A car battery is typically a lead-acid type of energy storage device, consisting of six independent cells from the negative terminal side of the battery to the positive terminal side of the battery. The energy storage for each cell is around 2 volts each, meaning that a fully-charged battery with all of the cells working properly will show a stored voltage of around 12 volts. When, after a full overnight charge, a battery volt reading indicates a total voltage of 10.5 volts or less, this is a strong indicator that a cell has gone bad or that it has a short between itself and another on either side of it. To determine whether a battery has a bad or shorted cell, provide a proper charge and to then test the battery after the charging cycle is complete.
Things You'll Need
- Automotive battery charging unit
- Digital multimeter
Disconnect the car battery, remove it from the automobile and set it next to an automotive battery charger. Connect the negative (-) charger cable to the negative (-) terminal of the battery and then connect the positive (+) charger cable to the positive (+) terminal of the battery. Plug in the charger and set it for an overnight full charge, then allow the charge to take place.
Turn off the battery charger, then unplug it. Quickly remove the cables from the battery terminals.
Turn on your digital multimeter and set it to measure 20 volts DC (direct current.) Touch the black test probe tip to the negative (-) terminal of the battery and then touch the red test probe tip to the positive (+) terminal of the battery. Allow a few seconds for the voltage reading to be displayed on your meter. If the voltage reads a full 12 volts or slightly more, your battery is healthy. However, if the voltage reads 10.5 volts or less, then your battery has either a bad cell or two cells shorted together.
Tips & Warnings
- If your battery isn't a "maintenance-free" type and has removable covers on the top, remove the covers and use a flashlight to shine down into each cell and then observe whether a cell is dry (without fluid) in the cell reservoir. If so, fill that cell to just below the bottom lip of the pour hole and follow all of the above steps once again. Improper maintenance of the fluids in a battery can damage a cell, causing it to short with another nearby cell or to simply stop working because of its inability to perform chemical storage processes.
- Always charge a car battery in a well-ventilated area; the charging process can cause the battery to emit hydrogen gas, which can explode in the event of a spark.
- Photo Credit Polka Dot Images/Polka Dot/Getty Images
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