows you to make changes to your post right underneath the 'batteries' tag. \$\endgroup\$ – Kevin Vermeer Nov 14 '11 at 12:24
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    \$\begingroup\$ It's not the depth or just the size which may cause worries. It's that you plan to fill a 5m\$^3\$ (huge!) reservoir with acid. \$\endgroup\$ – stevenvh Nov 14 '11 at 12:41
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    \$\begingroup\$ That's going to be a lot of lead, and concrete isn't going to contain the acid. This sounds like a disaster waiting to happen. At the very least, you'll have to line the concrete with something that can hold the acid. Please don't do this at all if there are any drinking water wells within a mile. \$\endgroup\$ – Olin Lathrop Nov 14 '11 at 14:28

There are vast resources on the web re lead acid batteries. Key parameterrs are provided below a loom around the internet and sorting the good references from the not so good would be a helpful part of your necessary education if you are going to do what you suggest.

Energy able to be stored in your water tank if it was converted to a large lead acid battery can be roughly determined from the Wh/l figure that I give further down.

  • Energy density = 60 - 75 Wh/l

VANADIUM REDOX BATTERY - Energy stored in liquid !!!

  • The main advantages of the vanadium redox battery are that it can offer almost unlimited capacity simply by using larger and larger storage tanks,

For a battery where the liquid IS the energy store and where adding more liquid adds more capacity see Vanadium Redox battery.

They note:

  • The vanadium redox (and redox flow) battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy.

    The present form (with sulfuric acid electrolytes) was patented by the University of New South Wales in Australia in 1986.

    There are currently a number of suppliers and developers of these battery systems including Ashlawn Energy in the United States, Renewable Energy Dynamics (RED-T) in Ireland, Cellstrom GmbH in Austria, Cellennium in Thailand, and Prudent Energy in China.

    The vanadium redox battery (VRB) is the product of over 25 years of research, development, testing and evaluation in Australia, Europe, North America and elsewhere.

    The vanadium redox battery exploits the ability of vanadium to exist in solution in four different oxidation states, and uses this property to make a battery that has just one electroactive element instead of two. The main advantages of the vanadium redox battery are that it can offer almost unlimited capacity simply by using larger and larger storage tanks, it can be left completely discharged for long periods with no ill effects, it can be recharged simply by replacing the electrolyte if no power source is available to charge it, and if the electrolytes are accidentally mixed the battery suffers no permanent damage.

The main disadvantages with vanadium redox technology are a relatively poor energy-to-volume ratio, and the system complexity in comparison with standard storage batteries.


Lead acid voltage per cell, as in any battery chemistry that you will probably encounter, is very largely a function of the battery chemistry, with other factors making a vey small difference to the cell voltage.

The example battery cited here on the Wikipedia Lead-Acid battery page gives values of key parameters which you would achieve if you implemented a competent design. For a battery of the size you suggest this would be at best impractical and liable to be near to impossible. So consider these as values you can aim at but will not achieve.

Note that a number of these values are somewhat dependant on sub technologies or mechanical construction methods.

  • Voltage per cell:
    Open circuit fully charged 2.10 - 2.13 V / cell.
    Open circuit, fully discharged 1.95 V - 2.0 V / cell
    Loaded, fully discharged 1.75 V/cell Gassing threshold 2.35 V / cell

  • Specific energy 30-40 Watt.hour/kg ~= 0.10 - 0.15 MJ/kg

  • Energy density = 60 - 75 Wh/l

  • Specific Power = 180 W/kg

  • Charge efficiency 40% - 98% very much dependant on application circumstances.

  • Cycle life 100 - 1000+ cycles very much dependant on construction and usage patterns.


