Here is a brief summary of what to do...
The first thing that you must realize is that all batteries cannot be rebuilt. They must undergo a few simple tests to determine their present condition.
You will need some simple equipment and supplies in order to perform the required tests and to recondition the batteries:  
1.  A DC voltmeter.
2.  An automotive battery load tester. It comes with full instructions from the manufacturer.  
3.  A battery Hydrometer.
4.  A small electric drill.  
5.  A good quality battery charger.
6.  A pair of homemade wire probes to clip to the test leads on your voltmeter. These are used to test each cell in the battery. They should be about eight inches long and can be made from brass brazing rod. This retards corrosion of the probes.
7.  A plastic funnel to add the chemicals into each cell.
8.  A supply of the China Depot battery treatment chemical to dissolve the buildup of sulfates in the bottom of the battery.
9.  A stop drill bit to drill holes in the top of sealed batteries.
10. A supply of plastic plugs to seal the holes in the top of the batteries after repairs are completed. A good quality face shield to protect you eyes and face. NEVER WORK ON BATTERIES NEAR AN OPEN FLAME OR WHILE SMOKING. THE GAS FROM THE BATTERY CAN EXPLODE! ALSO, THE BATTERY ACID CAN BURN YOUR SKIN AND CLOTHING. Connecting cables to charge more than one battery at a time in parallel or in series if you have a series charger. Do not overload your charger.
This should cover your equipment and supply needs.
Now we will explain how to evaluate the battery for rebuilding or for scrap.  

1.  Place the battery on your test bench. Connect your load tester to the positive and negative terminals. The positive terminal is always the larger terminal and should have a + sign on it or next to it. DO NOT  HOOK UP BACKWARDS AS YOU COULD DAMAGE THE LOAD TESTER. ALWAYS PUT THE  POSITIVE LEAD ON FIRST AND REMOVE IT LAST TO AVOID DANGEROUS SPARKS.

2.  Read the voltage shown on the load test meter. If the voltage is less than 12 volts chances are that the  battery cannot be rebuilt. In the case of a six-volt battery it should read six volts. A battery that reads  a volt less might bear further investigating. While the load tester is still hooked up you should perform a load test. Pressing the button on the tester and watching the needle does the test. Hold the button down for ten or fifteen seconds. If the needle drops like a rock and stays on zero the battery is probably junk. If the needle gradually climbs up to the yellow or green on the load test dial there is a good chance the battery can be rebuilt. Remember that each cell if it is working should read two volts. If the meter only reads ten volts total then you must have a dead cell. We don’t fix dead cells because we don’t take batteries apart.

3.  One more set of tests that you can perform just to be sure is the cell test. We do two of them. One with a voltmeter and one with the battery hydrometer. If the battery has caps, pry them off. If it is sealed, drill the holes in the top of each cell with a stop drill so you won't hit the plates and damage them. Make sure all the cells have water covering the top of the plates. This could be the problem to start with.

4.  Take the hydrometer and draw up the acid from the first cell. The scale that is marked on the hydrometer will tell you the condition of that cell. Make a note of the reading of that cell. Green is good, White is questionable and red is dead and probably will not come back with the treatment. Notice the color of the electrolyte while you are drawing the electrolyte from the cell. Draw it in and out several times so that you will disturb it and mix it up. This will give you a more accurate reading and also if the color is cloudy brown or black it means that the cell is in bad shape.

5.  The voltmeter test is another very reliable test. Take the voltmeter and set in on the range that best reads out for twelve volts or six volts as the case may be. Clip the test leads onto each of the homemade brass probes.      

a.  Place the positive test lead on the positive terminal of the battery.

d.  Follow this procedure right down the line to the last cell. REMEMBER THAT THERE IS ONLY A ZERO READING FROM THE LAST CELL TO THE NEGATIVE POST. Otherwise the total would be fourteen volts and not twelve. You have now pinpointed the low or dead cells in the battery.

