Lead acid batteries have been around for a hundred years and are primarily used in automobiles. These batteries are made up of stacked lead and lead oxide plates. The plates are surrounded by an electrolyte solution. The make-up of the solution is 35 per cent sulphuric acid and 65 per cent water. You can test some lead acid batteries using a hydrometer. When you do this, you are actually measuring the amount of sulphuric acid in the electrolyte solution of your battery. All batteries can be tested using a voltmeter.
Charge the battery fully. The alternator in your vehicle will fully recharge the battery after about thirty minutes of driving. You can also hook your battery up to a trickle charger. The charger will indicate when the battery is fully charged, usually with a green LED light.
Remove the surface charge. To do this you can turn your headlights on for several minutes.
Turn the headlights off. Let the battery sit for several hours.
Check your battery to see if it has removable cell caps. They will be made of plastic and will be on top of the battery. If your battery does not have these, you will not be able to use a hydrometer and should skip to the next section.
Unplug the battery cables. Make sure to tuck the cables away so that no part of the connectors are touching the battery cells.
Pry off the cell caps. You can do this with a flat head screwdriver.
Insert the hydrometer into each of the openings and take measurements. The reading on the hydrometer will tell you the specific gravity of each cell. There should be no more than .05 difference between the cells. The specific gravity should be above 1.225, otherwise the battery's days are numbered.
Attach the clip with the red cable to the positive cell of your battery.
Attach the clip with the black cable to the negative cell.
Check the meter reading. The meter will indicate what the voltage is. The voltage for a 12V battery should be no lower than 12.4 and the voltage for a 6V battery should be no lower than 6.2.
Wear rubber gloves and goggles when using the hydrometer.