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    \$\begingroup\$ Thank you for your response. I searched the internet but wanted to know opinions from most valuable specialists in the field. When I asked this question I was very serious. When we are talking about energy crises we all have to think in that way. So how to produce, contain and transport electrical energy in the cheapest way possible ? I was thinking about a cheapest method to build an electrical energy container to contain enough energy to sustain heating in winter and cooling in summer at least a little. So I don't think the idea is stupid. \$\endgroup\$ – Patrik Nov 14 '11 at 12:39
  • \$\begingroup\$ In my country and I think elsewhere batteries are not cheap. So, why not build something with your hands. Experimenting I will surely come up with something. \$\endgroup\$ – Patrik Nov 14 '11 at 12:50
  • \$\begingroup\$ @Patrik - I did not say or suggest "stupid". I did say "... at best impractical and liable to be neat to impossible." I still consider this to be the case. |By all means investigate battery construction further but expect that a moderately conventional solution is MOST likely (not certain) to make most sense. LA batteries will be made in Turkey now.Cost of lead and acid and other components and labor will be major factors. Talk to battery makers and find what they say the issues are. \$\endgroup\$ – Russell McMahon Nov 14 '11 at 14:05
  • \$\begingroup\$ I think there was a misunderstanding. I don't got angry and definitely accept your advices. Prices are still debatable things. But certainly glad that there are experts who can help a lot. \$\endgroup\$ – Patrik Nov 14 '11 at 16:28
  • \$\begingroup\$ So for 5 cubic meters, you could store roughly 100 kWH of energy. \$\endgroup\$ – endolith Nov 15 '11 at 18:03

What I haven't seen much of is this: SIZE MATTERS! Not for voltage, that will remain constant, all else remaining the same, but size DOES affect AMPERAGE! More plate surface area, (and electrolyte) equal more amps, all else remaining the same. So, the 'bigger' the battery, assuming the identical construction, compared to a smaller battery, will have the ability to provide more AMPS, and for long term use, AMPS matter.

Suppose on were using an inverter with 90% efficiency, (Unlikely, but for the sake of argument) And a battery with 100 amp hours available. Now, build a BIG battery, giving 10 times the amperage... It's obvious, that the bigger one can run the same inverter for as much as 10 times as long at the same voltage and amperage... All else considered the same. (It won't be, but that's another story)

As for a container of concrete, I agree, bad idea. Not unless you line it, with either an acid resistant plastic or glass, siliconed in place. My own idea to make a 'recycled battery' is to start with cheap and readily available fish tanks, ten gallon sizes are plentiful and cheap. Taking 'dead' (shorted cell) batteries apart, I can salvage all the good plates, and silicone in glass separators, or partitions, for making cells. I'm less concerned with the precise surface area available, as I have access to many many old lead acid batteries, and I can just add in another fish tank.

Paralleling them adds to the amps, while the voltage should remain pretty constant. I also plan to use a type of perforated plastic for my separators between plates, it's easy to get, cheap, and acid resistant. And it is already full of holes. Cut to size with simple scissors.

Tie the tops of plates together by soldering them together, and leave plenty of empty room at the bottom of the tank for cast off... The stuff that caused the shorted cell in the first place. I know I won't get optimal efficiency, or near best cycling... as these will be already partially spent plates. But, I have a LOT of them, to make up for that. I have the room.

And access to every other component I would need. Except one: Either a solar panel or wind turbine large enough, or even a combination of them, to keep the cells charged and topped of for when I really need them. I could use a generator... Which would be a good idea once in a while anyway, but I'd like to have a set it and forget system... Until the grid goes down, which it does around here. (Tornado alley)

My inverters, which are NO where NEAR 90% effecient, came to me cheap... Old UPS units that were tossed out because the gel cells in them died. I also have several of the so called Modified sine wave inverters meant for automotive use... And i know from experience, that do NOT run anything with a motor in them very well. The motor runs slow, and gets hot. Motors really REALLY want to 'see' a true sine waveform, not a choppy stepped wave, modified from a square wave. Some things don't mind, incandescent bulbs, for example, but who would be using those in a power critical situation? NOT ME!

A better solution, in my opinion, is a HUGE one to one transformer. Like the isolation transformers we used to use working on TVs and such. I believe that will somewhat smooth out the modified sine wave output of the inverters, albeit, with even more losses. The losses, I believe, would be made up for by increased efficiency. Or, close to it.

My idea for this transformer is one of two ideas... Either, back to back microwave oven transformers, (only the 120 Volt sides, the high voltage windings will be removed), and/or, a service garage type battery charger transformer, using the 12 volt coils back to back, two identical units, or as close to identical as i can manage. Their HUGE wires would limit losses due to resistance.

