Firstly, always disconnect the negative battery cable first and hook it up last so as not to short out the battery. To test a battery, you can check the indicator eye or see if the electrolyte level has gone down. Besides, hydrometer should be used to determine specific gravity of the electrolyte. Measure open voltage circuit using a digital voltmeter. Finally, prior to taking any voltage readings, it is mandatory to deplete surface charge. In addition, by using a battery load tester, one can evaluate whether this component can start an engine and run all car accessories at maximum capacity simultaneously. Battery drain test is required to detect constant drain in an electrical system. Hence, before removing a battery, disconnect cables and take off both holding fixture and covering of that unit as well as lift out this item from its place. The tray where the battery rests must be examined for corrosion and cleaned if necessary. It means you have to install new batteries that correspond with specifications given on old ones.