Rebuilding Batteries

Yep that's right - you can rebuild lead acid batteries.
Lead acid batteries can die from a number of causes but no matter what the cause they can be rebuilt.
According to all the battery manufactures and other sources of information I have read - the leading cause of battery death is due to sulfation (Although I have never seen one). The second cause is shorted cells (I have seen heaps of these).
The third is cell interconection failure due to corrosion (I have also seen a few of these).
Other causes are excessive top up with contaminated water, broken casings and general old age.
In my experience every failure I have seen can be attributed to anodic corrosion. The positive plates have simply corroded away.

No matter what the cause of death - materials can be salvaged from the battery and used to build new batteries.
When a lead acid battery dies you can bet it's the positive plates that are at fault especially if it has pasted positive plates. Anodic corrosion will eventually break down the positive plate grids leading to reduced capacity and eventual failure.
Oxides recovered from the corroded positive plates can be used to make paste for new plates. See battery recycling
However the negative plates will usually be in almost perfect condition although they maybe have a little sulfation.
The negative plates can be used to form a new cell. This means from the 6 cells in a 12 volt battery you can recover 3 working cells.

Drain all acid from the battery.
Do this by tipping it upside down in a bucket and leave it over night to drain as much acid as possible from the battery.
DO NOT DISCARD ACID DOWN THE DRAIN. The acid can be re-used. See battery recycling

Method 1.
Re-using the negative plates to form new cell.

Always ware protective or old clothing, gloves and eye protection when messing around with lead acid batteries.
Cut the top off the battery to be rebuilt. Use a hack saw or make a hot wire to cut through the plastic. Do this carefully if you intend to re-use the case. Cut around the terminals leaving at least 5mm of plastic around them.
With the top removed cut through the interconnects connecting the cells together. Again do this carefully and try to limit the amount of damage done.
Carefully remove each of the cells. Some cells maybe jammed in the case due to sulfation but you should be able to get them out.