Building a large lead acid battery?
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:

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.


Useful:


paper on 3.3 amps/100Ah battery capacity charge efficiencyNote: real world solar charge currents go up to 35 amps/100Ah. 
http://en.wikipedia.org/wiki/Lead-acid_battery
http://wattsupwiththat.files.wordpress.com/2011/11/ridley_rsa.pdf