f.  If the battery you are testing passes all of the tests you may then place it on the charger for a long slow charge. It is important to note that most of the batteries on the market today have lead calcium plates in them and when they get discharged down to a very low voltage they require a long slow recharge. YOU MUST ADD ONE TABLESPOON OF OUR CHINA DEPOT BATTERY TREATMENT CHEMICAL PURIFIER INTO EACH CELL TO DISSOLVE THE SULFATES THAT ARE BUILT UP ON THE  PLATES.

i.  It is a good idea to save the electrolyte from the batteries that you junk. You may want to add some from time to time. This would be used in the case where you have a perfectly good battery but through  someone's carelessness they wore a hole in it and you want to repair the hole and refill that cell. NEVER ADD ACID TO A BATTERY JUST TO SERVICE IT. YOU MIGHT BLOW IT UP!  The best way to repair a hole in a battery is to use some fiberglass cloth and a two part epoxy resin repair kit. Just mix up the resin as per instructions and use it to glue several layers of the patch cloth over the hole. It is very important that you clean off the area around the hole with solvent that won't leave a residue (Denatured alcohol) and scratch up the case so the epoxy will stick better. Many a good battery has been saved with this procedure. Most of this is just common sense and always clean the rebuilt batteries with a mild solution of baking soda and water to neutralize the acid, and also clean up the terminals so that every battery looks as good as new. 

FULL MANUAL

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MARKETING SUGGESTIONS We suggest that you charge $24.00 minimum with an exchange battery in trade for your rebuilt batteries. Your warrantee should be twelve months prorated at one twelfth per month ($2.00) in this example. Remember that the most successful companies today are the ones who give excellent customer service. Be fair with your customers and don't be afraid to replace the product or give them their money back. It always pays in the long run. We sell all of our scrap batteries by the truckload. Most of the big battery companies will buy them back and recycle them. They use the lead, the plastic, and the acid. Nothing is wasted. They will buy them by the trailer load. They will usually pick up if you have a full trailer load.

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BATTERY DESULPHATOR

Application   When batteries age and/or spend too much time in a discharged or abused state, sulphite crystals can form on the battery plates which impede the flow of electricity and is the main cause of poor battery performance. This unit is a basic battery desulphator with advanced control techniques. This unit emphasizes extremely low cost and premium performance. Model BD-2 Basic Operation Sulphite crystals form when the battery charge is reduced and/or the discharge/ discharge cycles are not well suited for the battery. These sulphite crystals act as insulators to the flow of electricity in the battery, seriously degrading the capability of the battery to have electricity flow in it, reducing its charging and discharge capability. Like most battery desulphators, this unit uses a sharp pulse of current forced into the battery suddenly to "jar" the sulphite crystals and cause internal resonance, both mechanical and electrical, to grind down the sulphite crystals that form so they can be re-combined into the battery acid. For increased performance, this unit precisely controls the magnitude of the sharp pulse of current, which is adjustable by the user. This adjustment is important as no 2 batteries are the same and no 2 battery setups are the same. As well, battery desulphators, as they work with current spikes, may interfere with equipment connected to the unit. This adjustment allow the user to tailor both the current spike and at what voltage it is present. Features Single unit 12 to 48 V nominal systems Adjustable pulse strength 0.0 Amp pulse to 10 amp pulse LED to indicate unit operating LED to indicate relative strength of battery Adjustable on/off voltage Can be adjusted to only operate when battery is near full. EXTREMELY low draw Electrical Specifications Base Model: BD-2 Nominal battery voltages (V) 12 - 48 V DC Maximum Input (V) 63 V DC Maximum Pulse Current (A) 10 amps Range of Pulse Current 0.0 Amps to 10 amps   Adjustable with small screwdriver Range of On/Off Control 0 V to 70 V   Adjustable with Voltmeter and small screwdriver Self Consumption < 0.05 ma from battery at full pulse Efficiency >96% over 20% pulse load   User Controls Pulse Current Magnitude On/Off Voltage Mechanical Specifications Weight: 0.3 lbs. 0.3 lbs. Size: 3" X 2" X 1.5" Connections: Power #16 AWG Flying Leads Operating Temperature: -40 to 60° C Humidity: 0 - 90% non-condensing LED Indicators Unit Operating Note: In the interest of continuous product improvement specifications subject to change without notice. Please e-mail us for a quote