The Golden rule to remember about automotive batteries(lead acid batteries)is that they discharge at the rate of 1%(one percent)(for a good,fully charged battery)a day,that means that if you let your car sit for a typical month(31 days)your battery is 31%(thirty one percent) discharged,after 100 days of sitting unstarted your battery is 100 percent discharged.It also depends on your climate,the colder the climate(the lower the temperature) the more CCA's(Cold Cranking Amps) you will need to crank over your engine,(meaning that a battery in a cold climate fifty percent discharged may not be capable of starting up a car) vs someone in a hot climate,their car would most likely crank over and start even if their battery had been sitting unstarted (discharged)for 90 days (ninety percent discharged). Hope this helps,when you go trying to take your Deloreans out of Hibernation,and the battery is appox.90 percent discharged(90 days). Claude 000570 To check a battery, disconnect a cable, charge it up and see if it holds the charge for a day or two (charges usually tend to leak off with a bad battery)( Or just take it into a local garage they'll usually put a tester on it and tell you, free of charge). Try your meter again and see if it's holding around 12.5 volts or so.