You've probably disposed of several "dead" car batteries thinking you can't rebuild them, but in many cases you actually can. Car batteries usually have six individual cells that contain lead plates and electrolyte fluid. If the electrolyte fluid falls below a certain level the cell effectively dies. If you find you have a car battery that doesn't...
M/F Place your car battery in a battery tray on a stable work surface. Put on goggles, rubber gloves and ensure you are wearing old clothes. The electrolyte fluid in the cells is sulfuric acid so you don't want it to get on your hands or in your eyes. It will also burn clothing.
M/F Remove the cell plugs so you access the electrolyte and repair the cell structure. Car batteries usually produce 12 volts so you have six cell plugs to remove as each cell produces 2 volts connected in series. Check how the cell plugs are fitted. You find three main types; plugs with slotted tops, knurled plugs and push on plugs. So use a flat-head screwdriver, your fingers, or pry off the plug using a screwdriver, depending on the way the plugs are fitted. Put the plugs in a small metal tray as they may have fluid on them.
M/F Look inside each cell. Notice two marks on the inside of each cell. These represent the minimum and maximum level for the electrolyte fluid. Once the fluid level drops below the minimum mark the flow of electrons between the positive and negative lead plates starts to reduce. This affects the cells ability to charge. If the level gets too low the lead plates corrode and often the damage is too severe to rebuild the structure. However, it's always worth trying.
M/F Pour distilled water into each cell slowly and carefully until it reaches the maximum mark. Do not overfill because the lead plates and fluid warms up and expands during charging so you need to leave expansion space. You must use pure distilled water. Mineral or tap water contain impurities and other unwanted chemicals that affect the lead plates and charging process.
M/F Replace the plug covers on each cell. Either screw them in place or push them tight using your fingers.
M/F Connect your battery charger to the battery terminals. Put the clamp on the end of the red cable onto the positive battery terminal labeled "+" then put the clamp on the end of the black cable onto the negative battery terminal labeled "-."
M/F Set your charger to a low charge rate; the lower the better. Slow charging allows the distilled water to mix with the existing electrolyte fluid and turn acidic. Once the acidity increases the electron flow speeds up and the rebuilding process starts. Turn on the charger and let it charge for at least 24 hours, or 36 if you possibly can.
M/F Check the side of the battery after 12 hours. Touch each place adjacent to the cell plugs using your hand. Each cell is getting warm, if the rebuilding process is working. If you find any of the cells are cold to the touch it probably means the cell is dead.
M/F Turn off the charger and disconnect the battery cables after 24 or more hours. Touch the side of the battery again to see if the cells are warm. The warmer the cells the better the rebuilding process has worked.
M/F Leave the battery to completely rest for 12 hours or more. If the rebuilding process has worked the battery retains its charge. If it hasn't the output voltage reduces quickly.
M/F Set your multimeter to read voltage. Place the end of the red wire from the multimeter onto the positive terminal of the battery. Place the end of the black wire from the multimeter onto the negative terminal of the battery.
M/F Read the output voltage. It reads 12 volts, plus or minus 5 percent, the rebuilding process has worked. If it reads much below 11 volts the battery is losing charge. The lower the output voltage, the worse the condition of the battery.