The car battery has 6 cells, each cell holds 2 volts, and connected in series adds to 12 volts. Just like your flashlight, each battery hooked in series results in increased voltage. Its possible one cell is not holding a charge, which slowly drains power from adjacent cells. In battery series connection, if one cell won't hold a charge, electrons will flow to lowest cell voltage in attempt to equalize voltage. So when one cell drops to less than 2 volts, the electrons from other cells will flow to this cell to recharge and bring voltage up to equalize voltage level. To diagnose for dead cell in 6-cell car battery, remove the 6 caps to expose the electrolyte liquid AFTER
you have recharged the battery for 20 minutes with 10-amp charger, or until "fully charged" indicator comes "on." With a digital voltage meter
set to 5 volts DC reading, connect the negative black probe to negative side of battery and touch the liquid electrolyte with the red positive probe. The voltage meter will display the voltage reading on each cell as you move the red positive probe from one cell to another. Make a note of each voltage reading from each cell. Wait one hour or more, then take another reading of each cell. The cell that has the lowest reading is the one that's draining power from the adjacent cells. A normal battery will show no change in voltage level between the first and second reading an hour later. A defective battery will show a drop in voltage to indicate one or more cell is slowly draining power to gradually render the battery completely dead 36 hours later.