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The Best Way to Check Lead Acid Batteries

A lead-acid battery kept in good condition ensures you won't have a problem starting your car on a cold morning. Lead-acid batteries in poor condition lose energy quickly at low temperatures, so even if you car starts fine on a warm day you can suddenly find the battery goes dead overnight. You should regularly check the condition of your battery by using a multimeter and a hydrometer.
Multimeter Check
Charge your lead-acid battery using a car battery charger until full. Disconnect the charger then turn on your car's lights for about 15 minutes. You need to slightly lower the charge level to get an accurate reading. Alternatively, leave your battery for about 12 hours after charging it.
Set your multimeter to read direct current voltage (DC). Use the metal prongs on the end of the two wires from the multimeter and place them on the lead-acid battery terminals. The prong on the red wires goes to the "+" battery terminal and the prong on the end of the black wire goes to the "-" terminal.
Read the measurement displayed on the meter. It reads about 12.7 volts if your battery is fully charged and in good condition. However, if it's below 12.4 volts then it means it's only retaining 75 percent of the charge and may need replacing shortly. A reading lower than 12.2 volts indicates your lead-acid battery is in poor condition and you should consider replacing it as soon as possible.
Hydrometer Check
Remove the caps from the battery cells. Some caps have slots, so use a reasonably large flathead screwdriver to remove them. If the caps don't have slots you can unscrew them using your fingers; rotate the caps counter-clockwise.
Place the tip of the hydrometer into the first battery cell. Make sure the tip is covered by the fluid in the cell. Remove the hydrometer and read the measurement on the side. Make a note of the reading.
Repeat the check on the remaining cells using the hydrometer and make a note of each reading. The readings will be within 0.05 of each other if all the cells are in the same condition. The readings are 1.265 if your battery is 100 percent charged. A reading below 1.125 means you need to consider replacing the battery soon. If any of the readings are below 1.190 you need to get a replacement quickly; your car may not start on a cold morning.
Replace the caps on the battery cells. Use a screwdriver or your fingers as appropriate.