Re: Can you use alternative acids to Sulfuric acid in a lead-acid battery?

Dave, I am afraid that there is good news and bad news.

The bad news is that the lead-acid battery is quite specific for sulfuric acid. It is based around 
reactions that turn metallic lead into lead sulfate & water at one electrode, and lead dioxide into lead 
sulfate & water at the other. All three lead substances are insoluble solids. There are good reasons 
why you need solids rather than solutions to achieve a battery with steady voltage and reasonable 
capacity. Otherwise we would all be using the pretty little zinc/copper cells that are typically used as 
the first illustration of electrochemistry in high schools.

There are only about five or six practical rechargeable battery systems that have been developed. 
Mostly they involve substances even nastier than lead and sulfuric acid (e.g. cadmium and caustic 
potash). Lead acid batteries are particularly good at providing a large number of discharge-recharge 
cycles. Unlike NiCad and AlCad batteries, they handle shallow discharge and recharge very well -- 
deep discharge rather less well. Lithium or metal hydride batteries can really only be used as sealed 
units.

The good news is that sulfuric acid is almost completely non-volatile. If your battery is well prepared, 
there should be almost no escape of sulfuric acid from your cells. Consider your car battery -- it 
travels open in a confined engine compartment that gets quite hot at times. Sometimes you get a little 
corrosion, but not a lot, and almost none at all if you look after your battery.

Battery acid is a fairly concentrated solution of sulfuric acid in water -- roughly one part pure acid to 
two parts water by weight. Any evaporation involves mainly the water. Sulfuric acid is a heavy, oily 
liquid that does not boil till well over 200 deg C. It is not hard to arrange to trap the tiny bit that might 
evaporate. For example (nylon) flywire across the top of the cell, supporting chips of slaked lime, or 
limestone, or washing soda should do the job.

But your battery will need to be well conditioned. Although very little sulfuric acid will evaporate, a bad 
cell can make a lot of acid spray if it starts gassing at all. Sometimes a side reaction can occur which 
produces gaseous hydrogen and/or oxygen. When the little bubbles of gas burst at the surface, they 
propel microscopic droplets of sulfuric acid into the air. The formation of this acid aerosol is what 
causes the corrosion that can occur around lead-acid batteries. If the battery is properly prepared and 
kept in good condition, gassing should not occur. It tends to be more of a problem on recharge than 
on discharge.

You have not said in your explanation why a "huge" rechargeable battery is required. Is it for aesthetic 
reasons, or because you require large voltage, or large current? For large voltages, there are probably 
better ways to go; for large currents you will need to be very careful of how you design your "wiring" 
(or it will get too hot). I presume you are familiar with those aspects of what you are trying to do.