There are two processes that occur in a car battery: the charging process and discharging process. The charging process converts the electrical energy input into chemical energy. The discharging process converts the chemical energy back into electrical current output. When a load is connected across a battery, the circuit of the electrolytic cell is completed. The circuit is completed by flow of current in form of cations and anions in the cells and in form of electrons through the external circuitry.
The key to maintaining a good battery output is to maintain the chemical balance of the cell. Water is an important factor in the chemical reaction inside the cell. Dropping water levels in a car battery can cause the voltage output to lower down. If the battery is overcharged, some extra chemical reactions may occur which otherwise, do not. This hampers the reversibility of the chemical reaction. That is, the charging/discharging processes that are reversible reactions cannot occur anymore. Therefore, overcharging the battery should be avoided by monitoring the voltage levels while charging.
The only way to gauge a battery's performance is to test the voltage output it is offering. The output voltage levels that it provides with and without load can give you an idea of battery health.
So to test a car battery, arm yourself with a voltmeter and set the dials on the 0-50 V range. Disconnect the battery from the car connections by following the details in the car manual to the word. While doing so attach a 9V alkaline battery to the car PCM as otherwise it loses its programmed settings.
Connect positive red lead of voltmeter to positive of battery terminal and the black lead to the negative terminal. Check the voltage level. If its in the range of 12.6 to 12.8, your battery is doing well and adequately charged. Anything below that like 10 V or lower means it is in need of charging.
Now reconnect the battery and start the car. Keep it in idling mode and check the voltage reading between the same points again. If it is around 10V, then the battery is okay. Anything below 10 V means that either the car battery needs charging, or some other problems are causing the voltage decline.
Next thing to check is the fluid levels in the battery. To do this you must disconnect the battery again of course. Open up the battery cover and peep into the compartments of cells inside. The electrode plates should be submerged in the electrolytic fluid. If they are not, you must add distilled water to the cell compartment only uptil the fluid level rises about a one fourth of an inch above the plate. Another reason causing low battery charging may be, due to car alternator problems. Get it checked with the mechanic as soon as possible.