The plus with microwave transformers is, they are so plentiful. Especially during spring cleanup. It's almost NEVER the transformer that makes them go bad and people throw them away.... In fact, I find most DO work. The people just wanted a newer or nicer one. Either way, it's unlikely the transformer is bad, and a quick check with a decent ohm meter will tell the tale in a matter of seconds. It's either good or it's not. (Usually) I built my first welder using microwave transformers, and have rewound many for specialized applications many times, including, heavy duty (for the home) car battery chargers.

So, I'm still gathering 'dead' batteries, and it's not hard to tell if it's just a shorted or 'dead' cell that killed it. Swelled sides will tell me too., what to look out for. In my case, I will not be trying to fit the plates as physically close as possible, just as close as PRACTICABLE. I loose a bit, I know, but, with say, 4 to 8 fish tanks... I really won't care. For the 24 volt applications, I double them up, parallel and serial, and for 12 volt applications, parallel connections.

WATTS are WATTS, either way. No way around it, which is a good thing here. 12 volts at 100 amps still only equal 1200 watts total. Assuming you can only realistically use half of total capacity, (Good practice if you want your batteries to last very long) then, that's only 600 watts. TOTAL. over time, we call it watt hours, and the slower you discharge the more you can take. Discharge at high rates, and the battery will go flat much faster.

Most batteries you see advertised, have a little disclaimer somewhere, saying that the rated capacity is only at something like a 20 hour rate. Not very realistic most times. That means, they artificially raise the apparent rating by a huge factor, when in fact, that 100 amp battery is really only a 50 amp or so. The 10 hour rate is more or less the standard.

And do not forget the square root of 2, rounded to 1.4.... That's what you have to put back in to replace what you took out. Roughly. If you take a 100 amps out, you need to use 140 amps to return it to a state of full charge. This is not counting a float charge, to protect against self discharge. That's in addition, not part of, the 140%.

Anyone have any experience using ANY means of making cheap inverters, or even not cheap UPS units, provide something closer to a true sine wave? Will transformers work, and if I do that, will I loose more than I gain? What are the gotchas in this?

I know I can use digital, aka, switching power supplies with no problems, but anything like a FAN... is a no go. I've tried it. My best luck came from using 12 volt DC fans. (In summer, when it's muggy as all heck, as it ALWAYS is when the storms knock the power out)

I have a generator, but it uses something like 12 gals of gasoline at less than half output, and MUST run at 3600 RAM even at NO load, meaning, it SUCKS the gasoline. Which is hard to get when there's no power for three days. Storing enough for a week long outage, ( the longest we've had in recent years) is both very expensive, and unsafe.

I'd rather switch to propane if I were going to do that. At least, it's rather safer than gasoline, and doesn't go stale... Having to rotate that amount of gasoline every two to three months is seriously expensive. and a pain the... neck. Also, theft becomes an issue when I'm the only kid on the block with any gas... But no one steals propane... Not the big household sized tanks. I do happen to have a 500 gal propane tank out back.

The Scarecrow said that: 10-7 and out.

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    \$\begingroup\$ Good answer but it may be a bit clearer with some line breaks / paragraphs. If you agree you can edit your own answers at any time and see what it looks like in the lower preview window while editing. \$\endgroup\$ – PeterJ Apr 30 '13 at 2:56
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    \$\begingroup\$ Wow! I would love to read your answer Scarecrow, but as it stands I'm afraid I might get lost and never make it to the end. \$\endgroup\$ – bhillam Apr 30 '13 at 3:15
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    \$\begingroup\$ Breathe! Please! \$\endgroup\$ – Anindo Ghosh Apr 30 '13 at 4:46

Personally, this is not a project I recommend doing for purely safety reasons. Not without extensive safety precautions and a lot of research. You are dealing with both toxic and corrosive substances. Unless you know exactly what you are doing the results could be fatal.

For example, the mixing of the electrolyte, sulfuric acid and water. Do it the right method and everything is OK. Do it the wrong way and a violent exothermic reaction results involving a great deal of heat with the possibility of an explosion. 5 cubic meters of battery is one helluva big battery and could do extensive damage if it were to explode.

There is also the hydrogen gas emitted while charging so ventilation is also an issue. The weight of the electrolyte and lead would probably be to much for the glass tank to contain.

There is also the weight to consider. A forklift battery is approx 1 cubic meter and weighs approx 1600-3200 lbs. What you are contemplating constructing is 5 times that volume.


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