Use a charger.
• Connect the red cable to the positive terminal, black to negative. If you have a "green eye" charge indicator on the battery, charge until it turns green. If not, disconnect the charger for half an hour and check the voltage (engine stopped) - 12.6 Volts indicates a full charge, 12.3 Volts half charged. Use a multimeter set to a DC Voltage range greater than 12 Volts full scale.
• Touch the red probe to the positive side of the battery, black to the negative side. A typical 5 amp charger may take more than 8 hours to charge the battery. More powerful chargers are for expert users, because they may blow up the battery! A tiny 1 amp cigarette lighter charger may be left connected indefinitely.
Make sure the battery terminals are clean.
A buildup of white powder between the terminal and the cable prevents a large current from flowing to the starter. Clean with a wet rag with baking soda sprinkled on. Use fine sandpaper if necessary. Be very careful when using metal tools near the positive terminal - avoid shorting the terminal to any metal part of the car.
Test the charging system with a Voltmeter.
With the car running put the multimeter on a DC Voltage range greater than 14 Volts and touch the probes to the battery terminals. The Voltage should be between 14.0 and 14.5 Volts for effective charging. If below 14, you probably need a new alternator. Above, you need a mechanic to adjust it.
If you suspect current is leaking and discharging the battery, check it with the current measuring part of a multimeter.
This is a little more complicated. Engine off for all this!
o Cover the positive battery terminal with a rag so you can't accidentally touch it.
o Use a wrench to disconnect the cable from the negative battery terminal.
o Set the meter to the largest DC current range (at least 10 Amperes; this may involve plugging the probe wire to a different jack in the meter).
o Use clips to connect the red probe to the disconnected cable and the black probe to the negative battery terminal. The current will then flow through the meter to the car, enabling the meter to measure the current. When first connected, the current may be large. An open door will draw at least 5 amperes for the interior lights, and the lights may stay on for several minutes after closing the doors.
o If you have an under hood light, disconnect it or remove the bulb. The current should go down to near zero. Most cars draw only 0.010 Amperes (10 mA) for the clock. Anything under 50 mA is alright. While watching the meter, rock the car, pat the fuse box, wiggle the doors (without opening). A current of even half an Ampere indicates a problem.
The current may be too small for the highest range of the meter.If you know the current is less than 1 Ampere, you can switch the meter down to the 1 Amp range. Be careful with lower ranges such as 100 mA - it is very easy to burn out that range on the meter with excessive current. If you have to move the probe lead to a different jack on the meter, you must use a clip lead to connect the black and red leads to each other during the move so the car maintains its connection to the battery and doesn't draw its larger initial current when connected again. At a current of .05 amperes (50 mA), a battery will take 700 hours (30 days) to half discharge. If your car's current is that low, you do not have a discharge problem. Do check that underhood light to make sure it goes off when the hood is closed. Or leave it disconnected.